CUET UG Chemistry: Organic Chemistry - Amines, Basicity Order, Reactions, Diazonium Salts

Must-Know (15–20 detailed bullets)

• Amines are basic due to the lone pair on nitrogen; basicity increases with electron-donating groups like alkyl groups. Example: methylamine (pKb = 3.36) is stronger base than ammonia (pKb = 4.75).
• In gaseous phase, basicity order of alkyl amines: tertiary > secondary > primary > NH? due to +I effect. In aqueous phase, order is: secondary > primary > tertiary > NH? due to solvation and steric effects.
• Aromatic amines (e.g., aniline, pKb = 9.38) are weaker bases than aliphatic amines due to resonance delocalization of lone pair into benzene ring.
• Electron-withdrawing groups (e.g., –NO?) decrease basicity of aniline; p-nitroaniline (pKb-13.0) is weaker base than aniline.
• Electron-donating groups (e.g., –CH?) increase basicity of aniline; p-toluidine (pKb-8.9) is stronger base than aniline.
• Gabriel phthalimide synthesis is used to prepare primary aliphatic amines; phthalimide reacts with KOH and alkyl halide, followed by hydrolysis.
• Hoffmann bromamide degradation reaction converts amides to amines with one less carbon atom; R–CONH? + Br? + 4NaOH-R–NH? + Na?CO? + 2NaBr + 2H?O.
• Nitration of aniline gives meta-nitroaniline under acidic conditions because –NH is meta-directing; direct nitration requires protection by acetylation.
• Carbylamine reaction is test for primary amines: R–NH? + CHCl? + 3KOH-R–NC + 3KCl + 3H?O; foul smell of isocyanide confirms primary amine.
• Secondary amines react with nitrous acid (HNO?) to form N-nitrosoamines (yellow oily liquid); R?NH + HNO?-R?N–N=O.
• Primary aliphatic amines react with HNO? to form aliphatic diazonium salts (unstable), releasing N? gas and forming alcohols; used in quantitative estimation.
• Primary aromatic amines form diazonium salts with HNO? at 273–278 K; C?H?NH? + NaNO? + 2HCl-C?H?NCl? + NaCl + 2H?O.
• Benzenediazonium chloride is stable at low temperature (0–5°C) but decomposes on warming to form phenol and N?.
• Sandmeyer reaction: ArNCl? + CuCl-Ar–Cl + N?; used to prepare chlorobenzene from aniline.
• Gattermann reaction uses Cu powder and HCl to convert diazonium salt to chloroarene; same product as Sandmeyer but milder conditions.
• Coupling reaction: Benzenediazonium chloride reacts with phenol in alkaline medium to form p-hydroxyazobenzene (orange dye).
• Diazonium salts are used in synthesis of azo dyes, methyl orange (pH indicator), and in replacement reactions (F, Cl, Br, I, CN, OH).
• Aniline cannot be prepared by Gabriel phthalimide synthesis because aryl halides do not undergo nucleophilic substitution easily; verify from NCERT.
• Amines have higher boiling points than hydrocarbons of similar mass due to intermolecular H-bonding; primary > secondary > tertiary (no H-bonding in tertiary).
• The pKb of ammonia is 4.75, methylamine is 3.36, dimethylamine is 3.27, aniline is 9.38 — used for comparing basic strength.

Difficulty Level

Intermediate — requires understanding of electronic effects, reaction conditions, and exceptions in aromatic vs aliphatic systems.

Common CUET Traps (3 bullets)

• Trap: Assuming tertiary amines are always strongest bases in water.
  Avoid: In aqueous phase, secondary amines are strongest due to balance of inductive effect and solvation; tertiary amines have steric hindrance.

• Trap: Thinking aniline undergoes electrophilic substitution at ortho/para without protection.
  Avoid: Aniline must be acetylated first because –NH? gets protonated in acid, making it meta-directing.

• Trap: Believing aliphatic diazonium salts are stable like aromatic ones.
  Avoid: Aliphatic diazonium salts are highly unstable and immediately decompose to alcohols and N? gas.

Practice MCQs (5 questions)

Q1. Which of the following is the strongest base in aqueous solution?
A) CH?NH?
B) (CH?)?NH
C) (CH?)?N
D) NH?

Answer: B
Explanation: (CH?)?NH has the lowest pKb (3.27) among amines due to combined inductive and solvation effects.
Why others fail: (CH?)?N has strong +I effect but poor solvation and steric hindrance, reducing basicity.

Q2. Which amine gives carbylamine test?
A) Dimethylamine
B) Aniline
C) N-Methylaniline
D) Trimethylamine

Answer: B
Explanation: Only primary amines (aliphatic or aromatic like aniline) give carbylamine test.
Why others fail: Secondary and tertiary amines do not react with CHCl?/KOH to form isocyanide.

Q3. What is the product when benzenediazonium chloride reacts with CuBr?
A) Bromobenzene
B) Benzene
C) Phenol
D) Nitrobenzene

Answer: A
Explanation: This is Sandmeyer reaction: ArN + CuBr-Ar–Br + N?.
Why others fail: Phenol forms only when diazonium salt is warmed with water, not with CuBr.

Q4. Which of the following has the highest pKb value?
A) Methylamine
B) Ammonia
C) Aniline
D) Dimethylamine

Answer: C
Explanation: Aniline (pKb-9.38) is weakest base due to resonance; higher pKb means weaker base.
Why others fail: Dimethylamine has lowest pKb (strongest base), so it's not correct.

Q5. What is the major product when aniline is treated with acetyl chloride followed by nitration and then hydrolysis?
A) o-Nitroaniline
B) m-Nitroaniline
C) p-Nitroaniline
D) 2,4,6-Trinitroaniline

Answer: C
Explanation: Acetylation protects –NH? group, making it ortho-para directing; para product dominates due to steric hindrance.
Why others fail: Without protection, nitration gives meta product; but here protection changes orientation.

Last?Minute Revision (15–20 one?liners)

• Basicity in water: 2° > 1° > 3° > NH? > C?H?NH?.
• Gabriel synthesis only for 1° aliphatic amines — fails for aryl halides.
• Hoffmann bromamide: R–CONH?-R–NH? with loss of one carbon.
• Carbylamine test: only for 1° amines; foul smell of isocyanide.
• HNO? with 1° aliphatic amine-N? gas + alcohol.
• HNO? with 1° aromatic amine-diazonium salt at 273–278 K.
• Diazonium salt stability: only aromatic, at 0–5°C.
• Sandmeyer reaction: CuCl or CuBr for haloarenes from diazonium.
• Gattermann: same as Sandmeyer but uses Cu/HX.
• Fluorobenzene from diazonium via Balz-Schiemann: ArNBF-ArF + BF? + N?.
• Coupling with phenol: alkaline medium-orange azo dye.
• Methyl orange: synthesized from diazonium + dimethylaniline.
• Aniline-nitrobenzene requires protection (acetylation first).
• –NH? group is o/p director; –NH is m-director.
• pKb of aniline > ammonia > methylamine (increasing basicity).
• Tertiary amines do not form H-bonds-lower boiling point than 1°/2°.
• Aromatic amines cannot be prepared by ammonolysis of aryl halides — unreactive.
• Diazonium salts used in synthesis of dyes, drugs, and substituted aromatics.
• Mnemonic: "CBS" — Carbylamine for Primary, Bromo for Secondary (yellow oil), Tertiary no reaction with HNO?.
• Sandmeyer uses Cu? salts; Gattermann uses Cu powder.