Amines — Class 12 MCQs with Answers

Amines — important questions & MCQs with answers (Class 12 Chemistry)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Functional group of amines:

   A−OHB−CHOC−NH₂ / −NHR / −NR₂D−COOH
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   Answer: −NH₂ / −NHR / −NR₂

   Explanation: Amines carry −NH₂, −NHR or −NR₂ (1°, 2°, 3° by substituents on N) — the lone pair on N, not −OH, −CHO or −COOH, is what makes the group basic and nucleophilic.

2. Q2 of 6Easy

   The carbylamine test is used to detect:

   APrimary amines onlyBSecondary aminesCTertiary aminesDQuaternary salts
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   Answer: Primary amines only

   Explanation: Only primary amines (aliphatic or aromatic) react with CHCl₃ and alc. KOH to give foul-smelling isocyanides (carbylamines).

3. Q3 of 6Medium

   In aqueous phase, basicity order of methylamines is generally:

   A1° > 2° > 3°B3° > 2° > 1°C2° > 1° > 3°DAll same
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   Answer: 2° > 1° > 3°

   Explanation: In water, solvation of the ammonium ion and steric hindrance override the inductive effect, giving (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N — the reverse of the gas-phase 3° > 2° > 1° order students often assume holds in solution.

4. Q4 of 6Medium

   Hofmann bromamide reaction converts:

   APrimary amide → primary amine (one C less)BAmine → amideCAcid → esterDAlcohol → ether
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   Answer: Primary amide → primary amine (one C less)

   Explanation: Hofmann bromamide degradation: RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O via an isocyanate intermediate, shrinking the chain by one carbon — the product is the amine, not an amide or ester.

5. Q5 of 6Hard

   Aniline is less basic than aliphatic amines because:

   ANo reasonBHeavierCMore volatileDLone pair on N is delocalised into benzene ring
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   Answer: Lone pair on N is delocalised into benzene ring

   Explanation: Aniline's N lone pair delocalises into the ring by resonance with the ortho/para carbons, leaving less electron density to bind H⁺ — so aniline is far weaker a base than aliphatic amines like methylamine.

6. Q6 of 6Hard

   Diazonium salts are:

   AStable foreverBUseful intermediates for many aromatic substitutionsCInertDFuels
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   Answer: Useful intermediates for many aromatic substitutions

   Explanation: Diazonium salts (ArN₂⁺) form only below 5 °C from primary aromatic amines and serve as reactive intermediates for Sandmeyer substitutions, azo coupling and deamination — they are not stable or inert.

Amines notes: key concepts (Class 12 Chemistry)

Amines covers the classification, preparation, basicity and reactions of ammonia derivatives, and the rich chemistry of aromatic diazonium salts. Named reactions run through it: Hofmann bromamide degradation and the Gabriel synthesis (for 1° amines), the carbylamine test, diazotisation, and the Sandmeyer and coupling reactions.

Classification

Amines are ammonia derivatives — primary (RNH₂), secondary (R₂NH) and tertiary (R₃N) by the number of alkyl/aryl groups on nitrogen; aniline is the key aromatic amine.

Preparation of amines

By reduction of nitro compounds and nitriles, ammonolysis of haloalkanes (gives a mixture), and the Gabriel and Hofmann routes that yield pure 1° amines.

Basic nature of amines

The lone pair on nitrogen accepts a proton, so amines are basic; alkyl (+I) groups increase basicity and stabilise the ammonium ion formed.

Basicity order in water

Not simply 3° > 2° > 1°: solvation of the cation and steric crowding compete with the +I effect, so for methylamines it is often 2° > 1° > 3° in water.

Aromatic amines are weaker bases

In aniline the nitrogen lone pair is delocalised into the ring by resonance, so it is less available to a proton — aniline is far less basic than aliphatic amines.

Reaction with nitrous acid

1° aliphatic amines give alcohols + N₂; 1° aromatic amines form stable diazonium salts at 0–5 °C; 2° amines give nitrosamines; 3° amines behave differently.

Carbylamine (isocyanide) test

Only 1° amines react with CHCl₃ and alcoholic KOH to give foul-smelling isocyanides (carbylamines) — a specific test for primary amines.

Hinsberg test

Benzenesulphonyl chloride distinguishes 1°, 2° and 3° amines by whether the product is soluble in alkali, insoluble, or does not react at all.

Acylation and alkylation

Amines react with acyl chlorides/anhydrides to give amides (acylation) and with alkyl halides to give higher amines and quaternary salts (alkylation).

Electrophilic substitution in aniline

The –NH₂ group is strongly activating and o/p directing; nitration needs protection (acetylation) of –NH₂ to avoid oxidation and get the para product.

Diazonium salts

Aromatic 1° amines with HNO₂ (NaNO₂/HCl) at 273–278 K give ArN₂⁺; the diazonium group is a superb leaving group for aromatic substitution.

Reactions of diazonium salts

The –N₂⁺ group is replaced by Cl, Br, CN (Sandmeyer), I, OH, H or F, or retained in coupling with phenols/amines to give coloured azo dyes.