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The IUPAC priority order of functional groups is: carboxylic acid > ester > acid halide > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > alkyne > ether > alkyl halide > nitro. Example: In 4-hydroxybutanoic acid, –COOH has higher priority than –OH.
When naming compounds with multiple functional groups, the principal functional group is given the lowest locant and suffix; others are prefixes. Example: 3-chloropropanoic acid – –COOH is suffix, –Cl is prefix.
Alkenes and alkynes are named with suffixes "-ene" and "-yne"; if both are present, "ene" comes before "yne" alphabetically, but numbering follows priority: alkyne > alkene only if no higher functional group exists.
The Cahn-Ingold-Prelog (CIP) rules assign R/S configuration based on atomic number priority at chiral centers. Example: In (R)-2-chlorobutane, Cl > CH2CH3 > CH3 > H.
E/Z isomerism applies to alkenes with two different substituents on each carbon of the double bond. E = opposite, Z = same side. Example: (Z)-1,2-dichloroethene has both Cl atoms on the same side.
Geometrical isomerism arises due to restricted rotation in alkenes or cyclic compounds. Example: cis- and trans-1,2-dimethylcyclopropane are stereoisomers.
Enantiomers are non-superimposable mirror images that rotate plane-polarized light equally but in opposite directions. Example: (R)- and (S)-lactic acid.
Diastereomers are stereoisomers that are not mirror images. Example: threose and erythrose in aldoses.
Meso compounds contain chiral centers but are optically inactive due to internal plane of symmetry. Example: meso-tartaric acid has two chiral centers and a plane of symmetry.
A molecule with 'n' chiral centers can have up to 2? stereoisomers, but fewer if meso forms exist. Example: 2,3-dichlorobutane has 3 stereoisomers (one meso, one pair of enantiomers).
In IUPAC, alcohols are named with suffix "-ol", and the chain is numbered to give the OH group the lowest possible number. Example: pentan-2-ol, not pentan-4-ol.
Aldehydes use suffix "-al", and the carbonyl carbon is always C1. Example: butanal, not 1-butanal.
Ketones use suffix "-one", and the chain is numbered to give the carbonyl the lowest number. Example: pentan-2-one.
Amines are named as alkanamines; primary amines use "-amine" as suffix. Example: propan-1-amine.
For compounds with equal priority functional groups, numbering is done to give the lowest set of locants (lowest sum rule). Example: 2-hydroxypentanal – aldehyde gets priority, so C1 is carbonyl.
In substituted benzene compounds, common names like toluene, phenol, aniline are retained, but derivatives use prefixes. Example: 4-nitrotoluene, not 1-methyl-4-nitrobenzene.
The term "retention of configuration" refers to the preservation of spatial arrangement during a reaction, as in SN2 inversion vs SN1 racemization.
Stereoisomers have the same molecular formula and connectivity but differ in spatial arrangement. Example: cis- and trans-2-butene.
IUPAC naming requires hyphens to separate numbers and letters, and commas to separate numbers. Example: 3-methylhexane, not 3methylhexane or 3, methylhexane.
Functional group priority determines the suffix; all others are substituents. Example: in 4-oxopentanoic acid, –COOH is principal, –CHO (as oxo) is prefix.
Intermediate — requires memorization of priority order and application in naming, plus conceptual clarity in stereochemistry.
Trap: Students assign R/S configuration based on clockwise/anticlockwise order without checking atomic number priority. Avoid: Always assign priority 1?4 by atomic number first, then determine direction.
Trap: Naming alkenes with E/Z without checking if both carbons have two different groups. Avoid: E/Z only applies when each double-bond carbon has two different substituents; otherwise, no isomerism.
Trap: Assuming all chiral molecules are optically active. Avoid: Meso compounds have chiral centers but are optically inactive due to symmetry.
Q1. Which of the following is the correct IUPAC name for CH?CH?COCH?CH?OH? A. 4-hydroxypentan-2-one B. 2-hydroxypentan-4-one C. 4-oxopentan-2-ol D. 2-oxopentan-4-ol
Answer: A Explanation: The ketone group has higher priority than alcohol, so suffix is "-one"; chain numbered to give carbonyl lowest number. Why others fail: Option C incorrectly uses "oxo" as suffix; alcohol should be prefix when ketone is present.
Q2. Which functional group has the highest priority in IUPAC nomenclature? A. –OH B. –CHO C. –COOH D. –NH?
Answer: C Explanation: Carboxylic acid (–COOH) has the highest priority among common functional groups. Why others fail: Aldehyde (–CHO) is second to carboxylic acid, making B a tempting but incorrect choice.
Q3. The correct E/Z designation for the following compound is: ClCH=CHCH? (with Cl and CH? on same side) A. E-isomer B. Z-isomer C. Cis-isomer only D. Trans-isomer
Answer: B Explanation: Higher priority groups (Cl and CH?) are on the same side, so it is Z. Why others fail: Students confuse Z with cis; while often correlated, E/Z is based on priority, not just position.
Q4. A compound with two chiral centers and a plane of symmetry is: A. Optically active B. A meso compound C. A racemic mixture D. An enantiomer
Answer: B Explanation: A meso compound has chiral centers but is optically inactive due to internal symmetry. Why others fail: Option A is tempting because chiral centers usually imply optical activity, but not in meso forms.
Q5. What is the maximum number of stereoisomers possible for 2,3-dibromobutane? A. 2 B. 3 C. 4 D. 1
Answer: B Explanation: 2,3-dibromobutane has two chiral centers, but one meso form exists, so total stereoisomers = 3 (one meso + one pair of enantiomers). Why others fail: Option C (4) assumes no meso form, a common error when symmetry is overlooked.
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