PCAT Exam: Chemical Processes - Organic Chemistry

Organic chemistry is a branch of chemistry that involves the study of the structures, properties, and reactions of organic compounds. Organic compounds are molecules that have carbon as the backbone molecule. Organic chemistry studies range from understanding the nature of various molecules to describing chemical reactions in laboratories and living organisms.

Structures and Properties
Organic compounds are carbon-based compounds, meaning the carbon (C) atoms form the skeleton or backbone of the molecule. In organic compounds, the C atoms bind to other atoms, mostly hydrogen (H), oxygen (O), and nitrogen (N). Organic molecules can also contain other atoms, including sulfur (S), phosphorus (P), and halogens (X) such as fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). Functional groups are the different atoms and how they are bonded together. Functional groups give the organic compound its molecular properties and reactivity.
In organic chemistry, functional groups help predict the chemical behavior of an organic compound because molecules possessing the same function groups undergo similar patterns of chemical reactions. Adjacent functional groups may also influence this reactivity. Functional groups are used to classify organic compounds into different chemical classes.

An example of this is alcohols. The molecular and structural formula of an alcohol is shown below. The functional group is the –OH group.



Structural Formulas and Bonding
Hydrocarbons are organic compounds that consist of only H and C atoms. They are classified into two major categories: aliphatic hydrocarbons (open chain) or cyclic hydrocarbons (closed chain). These groups are further divided. The overall classification of hydrocarbons is illustrated in the chart below:



Aliphatic Hydrocarbons (Open Chain)
Aliphatic hydrocarbons have two open ends, meaning the C atoms at two terminals of the chain do not connect together. Because an aliphatic hydrocarbon chain has an open structure, it is often called an open chain hydrocarbon. Aliphatic hydrocarbons can contain single, double and/or triple C-C bonds. These different types of bonds classify aliphatic hydrocarbons into saturated hydrocarbons and unsaturated hydrocarbons.
Saturated hydrocarbons are the simplest form of hydrocarbons, and a type of aliphatic hydrocarbons. They contain only single bonds and are called alkanes. The single bonds are σ-bonds. Therefore, saturated hydrocarbons are more stable (or less susceptible to chemical reactions) than unsaturated hydrocarbons, and there is full rotation between all C-C bonds. In these molecules, the carbon bonds have the maximum number of H atoms possible, and are fully saturated with H atoms. Saturated hydrocarbons can have a linear or branched structure. In a linear structure, all of the carbon atoms form one single line. In a branched structure, the carbon chain splits off, creating multiple “branches” of carbon chains. The general molecular formula of saturated hydrocarbons (or alkanes) is CnH2n+2, where n is the number of C atoms in the chain.

Below are examples of short-chain saturated hydrocarbons.


Unsaturated hydrocarbons are hydrocarbons that contain one or more double or triple bonds between C atoms in the hydrocarbon chain. They are further categorized into two groups: alkenes and alkynes.
Alkenes are hydrocarbons with double bonds. They have the general molecular formula of CnH2n. In their double bonds, one bond is a σ-bond and the other is a π-bond. Because of the presence of π-bonds, alkenes are more chemically reactive than saturated hydrocarbons. Additionally, there is no rotation between the two C atoms in a double bond. Below are examples of the short-chain alkenes ethene and propene:


Alkynes are hydrocarbons with triple bonds. They have the general molecular formula CnH2n-2. The triple bonds in alkynes consist of one σ-bond and two π-bonds. Alkynes are quite unstable and are more reactive than alkanes and alkenes. Like alkenes, no rotation exists between the two C atoms in a triple bond.

Below are examples of the alkynes ethyne and propyne:



Cyclic Hydrocarbons (Closed Chain)
Cyclic hydrocarbons have the terminal C atoms connected to each other, forming a closed, cyclic structure. Therefore, they are closed chain hydrocarbons. These hydrocarbons might have single, double, or triple bonds. Cyclic hydrocarbons are classified into alicyclic hydrocarbons and aromatic hydrocarbons.
Alicyclic hydrocarbons have one or more carbon rings and can contain either saturated or unsaturated bonds. They have similar properties to aliphatic hydrocarbons. Below are examples of alicyclic hydrocarbons:
                  
Aromatic hydrocarbons are cyclic hydrocarbons with σ-bonds and delocalized π-electrons between the carbon atoms that form the ring. Delocalized π-electrons are the result of a structure with resonance. This structure is often written as a molecule with a double bond between every other carbon atom, but this description is not entirely accurate. The double bonds are “shared” between all of the carbon atoms in the ring. This creates a very stable, unreactive structure. The geometry of these rings is flat and rigid. These compounds are also called arene, or aryl, hydrocarbons. Benzene is the simplest aromatic hydrocarbon. Historically, individuals thought that aromatic hydrocarbons contained a pleasant odor, resulting in the descriptive name. Aromatic hydrocarbons constitute a major part of crude oil and are highly flammable in nature.



Naming of Hydrocarbons
The naming of hydrocarbons is performed by the IUPAC (International Union of Pure and Applied Chemistry). According to this system, saturated hydrocarbons (alkane) are named by adding “ane” to the end of the prefix that corresponds to the number of C atoms present in the chain. An “alkyl” group is the name for a hydrocarbon with one H atom removed. These often serve as functional groups, and their name corresponds to the name of the alkane with the same number of C atoms.

The table below lists alkanes and corresponding alkyl groups based on the number of C atoms in the chain.
 

 When alkanes are branched chains, the following rules are followed:
1. Find the longest chain of C atoms. This is the parent hydrocarbon. Name the alkane by using the rules listed above.
2. Identify all of the alkyl groups attached to the parent chain.
3. Number the parent chain so the first alkyl group is located closest to the number 1. If there are different alkyl groups equidistant from each terminal C atom, assign the lowest number to the one with the most substitutions or the one that that will come first in the name.
4. Name each substitution, starting with the number of the C atom, denoting its location and the name of the alkyl group. If more than one of the same alkyl group exists, list them all with commas separating the C atom number. In some cases, they may be on the same C atom.
Please note this is a simplified version of the rules. More rules exist for side chain priority.


Similar to saturated hydrocarbons, unsaturated hydrocarbons are named using the longest C chain, but with adding “ene” (double bond) and “yne” (triple bond) to the end. The number of the C atom containing the double bond is added to the end of the name. The terminal C atom closest to the unsaturated bond is numbered 1. Below are examples of unsaturated hydrocarbons:


If the hydrocarbon chain contains two or three double bonds, then they are called “alka-di-ene” and “alka-tri-ene,” respectively. Similarly, the unsaturated chains containing two or three triple bonds are called “alka-di-yne” and “alka-tri-yne,” respectively.


The C chain might contain various functional group(s), and they are named accordingly by adding the name of the functional group to that of the parent carbon chain.

Common functional groups found in organic compounds are listed in the table below:

 
Isomers and their Classifications
Isomers are two or more molecules with the same molecular formula, but with different arrangements of atoms in the molecule, which can give them different chemical and physical properties.

For example, ethanol (CH3-CH2-OH) and dimethyl ether (CH3-O-CH3) have the same molecular formula, C2H6O. However, they have different physical and chemical properties, due to the variation in their structural formula.
Isomerism is divided into two major categories: structural (constitutional) isomerism and stereoisomerism (spatial isomerism).
Structural isomerism is a type of isomerism in which the isomers have identical molecular formula, but the atoms in the molecules have different arrangements.

There are four types of structural isomerisms: chain isomerism, position isomerism, functional group isomerism, and tautomerism.

1. Chain isomerism is also called skeletal isomerism. Chain isomerism occurs when the backbone atoms (usually C atoms) have different arrangements, such as straight or branch structures.

Using the figure below as an example, butane and 2-methyl propane (isobutene) have the same molecular formula, C4H10, but different chain structures. This results in different physical and chemical properties.


2. Position isomerism, also termed regioisomerism, occurs when unsaturated (double or triple) bonds or functional groups have different locations in the chain. For example, 1-butene and 2-butene are position isomers because they have the same molecular formula (C4H8), but the double bonds are located at different positions. The same is true for 1-propanol and 2-propanol (isopropyl alcohol), where the functional group (-OH) is located on different C atoms. Position isomerism is also possible in aromatic compounds, based on the relative positions of the substituent atoms/groups in the benzene-ring structure.


3. Functional group isomerism refers to isomers that have the same molecular formula but different functional groups. For example, ethanol (CH3-CH2-OH) and dimethyl ether (CH3-O-CH3) are functional isomers because they have same molecular formula (C2H6O), but have different functional groups—hydroxyl (-OH) and ether (-O-) groups.


4. Tautomerism refers to a special type of structural isomerism resulting from spontaneous inter-conversion of functional groups to form two different compounds. This process primarily results in migration of a proton and switching between single and double bonds. Tautomerization is a chemical reaction that reaches equilibrium between the two tautomers (isomers). One example of tautomerism is “keto-enol tautomerism.” It causes conversion of keto (-C=O) group to from a double bond (“ene” from alkene) and an alcohol group (-OH) (“ol” from alcohol). Therefore, it is called keto-enol tautomerism.

See the example below in which a ketone, acetone (propanone), undergoes tautomerization to form an alcohol, 2-propenol:
 
Stereoisomerism is the type of isomerism in which molecules have similar molecular and structural formula, but they differ in their three-dimensional configurations and orientation of atoms in space.

There are two types of stereoisomers: diastereomers and enantiomers.

1. Also called configurational or cis-trans isomers, the diastereomers have different orientations of atoms/groups along the C-C bond axis. Diastereomers generally contain a double bond or cyclic structure that has restricted bond rotation. The different isomers are designated “cis” and “trans.” “Cis” isomers have similar atoms or groups on the same side of the C-C bond axis. “Trans” isomers have similar atoms or groups positioning on the other side or across the C-C bond axis.

Examples of cis and trans isomers in double bond and cyclic structures are presented below:


2. Enantiomers are stereoisomers that have a different arrangement around a chiral (asymmetric) center (usually a C atom). Enantiomers are mirror images of each other that cannot be superimposed on top of each other. A non-chemistry example this concept is your hands. If you hold them facing each other, you can see they are mirror images of each other. However, it is impossible to superimpose your left hand on top of your right hand.

When these isomers are in a symmetric environment, they have the same physical and chemical properties, but they behave differently in symmetric environments. Also, enantiomers can rotate plane-polarized light.
Most enantiomers are formed from a chiral carbon, which is a C atom that has four different atoms or functional groups attached to it. They are also called an asymmetric carbon. The four different atoms/groups can be arranged in two different ways around the chiral carbon, creating left-handed and right-handed configurations. These configurations are mirror images and do not superimpose. The number of optical isomers that can be formed from an organic compound is 2n, where n equals the number of chiral C atoms in the molecule.

There are two naming conventions for enantiomers. One stems from their ability to rotate plane-polarized light. The enantiomer that rotates the light to the right is called the dextrorotatory or (+) or d-isomer. The enantiomer that rotates plane-polarized light left is called the levorotatory or (-) or l-isomer. The other naming method uses the Cahn-Ingold-Prelog priority rules, which ranks the functional groups based on their atomic number (a higher number has a higher priority). If the molecule is rotated so that the lowest priority group (4) faces you, the remaining three groups will either increase priority in a clockwise direction (the R enantiomer), or will increase priority in a counterclockwise direction (the S enantiomer).

There is no correlation between d/l and R/S naming conventions.
 
The example below shows a chiral carbon-containing compound, lactic acid [CH3-CH(OH)-COOH]:


Normal visible light is a mixture of electromagnetic waves.

The waves oscillate (up and down motion of the light wave) perpendicular to the direction the wave moves. In normal visible light, this oscillation can occur in all planes (directions) around the wave. Plane polarized light is light that is monochromatic (has only one wavelength), and all of the waves oscillate on the same plane. Plane polarized light is created by using a lamp that only produces light with one wavelength and then passing that light through an optical filter called a polarizer.


Optically-active compounds have the ability to rotate the plane polarized light. The instrument used to measure the angle of rotation of plane polarized light, while passing through an optically-active compound, is called polarimeter.

The overall process of studying the properties of optically-active compounds is presented in the figure below:


A racemic mixture contains an equal amount of d-(also called R) and l­-(also called S) isomers, which do not rotate plane polarized light. For example, the mixture of equal ratio (1:1) of d­-lactic acid and l-lactic acid is a racemic mixture. A racemic mixture is presented by dl or ± or RS. If the mixture is not a 1:1 ratio, a slash is used instead (i.e. d/l or (+)/(-) or R/S).



Catenation
Carbon atoms can undergo a unique process called catenation, which make them able to form a vast number of organic compounds found in nature. Catenation refers to the linkage of the same kind of atom to form longer chains and structures. Carbon catenation helps build numerous types of structures, containing open chain, branch chain, or cyclic configurations in organic compounds. C atoms can form single bonds (a, in the figure below), double bonds (b), or triple bonds (c). C atoms can also form ring structures (d).


The success of C atoms to undergo catenation is because of high stability of the covalent C-C bonds. This high bond stability is a result of the relative electron affinity of C atoms and the small atomic radius of the atoms to face across the intra-atomic orbitals and form C-C bonds (σ (sigma) and (pi) bonds).

Covalent Bonds
A covalent bond is a type of chemical bond that is formed through sharing electron pairs between atoms. Contrary to ionic bonds (found mostly in inorganic molecules, e.g. Na+Cl-) that involve electron transfer between atoms, covalent bonds involve electrons shared between atoms. A covalent bond formation, through the sharing of electrons, helps the corresponding atoms attain a stable electronic configuration at the outer valence shell.
The C atom has four valence electrons in the outer electron orbital. The octet rule states that most atoms combine to result in eight valence electrons in the outer electron orbital. In order to get a stable atomic configuration, carbon can share those four electrons to form four covalent bonds. Below is an example of covalent bond formation in methanol (methyl alcohol). In this example, a C atom is attached to H and O through the sharing of electrons.


Atomic orbitals are the space, relative to the atom’s nucleus, where one or more electrons persist. Covalent bonds are formed through an overlapping of the atomic orbitals. There are two types of covalent bonds: sigma (σ) and pi (π). Typically, a single bond is a σ-bond, and a multiple bond has both σ-bonds and π-bonds. For example, a double bond has one σ-bond and one π-bond, while a triple bond has one σ-bond and two π-bonds.

Sigma (σ) Bonds
A sigma (σ) bond is the strongest type of covalent bond. It forms from the direct and linear overlapping of atomic orbitals.
1. Formation of σ-bond by overlapping s-orbitals:


The nature of a σ-bond allows for rotation around the bond.

Pi (π) Bonds
Pi (π) bonds are formed as a result of parallel overlapping between two p-orbitals, which is a weaker and more diffuse bond than the linear σ-bond. π-bonds are weaker than σ-bonds and more susceptible to fission in chemical reactions. The parallel nature of a π-bond prevents rotation around the bond. Any rotation would cause the π-bond to break.



Properties of Organic Compounds

Alkanes
Alkanes are saturated hydrocarbons with a general molecular formula of CnH2n+2. Some examples of alkanes are methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H8).

The physical properties of alkanes are as follows:
1. Physical State: Lower molecular weight alkanes (C1 to C4) are odorless gases at room temperature (i.e., methane, ethane, propane, and butane). Higher alkanes with C numbers from C5 to C17 are colorless liquids with a petroleum odor. The alkanes with a C number equal or greater than C18 are colorless, odorless, and a wax-like solid at room temperature.
2. Melting and Boiling Points: The melting and boiling temperatures of straight chain alkanes increase as the number of C atoms increases. Branched chain alkanes have lower melting and boiling points than the straight chain alkanes with the same number of carbons. For example, pentane, iso-pentane, and neo-pentane have the boiling points of 36oC, 28oC, and 10oC, respectively. This happens because the branched chain alkanes have a less surface area that contacts each other (the molecules cannot pack in as tightly), resulting in weaker intra-molecular attraction (van der Waals force). Therefore, this causes lower melting/boiling points.
3. Solubility: Alkanes are insoluble in water and other polar liquids. Instead, they are soluble in organic liquids. Alkanes are non-polar, so they are soluble in other non-polar liquids (e.g. benzene, ethanol, ether etc.).
4. Relative Density: The relative density, or specific gravity, of alkanes is less than 1. Therefore, alkanes are not as dense as water. With the increase in the chain length of alkanes, the relative density is gradually increased, but always remains less than 1.
5. Combustion: Alkanes are flammable, catch fire easily, and burn with a blue flame. The complete combustion of an alkane leads to the formation of carbon dioxide and water. During combustion, the supply of oxygen has to be sufficient. Insufficient oxygen leads to the production of carbon monoxide, and the heat generated is less if sufficient oxygen is unavailable.

Alkanes only have sigma (σ) bonds, which are relatively stable in chemical reactions. Therefore, alkanes generally do not react with acids, alkalis, and oxidizing/reducing agents. However, in the presence of high temperature and pressure, σ-bonds undergo fission to produce free radicals. Free radicals are highly reactive, since they carry unpaired valence electrons. Once alkyl-free radicals are formed, they interact with the attacking reagents to form new compounds. The chemical reactions of alkanes are categorized three ways: substitution reactions, thermal decomposition reactions or pyrolysis, and isomerizations.

Alkenes
Alkenes are unsaturated hydrocarbons with one or more double bonds. Alkenes are represented by the general formula CnH2n. Examples of alkenes are ethene (CH2=CH2) and propene (CH3—CH=CH2).

The physical properties of alkenes are as follows:
1. Physical State: Lower molecular weight alkenes (C2 to C4) are gases at room temperature (i.e., ethene, propene and butene). Higher alkenes with the carbon number of C5 to C15 are liquids. C16 and higher alkenes are solid. Ethene has a sweet odor, but other alkenes are colorless and odorless.
2. Melting and Boiling Points: The melting and boiling points of alkenes increase with the molecular weight. However, branched chain alkenes have lower melting and boiling points than those of the corresponding straight chain alkenes.
3. Solubility: Alkenes are insoluble in water, but soluble in organic solvents like benzene, ethanol, ether, etc.
4. Relative Density: The relative density of alkenes is less than 1. Therefore, they are less dense than water. With the increase in the chain length of alkanes, the relative density increases, but always remains less than 1.
5. Combustion: Alkenes are flammable, and they burn with a yellow flame. Complete combustion of an alkene in the presence of adequate oxygen produces carbon dioxide and water. When enough oxygen is not available, partial combustion of alkenes primarily produces carbon monoxide.
Alkenes are unsaturated hydrocarbons, which have double bonds. The double bond consists of one sigma (σ) bond and one pi (π) bond. The π-bond is prone to electrophilic addition reactions. Alkenes undergo three types of reactions: addition reactions, oxidation reactions, and polymerizations.

Alkynes
Alkynes are unsaturated hydrocarbons with one or more triple bonds. They are represented by the general formula CnH2n-2, where n equals 1, 2, 3 . . .n. Examples of alkynes are ethyne and 2-butyne.

The physical properties of alkynes are as follows:
1. Physical State: Lower molecular weight alkynes (C2 to C4) are gases at room temperature (i.e., ethyne, propyne and butyne). Alkynes with the carbon number of C5 to C12 are liquids. Higher alkynes are colorless and odorless solids.
2. Melting and Boiling Points: The melting and boiling points of alkynes are higher than that of alkanes and alkenes of similar C-chain length. With the increase in molecular weight of the alkynes, the melting and boiling points increase accordingly.
3. Solubility: Alkynes are insoluble in water, but soluble in organic solvents.
4. Relative Density: The relative density of alkynes is higher than that of alkanes and alkenes, but still less than water.
5. Combustion: Alkynes are flammable, and they burn with a yellow flame. Like alkanes and alkenes, complete combustion of an alkyne in the presence of adequate oxygen produces carbon dioxide and water. When oxygen is inadequate, combustion of alkynes produces carbon monoxide.

A triple bond in an alkyne contains one σ-bond and two π-bonds. The presence of π-electrons in alkynes makes them suitable to interact with electrophiles. Alkynes undergo the following types of chemical reactions: addition reactions, oxidation reactions, and polymerizations.

Aromatic Compounds
Aromatic compounds are also called arenes. The term “aromatic” is a historical term related to the belief that aromatic compounds have a pleasant aroma. We now know this is not true. These molecules all contain a planar ring structure with conjugated, delocalized π-electrons. This configuration renders the aromatic structure unusually stable. Therefore, they are relatively more resistant to chemical fission or participation in chemical reactions. Aromatic compounds include benzene, benzene derivatives, and compounds that behave like benzene.


Aromatic compounds are classified into three major groups:
1. Benzene and substituted benzene compounds
2. Polycyclic or fused aromatic compounds
3. Heterocyclic aromatic compounds

Benzene and substituted benzene compounds contain a benzene ring with/without substituted groups or atoms in the ring. Examples are benzene, toluene (methyl benzene), ethyl benzene, and phenol.


Polycyclic aromatic compounds contain two or more benzene rings fused together while sharing two adjacent C atoms.

Examples of this type of compounds are naphthalene, anthracene, and phenanthrene.
       
Heterocyclic aromatic compounds contain one or more heteroatoms (other than C atoms), like O, N, S, as a part of the aromatic ring. This causes a decrease in the ring’s aromaticity and stability, and therefore increases its reactivity.

Some examples of heteroaromatic compounds are furan, pyridine, imidazole, oxazole, and thiophene (or thiofuran).


Benzene is a colorless liquid with a sweet smell. Benzene’s melting and boiling points are 5.5oC and 80.1oC, respectively. Its specific gravity is less than water, and therefore it is less dense than water. It is a constituent of crude oil and it is flammable. It burns with a black flame. Because of its high stability, it is a good solvent for organic compounds.
Benzene has a hexagonal planar structure. Six H atoms remain attached to C atoms by σ-bonds. The functional group of an aromatic hydrocarbon formed by removing one H atom is called aryl functional group. For example, the function groups of benzene (C6H6) is called phenyl group (C6H5-).
The high stability of the benzene ring makes the molecule resistant to any reaction that would destroy the aromaticity of the molecule. Benzene can undergo two types of reactions: addition reactions and substitution reactions.

Alcohols
Alcohols are organic compounds that contain one or more hydroxyl (-OH) group in the aliphatic chain. Examples of alcohols are methanol (CH3—OH) and ethanol (CH3—CH2—OH). Alcohols can exist on an aliphatic side-chain of an aromatic ring. These alcohols are called aromatic alcohols or aryl alcohols. Examples are phenyl methanol (C6H5—CH2—OH) and phenyl ethanol (C6H5—C2H4—OH).
Most alcohols contain one hydroxyl group. For example, methyl alcohol is CH3-OH, ethyl alcohol is CH3—CH2—OH, and propyl alcohol is CH3—CH2—CH2—OH.

Depending on the position of –OH on the C chain, monohydric alcohols are further classified as:
1. Primary or 1o alcohols have an –OH group attached to a C atom bound to two or three H atoms. See the classification examples below:


2. Secondary or 2o alcohols have an –OH group attached to a C atom bound to only one H atom. See the examples below:


3. Tertiary or 3o alcohols have an –OH group attached to a C atom not bound to any H atoms. All of the side chains have been substituted. Examples are illustrated below:

Some alcohols contain more than one -OH group. Alcohols with two –OH groups are called “diols,” three –OH groups are called “triols” and those with four –OH groups are called “tetraols.”

Examples of alcohols with more than one –OH group are shown below.
OH – CH2 – CH2 – OH       OH – CH­2 – CH2 – CH2 – OH                             OH – CH2 – CH(OH) – CH2 – OH
     Ethane-1,2-diol       Propane-1,3-diol        Propane-1,2,3-triol (glycerol)

The physical properties of alcohols are as follows:
1. Physical State: The lower molecular weight alcohols (C1 to C12) are colorless liquids, and higher molecular weight alcohols are wax-like solids. Lower molecular weight alcohols have pleasant odors.
2. Melting and Boiling Points: The higher electronegativity of the O in the –OH group makes alcohols polar. The polar molecules produce intra-molecular attractions, resulting in increases in melting and boiling points. In alkanes, this type of attractive force is absent. For example, the boing points of methane and ethane are -161.5oC, and -89oC, whereas the boiling points of methanol and ethanol are 64.7oC and 78.4oC, respectively.
3. Solubility: The –OH group makes alcohols polar. Methanol, ethanol and propanol are readily-soluble in water. However, as the size of the C chain increases, the solubility in water decreases because the C chain is non-polar.
4. Relative Density: The relative densities of alcohols are less than water.
5. Combustion: Alcohols are flammable, and they burn in the presence of oxygen to produce carbon dioxide and water.
Alcohols can act as a weak acid by donating a proton (H atom) in aqueous solutions.


Therefore, alcohols can interact with different chemical species including alkali metals, carboxylic acids, acyl halides, and oxidizing agents.
In reactions with alkali metals, the –OH group releases its proton to alkali metals to form alkoxide and hydrogen.


Alkoxides can react with water to reproduce an alcohol.


Alcohols can react with organic acids (carboxylic acids) to produce esters and water. For example, the reaction between ethanol and acetic acid produces an ester, ethyl acetate.


Alcohols react with acyl halide to form esters. For example, the reaction between ethanol and acetyl chloride forms ethyl acetate ester.

               

Phenols
Aromatic hydrocarbons that have one or more H atoms replaced by hydroxyl (-OH) groups are collectively called phenols. When a single H atom is replaced by an –OH group, it is called a carbolic acid (generally termed as phenol). Phenols are generally slightly acidic in nature. See examples below of certain phenolic compounds with different substituents in the ring:
         
Phenol, carbolic acid, specifically, is colorless crystalline solid. The –OH group in phenol is polar and forms intra-molecular hydrogen bonds. Therefore, phenol’s molecular weight (94.1 g/mol), melting point (40.5oC), and boiling (181.7oC) points are higher than that of other organic compounds, such as toluene, with a similar molecular weight (92.1 g/mol), melting point, (-95oC), and boiling point (111oC).
As a class of organic compounds, phenols do not dissolve in water at room temperature. At higher temperatures hydrogen bonds break down, and phenols dissolve in water. Similarly, phenols are insoluble in other polar solvents, like alcohol and ether, at room temperature but dissolve at higher temperatures.
 

The chemical properties of phenols are as follows:
Phenols are more reactive than benzene due to the presence of the –OH group attached to the ring. However, the –OH group of a phenol is less reactive than that of an alcohol. This is because the delocalized π-electrons stabilize the O atom and the –OH group remains strongly attached to the benzene ring. Therefore, unlike alcohols, phenols do not react with the Lucas reagent. The bond between the –OH molecules in phenols is more polarized than that in alcohols and thus, they can readily donate a proton. Because of this, phenols are more acidic than alcohols.


Phenols react with different chemicals.

The major reactions of phenols are discussed below.
1. Reduction of Phenol by Zinc: When a phenol is passed through zinc crystals at high temperatures, it is reduced to form benzene, with the formation of zinc oxide.
            
2. Reduction of Phenol by Hydrogen: When a phenol is treated with H2 at 150°–175°C in the presence of a nickel catalyst, three molecules of H2 are attached to form cyclohexanol.
      
3. Reaction with Ammonia: At a high temperature and pressure, phenol reacts with ammonia to produce aniline (also called phenylamine or aminobenzene). The reaction uses zinc chloride as the reaction catalyst.


4. Formation of Esters: Like alcohols, phenols can also form esters. However, the reaction progresses slowly. Therefore, a phenol is first converted to a phenoxide, which further reacts with acyl halide or acid anhydride to form an ester. For example, sodium phenoxide reacts with acetyl chloride (CH3COCl) or acetic anhydride (ethanoic anhydride) [(CH3CO)2O] to form phenyl acetate ester.  


The above reactions can be used to commercially-synthesize aspirin (acetylsalicylic acid, ASA) and acetaminophen, which are widely used to treat fevers and pain.

The syntheses of aspirin and acetaminophen are shown below:


5. Williamson Ether Synthesis: This refers to the formation of an ether from an organohalide and an alcohol. For example, phenol reacts with sodium hydroxide to form sodium phenoxide, which further reacts with methyl bromide to form an ether, methoxybenzene.


6. Friedel-Crafts Alkylation: In the presence of anhydrous aluminum chloride, phenols react with alkyl halides to form ortho- and para-alkyl phenol. For example, phenol reacts with methyl chloride to form ortho-methyl phenol and para-methyl phenol.


7. Friedel-Crafts Acylation: In the presence of anhydrous aluminum chloride, phenol reacts with acetyl chloride to form ortho- and para-acetyl phenol.


8. Reimer-Tiemann Reaction: When phenol is mixed with chloroform and sodium hydroxide and heated to around 60oC, then phenol is converted into salicylaldehyde or 2-hydroxybenzaldehyde.



Ethers
Ethers are organic compounds with two alkyl or aryl groups connected by an oxygen atom. They can be represented by R—O—R1, where R and R1 are alkyl or aryl groups.

Ethers can be classified into the following categories:
1. Simple ethers have the same alkyl group on each side of the O atom. Examples are dimethyl ether (CH3—O—CH3) and diethyl ether (C2H5—O—C2H5).
2. Mixed ethers have different alkyl groups on each side of the O atom. Examples are methyl ethyl ether (CH3—O—C2H5) and methyl propyl ether (CH3—O—C3H7).
3. Cyclic ethers form a ring structure. Three-membered ether rings (pictured below) are called epoxide compounds and are highly reactive (e.g. ethylene oxide).


4. Aromatic ethers contain at least one aryl group connected to the O atom (the other group can be an aryl or an alkyl group). Examples are methyl phenyl ether or methoxy benzene (C6H5—O—CH3) and diphenyl ether (C6H5—O—C6H5).

The physical properties of ethers are as follows:
Low molecular weight ethers are gases. However, higher molecular weight ethers are colorless liquids. Ethers have slight solubility in water because of the formation of H bonds between the H atom of water and O atom of the ether. The solubility decreases with the increase in molecular weight.
Ethers have lower melting and boiling points than alcohols of similar molecular weight. This is because, unlike alcohol, ether molecules do not readily form intra-molecular hydrogen bonds. For example, dimethyl ether (CH3—O—CH3) and ethanol (CH3—CH2—OH) have the same molecular formula, C2H6O, but the boiling points of ethanol and dimethyl ether are 78.4oC and –24oC, respectively. At room temperature, dimethyl ether is a gas, but ethanol is a liquid.
Ethers are less dense than water because they have a lower specific gravity. With an increase in molecular weight, their specific gravity increases. However, it remains less dense than water.
Ethers are one of the least reactive classes of organic compounds. They are less reactive than alcohols and phenols. When an ether is treated with a hydrogen halide, it undergoes fission to form alkyl halide and alcohol. In the case of a mixed ether, the halogen atom joins with the smaller alkyl group. For example, reaction between methyl ethyl ether and hydrogen iodide forms methyl iodide and ethanol.


When treated with an excess of HI at a higher temperature, both alkyl groups are converted into alkyl halides.



Carbonyl Compounds
Aldehydes and ketones are collectively termed carbonyl compounds because they contain the carbonyl (=C=O) functional group. The carbonyl group for aldehydes is on a terminal C atom. Some examples of aldehydes are formaldehyde or methanal (H—CHO), and acetaldehyde or ethanal (CH3–CHO). The carbonyl group of ketones is on a central C atom. Examples of ketones are acetone or propanone (CH3—CO—CH3), and benzophenone or diphenylketone (C6H5—CO—C6H5).


The physical properties of carbonyl compounds are as follows:
Formaldehyde (H—CHO) is a colorless gas, while acetaldehyde has a boiling point of 20.2oC. Aldehydes with C3 to C9 carbon numbers are colorless liquids, and higher carbon aldehydes are colorless solids. Ketones up to C11 are liquids, and C12 and higher ketones are solids. Lower C number aldehydes have an unpleasant odor, whereas higher C number aldehydes have a pleasant smell. Ketones have a pleasant smell as well.
Aldehydes and ketones with short C chains are fairly soluble in water through the formation of H bonds with water. Solubility decreases with the increase in molecular weight, and larger carbonyl compounds are insoluble in water. Carbonyl compounds are soluble in organic solvents.
Unlike alcohols, carbonyl compounds cannot readily form intra-molecular hydrogen bonds, resulting in relatively lower melting and boiling points.

The chemical properties of carbonyl compounds are as follows:
The bond between the C and O atoms in the carbonyl group is polarized (–Cδ+=Oδ-), making the carbonyl compounds highly reactive. The relatively positive-charged C atoms in carbonyl compounds are susceptible to nucleophile (electron-rich substrate) attack.

Various chemical reactions of carbonyl compounds are summarized below.
1. Grignard Reactions: This reaction is used to produce a new C–C bond. The Grignard reagent reacts with the carbonyl group and is an organometallic compound with a general formula of R—MgX, in which R represents the alkyl or aryl groups, and X represents a halogen.
The Grignard reagent reacts with formaldehyde to form primary (1o) alcohols. For example, the reaction between formaldehyde and methyl magnesium bromide produces an unstable intermediate, which is further hydrolyzed to form ethanol.


The Grignard reagent reacts with other aldehydes (except formaldehyde) to form secondary (2o) alcohols. For example, the reaction between acetaldehyde and methyl magnesium bromide forms a secondary alcohol, 2-propanol.


The Grignard reagent reacts with ketones to form tertiary (3o) alcohols. For example, the reaction between acetone and methyl magnesium bromide forms a tertiary alcohol, 2-methyl-2-propanol.


2. Oxidation Reactions: Aliphatic aldehydes are readily oxidized by strong oxidizing agents, like potassium dichromate (K2Cr2O7) and sulphuric acid (H2SO4), to form carboxylic acids. However, aromatic aldehydes are not easily oxidized.


Ketones do not oxidize easily. When ketones are heated with strong oxidizing agents, a mixture of different organic acids is formed.


3. Reduction Reactions: Reduction of aldehydes by a mild reducing agent, like lithium aluminum hydride (LiAlH4), sodium borohydride (NaBH4), or sodium amalgam (NaHg), produces primary alcohols, whereas ketones are reduced to secondary alcohols.


Reduction of carbonyl compounds with a strong reducing agent, like zinc amalgam and concentrated hydrochloric acid (ZnHg + HCl), causes conversion of the =C=O group into a –CH2 – group, resulting in formation of saturated hydrocarbons. This reaction is termed a Clemmensen reduction.


4. Aldol Condensation Reaction: In the presence of dilute acid or alkali, two molecules of an aldehyde or ketone containing α-hydrogen can undergo a condensation reaction to form a β-hydroxy aldehyde or β-hydroxy ketone. This reaction is termed “aldol condensation,” as the product carries both the aldehyde (–CHO) and alcohol (–OH) functional groups. Aldol condensation helps form a new C–C bond. For example, in the presence of dilute NaOH, two molecules of acetaldehyde condense to from an aldol, 3-hydroxybutanal (or β-hydroxybutanal).


When aldol is heated in the presence of an acid (e.g. HCl), a water molecule is removed causing the formation of α-β unsaturated aldehyde or ketone.


Carbonyl compounds that do not have α-hydrogen cannot participate in aldol condensation. For example, formaldehyde (H-CHO) and trimethyl-acetaldehyde [(CH3)3C–CHO] do not have an α-hydrogen, so they cannot undergo aldol condensation.
5. Cannizzaro Reaction: In the presence of a concentrated base (e.g. NaOH), aldehydes lacking an α-hydrogen undergo oxidation-reduction reaction. In such reactions, one aldehyde molecule is oxidized to form a salt of carboxylic acid and the other is reduced to form an alcohol. For example, when treated with NaOH, two molecules of formaldehyde undergo an oxidation-reduction reaction to form methanol and sodium formate.



Biomolecules
Humans, animals, and plants are built with different biomolecules and vital life functions rely on them. Biomolecules are organic polymers that perform various functions in the human body. Among their many functions in the human body, biomolecules do the following: provide structure, provide nutrition and energy to cells, perform various enzymatic reactions, regulate the body’s defense mechanism, and control genetic functions through heredity.
Classes of important biopolymers include:
1. Carbohydrates, such as starch (in animals) and cellulose (in plants).
2. Proteins, such as nucleoprotein, plasma protein, hormones, enzymes, and antibodies.
3. Nucleic acids, such as ribonucleic acid (RNA) and deoxyribonucleic acid (DNA).
4. Triglycerides, which are formed from fatty acids with a glycerol backbone.
All biomolecules are polymers, which are formed from repeated units of basic monomers. These polymers can be broken down (hydrolyzed) into their respective monomers.

Proteins can be hydrolyzed under acidic condition to form amino acids.


Nucleic acids are structurally associated with some proteins to form nucleoproteins. The hydrolysis of nucleic acids and nucleoproteins is illustrated below:


Carbohydrates are represented by a general formula, (C6H10O5)n, where 40 ≤ n ≤ 3000. When hydrolyzed, starch produces the monosaccharide glucose (C6H12O6).             


However, glucose remains stored as glycogen in the liver and muscle. Glycogen stores energy in the body.


Carbohydrates are classified into three categories:
1. Monosaccharides: Monosaccharides are the monomers of carbohydrates. They are represented by a general formula (CH2O)n, where n = 3 – 6. Examples of monosaccharides are glucose (dextrose), fructose, and galactose.
They are named based on the number of C atoms in the molecule, such as triose (C3), tetrose (C4), pentose (C5) and hexose (C6). A monosaccharide with an aldehyde group is called an “aldose,” and one with a ketone group is called a “ketose” (e.g. fructose).


2. Disaccharides: Disaccharides are two monosaccharides joined together. When hydrolyzed, disaccharides produce two monosaccharide molecules. See the examples below:


3. Polysaccharides: Polysaccharides are high molecular weight carbohydrates. When hydrolyzed, they produce many molecules of monosaccharides. Examples of polysaccharides are starch, glycogen, and cellulose.
Carbonyl compounds, aldehydes and ketones, can react with –OH group of an alcohol to form hemiacetal and acetal.


Glucose and fructose contain both carbonyl and hydroxyl groups, and therefore can form intramolecular hemiacetal to produce a cyclic structure. This hemiacetal formation takes place between C1 and C5 carbons to form a stable heterocyclic structure. The cyclic structure forms a pyranose ring, which is a six-membered ring consisting of five C atoms and one O atom. In a cyclic structure, C1 might have an –OH group at the right or left side, and therefore may be termed α-D-glucose and β-D-glucose, respectively.

View the structures below as an example:
                       
Reducing sugars are ones with a free aldehyde or ketone groups, and they can act as reducing agents. All monosaccharides, including glucose, fructose, and galactose, are reducing sugars. Many disaccharides, including lactose and maltose (except sucrose), are also reducing sugars. The reducing sugars are able to reduce Fehling's solution and Tollens' reagent.
When Fehling's solution is treated with a reducing sugar, the deep blue color of Fehling's solution fades and then forms a reddish precipitate. Fehling's solution is prepared by mixing a copper sulphate solution with potassium sodium tartrate in NaOH. See the reaction below as an example:


When a solution of reducing sugar is heated with Tollens' reagent, silver is precipitated and forms silver mirror on the inner surface of the reaction vessel.

See the reaction that follows:



Reactions of Organic Compounds
In inorganic compounds, chemical reactions occur through the formation of electrically-charged ions and their polarization. However, organic compounds contain covalent bonds, and the chemical reactions of organic compounds involve simultaneous breakage and formation of these covalent bonds. The functional groups present in the molecule are important in determining the nature and rate of reactions. In addition to the functional groups, the reactions are also determined by the type of bonds present in the molecule (i.e. σ-bonds and π-bonds), and the nature of other reactants present.
Organic chemical reactions occur when an attacking reagent interacts with an organic molecule (substrate) to form a new organic compound (product). This can be explained with the reaction between chloromethane (CH3—Cl) and sodium hydroxide (Na+OH-). In this reaction, CH3—Cl is the organic substrate, OH- is the attacking reagent, and CH3-OH (methanol) is the product. There are three steps in this chemical reaction:


These chemical reactions take place through fission or breakdown of the C—C bonds and formation of carbon radical intermediates.

There are two types of carbon radicals that are formed in chemical reactions: carbocation and carbanion.

A carbocation is a positively-changed carbon ion (i.e. cation). Examples are methyl carbocation (+CH3) and ethyl carbocation (CH3—+CH2). Carbocation are formed due to an unequal distribution of the shared electrons from a bond cleavage. For example, in chloromethane (CH3—Cl), the chlorine (Cl) atom is more electronegative than the C atom; therefore, electrons are drawn towards the Cl. Fission of the bond causes the Cl atom to take the shared electron of the C atom, which results in the formation of an electropositive carbocation.


Carbocation groups are named after the parent alkyl group by “ium” (i.e. alkyl + ium = alkylium). For example, +CH3 is called methyl carbocation or “methylium.” Similarly, CH3—+CH2 is called ethyl carbocation ion or “ethylium.”
The positively-charged carbocations are highly reactive and can readily bind with the electron-donating atoms or groups to form new organic compounds. Carbocations are stabilized by nearby electron donating groups, including alkyl groups. In terms of stability, the primary (1o) carbocation ion is less stable than secondary (2o) carbocation, which is less stable than tertiary (3o) carbocation. A 1o carbon is one that is bound to one other carbon, a 2o carbon is bound to two carbon atoms and a 3o carbon is bound to three carbon atoms.

See the stability order below of carbocation where “R” refers to alkyl group(s).
 
A carbanion is a negatively-charged carbon atom that carries an unshared pair of electrons. Examples of carbanions are methyl carbanion (-CH3) and ethyl carbanion (CH3—-CH2). A carbanion is formed through fission of an σ-bond and concentration of an unshared electron pair in a C atom. This type of fission takes place when the C atom carries relatively higher electronegativity compared to the attached atom or group.


The stability of a carbanion is increased in the presence of an electron-withdrawing (attracting) group that pulls the electron density toward itself and stabilizes the carbanion (e.g. –NO2, –CO, –CN etc.). On the other hand, the stability is decreased in the presence of the electron-donating (repelling) alkyl (R) groups, which push the electron cloud toward the carbanion (e.g. –CH3, –C2H5 etc.). Therefore, the stability order is as follows:


In chemical reactions, a carbocation or carbanion binds with the attacking reagent to form a new organic compound. The nature of the reaction depends significantly on the attacking reagent. The mechanism of interaction between the organic substrate and the attacking reagent can be illustrated in the steps below:


Depending on the nature, attacking reagents (Z) can be categorized into an electrophile or electrophilic reagent and a nucleophile or nucleophilic reagent.

Electrophiles
Electrophiles are attacking species that have a strong attraction to electrons. Electrophiles are naturally electron-deficient, and therefore, they have electron-accepting characteristics. There are two types of electrophiles: positive (or charged) electrophiles and neutral electrophiles.
Positive electrophiles (E+) are positively-charged, so they have a high affinity for electrons. Examples are proton (H+), alkylium (R+), nitronium (NO2+), nitrosonium (+NO) etc., where R = alkyl groups.
Neutral electrophiles (E) do not have an electric charge but have a strong affinity for electrons to complete the octet in their valence shell. Examples of neutral electrophiles are aluminum chloride (AlCl3), boron trifluoride (BF3), and ferric chloride (FeCl3).



Nucleophiles
Nucleophiles are rich in electrons, so they act as electron donors. Nucleophiles are also categorized into negative (or charged) nucleophile and neutral nucleophile.
Negative nucleophiles (Nu-) are negatively-charged attacking reagents. Examples include methyl carbanion (-CH3), chloride (Cl-), bromide (Br-), hydroxide ion (-OH), and cyanide ion (-CN).
Neutral nucleophiles (Nu) carry an unshared pair of electrons, but do not carry any negative charge. Examples are ammonia (NH3), water (H2O), and alcohol (R-OH).


Based on the nature of the attacking agents, organic reactions can also be classified into following categories:
1. An electrophilic addition is an addition reaction in which an electrophile (electron-attracting species) is added to a molecule that is mostly carrying unsaturated bonds. For example, the electrophilic proton (H+) attacks the double bond of ethene (CH2=CH2) to form ethane (CH3—CH3).
2. An electrophilic substitution is an elimination reaction in which an electrophile is eliminated from an organic molecule, e.g. elimination of protons from ethane (CH3-CH3) causes the formation of ethene (CH2=CH2).
3. A nucleophilic addition is an addition reaction in which an electron-rich reactant (i.e. nucleophile) attaches to an electrophile to form a new bond.
4. A nucleophilic substitution is an elimination reaction that causes the removal of a nucleophile from an organic molecule.             

Oxidation and Reduction
Oxidation/reduction, or redox reactions, are reactions in which the oxidation state of an atom changes. This oxidation state change relates to the number of electrons lost or gained during the reaction. In the oxidation half of the reaction, an electron is lost. In the reduction half of the reaction, an electron is gained. These two parts are referred to as “half-reactions” because oxidation is always accompanied with reduction. Usually, the change in the oxidation state of the C atom determines if an organic redox reaction is termed an oxidation or a reduction reaction.

1. Alkanes: Alkanes are generally stable and unreactive. Therefore, alkanes rarely participate in chemical redox reactions. The most common redox reaction of alkanes is combustion, in which the burning of hydrocarbon chains in the presence of oxygen produces carbon dioxide, water, and energy. In the absence of adequate oxygen, oxidative combustion of hydrocarbons produces carbon monoxide, water, and energy. This is considered an oxidation reaction because the C atom loses electrons. In this equation, the O atoms are reduced. Linear and small chain alkanes are more readily oxidized than larger and branched chain alkanes. The figure below shows the oxidation of methane and is labeled with the oxidation numbers of each atom to illustrate the change in oxidation state. Here, carbon is oxidized and oxygen is reduced.


2. Alkenes: In the presence of a weak oxidizing agent such as dilute potassium permanganate (KMnO4) in alkaline (KOH) solution, alkenes undergo oxidation to produce glycols. This reaction is utilized to detect unsaturation (double and triple bonds) in organic compounds. While observing this test, called Baeyer’s Test, the pink color of potassium permanganate gradually fades.


However, in the presence of a strong oxidizing agent such as concentrated KMnO4 in acidic (H2SO4) media, alkenes are oxidized to produce organic acids.


In the presence of catalysts like platinum (Pt), palladium (Pd), or nickel (Ni), alkenes are reduced to alkanes.


3. Alkynes: When alkynes are treated with an oxidizing agent like alkaline potassium permanganate (KMnO4) in the presence of a high temperature, the π-bonds undergo fission and oxidation to form carboxylic acids. The type of carboxylic acids formed depends on the position of the triple bond in the C chain.
Alkynes can be reduced by H2 at room temperature in the presence of Pt or Pd, or at a high temperature (150o – 180oC) in the presence of Ni.


4. Aromatic Compounds: The alkyl side chain on the benzene is susceptible to oxidation. For example, the –CH3 group of toluene could be oxidized by K2Cr2O­7/H2SO4 or KMnO4 or dilute heated HNO3 to form benzoic acid.


When treated with a weak oxidizing agent like chromyl chloride (CrO2Cl2), -CH3 group of toluene is oxidized to form benzaldehyde. This is called the Étard Reaction.


Under similar conditions, toluene is reduced to hexahydrotoluene.


5. Alcohols: Alcohols are oxidized to produce carbonyl compounds like aldehydes and ketones. The type of carbonyl compounds formed depends on the nature of alcohol (1o, 2o, or 3o) and oxidizing agent.
6. Carbonyl Compounds: Carbonyl compounds can be oxidized to organic acids, whereas reduction of carbonyl compounds produces alcohols.

Hydration and Dehydration
Hydration refers to the addition of water to a compound. Conversely, dehydration is the removal of water from a molecule.
1. Alkenes: Alkenes can be hydrated in the presence of concentrated sulphuric acid to form alcohol. In such reactions, an unstable intermediate, alkyl hydrogen sulphate, is formed. This is further degraded to yield an alcohol.


The above reaction follows Markovnikov's Rule. According to this rule, when an asymmetric alkene interacts with an asymmetric reagent, the H atom of the reagent attaches with the C atom that carries the greater number of H atoms. An example below shows how propene reacts with hydrogen bromide to yield 2-bromopropane as the principal yield in the reaction:


2. Alkynes: When treated with sulphuric acid and in the presence of mercuric sulphate catalyst, alkynes are converted into carbonyl compounds. The reaction proceeds through the formation of an unstable intermediate that undergoes rearrangement to form the final carbonyl product.


2. Alcohols: Alcohols undergo dehydration reactions in the presence of a Lucas reagent. This reaction causes the elimination of –OH group of alcohol, resulting in the formation of organic halide.

3. Carbonyl Compounds: Aldehydes and ketones can be hydrated to produce alcohols. Hydration of aldehydes produces 1o and 2o alcohols, whereas that of ketones produces 3o alcohols.

Hydrolysis
Hydrolysis refers to chemical reactions that involve cleavage of bonds in a molecule by the addition of water. In such reactions, a chemical bond in the organic molecule undergoes fission. The -OH group of water attaches to one part of the organic molecule, and proton (H) attaches to the other. The reactions are catalyzed by an acid or alkali. Hydrolytic bond cleavage is limited to certain classes of organic compounds, including amides, esters, ethers, and alkyl halides.

1. Amides: A primary amide is hydrolyzed to form a carboxylic acid and ammonia, whereas a secondary amide produces a carboxylic acid and primary amine.


2. Esters: Hydrolysis of an ester produces a carboxylic acid and an alcohol.


3. Ethers: Ethers are hydrolyzed to form alcohols.


4. Alkyl Halides: Hydrolysis of alkyl halides produces alcohol.


Organic compounds also undergo a variety of reactions that involve addition, substitution, and elimination of atom(s) or group(s).

Addition, Substitution, and Elimination Reactions

Addition Reactions
In addition reactions, two different molecules are combined to form a new organic compound. Generally, unsaturated compounds with π-bonds undergo addition reactions. These reactions involve breaking a π-bond to form more stable σ-bond.

An example is the reaction between ethene and bromine, which causes the formation of 1,2-dibromo ethane.


Addition reactions are chemical reactions that involve the addition of a molecule to an organic compound that contains a double bond. Alkenes can undergo addition reactions with hydrogen (H2), halogens (X2), hydrogen halide (HX), hypohalous acid (HOX), and sulphuric acid (H2SO4). In such reactions, the above electrophilic reagents interact with π-electrons to form new compounds.

Below are examples of addition reactions of alkenes with various reagents:


The π-bond in alkenes is readily oxidized by different oxidizing agents. In such reactions, the different compounds formed include glycols, ketones, carboxylic acid, and carbon dioxide. The reaction pattern, however, depends on the nature of the oxidizing agent. For example, in the presence of a weak oxidizing agent, such as dilute potassium permanganate (KMnO4), in alkaline (KOH) solution, ethene undergoes oxidation to produce an alcohol, ethylene glycol.


However, in the presence of a strong oxidizing agent, such as concentrated KMnO4, in acidic (H2SO4) conditions, propene undergoes oxidation to produce carboxylic acid, acetic acid.
CH3—CH=CH2 + 5[O] → CH3—COOH + CO2 + H2O
Like alkenes, alkynes can undergo addition reactions with hydrogen (H2), halogens (X2), hydrogen halide (HX), hypohalous acid (HOX), and sulphuric acid (H2SO4). In such reactions, these electrophilic reagents interact with π-electrons to form new compounds. Examples of addition reactions are illustrated below:


Benzene also undergoes addition reactions including hydrogen addition and halogen addition.
 

Hydrogen Addition: In the presence of a nickel (Ni) catalyst and at temperature 200oC, hydrogen (H2) undergoes an addition reaction with benzene (C6H6) to form cyclohexane (C6H12). In this reaction, three molecules of H2 attach with a molecule of benzene.


Halogen Addition: In the presence of ultraviolet radiation, three molecules of chlorine (Cl2) add with benzene (C6H6) to form hexachlorocyclohexane (C6H6Cl6).



Substitution Reactions
A substitution reaction is when an atom or group is substituted by another atom or group to form a new compound. An example is a reaction between chloroethane and sodium hydroxide, where the chloride group of chloroethane is replaced by the hydroxyl group to form ethanol.


In substitution reactions, one functional group is replaced with another. For alkanes, H atoms (R-H) are replaced by attacking reagents such as halogens (X) or nitro (-NO2) groups. Introduction of a halogen is called halogenation, and introduction of a nitro group is called nitration.
Halogenation of alkanes requires UV light and high temperatures (300o–400o C) because alkanes are very stable.

Halogenation can occur at every available H atom. For example, halogenation of alkanes by chlorine (Cl2) will produce methyl chloride (CH3Cl), dichloromethane (CH2Cl2), trichloromethane or chloroform (CHCl3), and carbon tetrachloride (CCl4), respectively.

The steps of reaction are illustrated below.
1. CH4 + Cl2 → CH3Cl + HCl
2. CH3Cl + Cl2 → CH2Cl2 + HCl
3. CH2Cl2 + Cl2 → CHCl3 + HCl
4. CHCl3 + Cl2 →CCl4 + HCl

The chlorination reaction takes place in three steps:
1. Chlorine free-radical is formed in the presence of high temperature and UV radiation.
2. A chlorine free-radical attacks a C—H bond of alkane to form an alkyl (R) free-radical and HCl.
3. The other chlorine free-radical binds with the alkyl free-radical to form chlorinated (halogenated) alkane (R—Cl).


Because a carbon free-radical is formed, the rate and extent of halogenation depends on the stability of the carbon free-radical. For example, a H atom attached to a 3o carbon is more reactive than that attached with 2o and 1o carbon, respectively. Therefore, in the following reaction, the yield of 2-chloro propane (55%) is more than that of propyl chloride (45%).


Halogenation with any halogen occurs through the same general steps. However, the different halogens have different reactivities. The order of reactivity is as follows: F2 > Cl2 > Br2 > I2.
Benzene and aromatic compounds undergo substitution reactions. Since benzene is quite stable, each H atom is substituted sequentially. The substituent groups/atoms take specific positions on the benzene ring during those substitution reactions.
1. Halogenation: Halogen substitution is a catalyst-dependent reaction, commonly using ferric chloride (FeCl3), ferric bromide (FeBr3) and aluminum chloride (AlCl3) as catalysts. Halogenation is an electrophilic addition reaction. For example, in the presence of anhydrous AlCl3, chlorine (Cl2) substitutes an H atom to form chlorobenzene (see below).


These substitutions can eventually replace all of the H atoms. In the example above, continued reaction would form dichlorobenzene, trichlorobenzene, tetrachlorobenzene, pentachlorobenzene and hexachlorobenzene, respectively.


2. Nitration: This electrophilic substitution reaction replaces H atoms with a nitro (–NO2) group. For example, when benzene is treated with concentrated sulphuric acid (H2SO4) and concentrated nitric acid (HNO3) at around 50oC, one H atom is substituted by a –NO2 group to produce nitrobenzene and water.
   
When the reaction is carried out at 100oC in the presence of concentrated H2SO4 and HNO3, 1,3-dinitrobenzene (meta-dinitrobenzene) is formed.


3. Friedel-Crafts Alkylation: When benzene is treated with an alkyl halide (R—X) in the presence of a Lewis acid catalyst (e.g. anhydrous AlCl3 or FeCl3), one H atom of the ring is substituted by the alkyl group to form alkyl benzene. This is called Friedel-Crafts alkylation. For example, in a reaction between benzene and methyl chloride, methyl benzene (toluene) is formed.


4. Friedel-Crafts Acylation: When benzene reacts with an acyl halide (RCO—X) in the presence of a Lewis acid, like AlCl3 or FeCl3, an acyl group (RCO-) is introduced in the benzene ring to form an aromatic ketone. For example, the reaction between benzene and acetyl chloride produces acetophenone (methyl phenyl ketone).


When a single H atom is substituted from the benzene ring, it makes mono-substituted benzene derivatives. Examples are methylbenzene (toluene), chlorobenzene, phenol, phenyl amine (aniline), benzaldehyde, and benzoic acid.


When a single H atom is substituted, there are still five H atoms that could be substituted by different atoms or groups. The C atom with first substitution is numbered 1, and other C atoms in the ring are numbered and named accordingly. Since benzene has a cyclic, symmetrical structure, 1:2 and 1:6 positions are equivalent. Similarly, 1:3 and 1:5 positions are equivalent. The 1:2 and 1:6 positions are nearest, and called “ortho” or “o.” The 1:3 and 1:5 positions are termed “meta” or “m”, and the 1:4 position is the farthest, and termed “para” or “p.”


The following example shows three possible isomers of dichlorobenzene based on the relative positions of the substituent atoms.
   
The substituents on benzene ring could have two types of effects: an inductive effect or a mesomeric effect.

1. Inductive Effect (I): The electronegativity of substituent atoms or groups can cause polarization of the bond between the C atom in benzene ring and the substituent group. When the electronegativity of the substituent (e.g. halogens, X: F, Cl, B, I) is higher than carbon, it pulls the σ-electrons and causes polarization of the bond. This is called a negative (–) inductive effect. In contrast, if the electronegativity of substituent atoms or groups (e.g. alkyl groups, R: –CH3, –C2H5) is lower than the ring C atom, it pushes the σ-electrons toward C atom, and is called a positive (+) inductive effect.


 

2. Mesomeric Effect (M): The electron cloud in a π-bond can be influenced by the nature of substituent atoms or groups. The delocalization of π-electrons toward the relatively higher electronegative atom or group is called a negative mesomeric effect. The opposite effect by an electron repelling/pushing atom or group is called a positive mesomeric effect.


The nature of substituent group or atom determines the position of further substitutions in the ring.

Therefore, the substituent groups/atoms can be categorized into two types: ortho-para directing and meta directing.
 

1. Ortho-Para Directing Atoms/Groups: Electron-donating substituents like -CH3, -OH, and -NH2 donate electrons to the benzene ring and make it more reactive. Due to the positive mesomeric effect, they push into the electron cloud and the electron density gets relatively higher at ortho-para positions. Therefore, the ortho-para position becomes more reactive, allowing further substitutions on those positions. In the case of halogens, the effect is relatively complicated. X atoms have a negative inductive effect, but they provide a positive mesomeric effect by pushing unpaired electrons to the benzene ring. Therefore, halogens are ortho-para directing substituents.
2. Meta Directing Atoms/Groups: Certain atoms/groups, due to their negative mesomeric effects, pull the electron cloud from the benzene ring. These atoms and groups make benzene ring less reactive. Examples are –NO2, -CHO, -COOH, and –SO3H. By withdrawing the electron cloud, these groups decrease the electron density significantly at the ortho-para positions. However, meta position is less affected, and thus, the second electrophile attacks the meta position. These groups are called meta-directing substituents.

Elimination Reactions
In elimination reactions, atoms or groups are eliminated from two adjacent C atoms to form π-bonds. This is opposite to addition reactions. As an example, in the presence of concentrated sulphuric acid, a water molecule is removed from ethanol to form a C-C π-bond, i.e. ethene.



Isomerization
Isomerization refers to a rearrangement of atoms or groups in an organic molecule to form an isomer. The newly formed compound has a similar molecular formula, but has a different structural arrangement than the parent compound.

As an example, in the presence of aluminum chloride and hydrochloric acid, butane undergoes atomic rearrangement to form 2-methyl propane.


Alkanes can undergo isomerization to produce isomers. In such reactions, straight chain alkanes can be converted into corresponding branched-chain isomers.

For example, in the presence of aluminum chloride (AlCl3) and hydrochloric acid (HCl) and at 250o–300oC, butane is converted into iso-butane (2-methyl propane).
CH3-CH2-CH2-CH3 → CH3-CH(CH3)-CH3
ButaneIsobutane

Polymerization
In the presence of a high temperature, pressure, and a catalyst, molecules of a same alkene can join together to form a large molecule. This process is called polymerization and it results in the formation a polymer from a monomer (single unit). Polymerization is used widely in industries to synthesize compounds with modified physical properties and shear withstanding capacities. For example, polymerization of ethene (ethylene) causes the formation of polyethylene, which is widely used as a plastic material in manufacturing industries.

Note that n equals the number of monomers.
nCH2=CH2 → (-CH2-CH2-)n
Like alkenes, alkynes also undergo polymerization to produce larger molecules. Alkynes can produce both aliphatic and aromatic polymers during this process.

The type of polymer formed depends on the reaction conditions. ­­­
1. Formation of Aliphatic Polymers: In the presence of cuprous chloride (CuCl) and ammonium chloride (NH4Cl), ethyne (or acetylene) undergoes polymerization to first form vinylacetylene, and the further addition of a molecule forms divinylacetylene.
 
2. Formation of Aromatic Polymers: When ethyne (or acetylene) is passed through a heated iron pipe at a temperature of 400°–500°C, then three molecules join together to form aromatic compound benzene.