Amines (Class 12): Basicity, Reactions and How to Tell Them Apart

Why the order changes with the medium

Alkyl groups are electron donating, so on inductive grounds alone a tertiary amine should be the strongest base. In water that is not what is observed, because basicity also depends on how well the protonated ion is stabilised by hydrogen bonding with solvent. A tertiary ammonium ion has only one N–H to hydrogen bond with, and its three alkyl groups get in the way. The net result for simple alkyl amines in water is 2° > 1° > 3° > NH₃.

Aniline, and why it is a weak base

The nitrogen lone pair in aniline is delocalised into the benzene ring, so it is far less available to accept a proton. That single fact answers a large family of questions: any substituent that withdraws electrons from the ring makes aniline weaker still, and any that donates makes it slightly stronger. It also explains why aniline cannot be nitrated directly under strongly acidic conditions without protection — the protonated form is meta directing, which is not what the question wants.

Reactions that appear year after year

• Hofmann bromamide degradation — an amide loses one carbon to give a primary amine. The carbon count dropping by one is the giveaway in a conversion question.
• Gabriel phthalimide synthesis — gives pure primary amines, and importantly does not work for aromatic amines, which is itself a common one-mark question.
• Diazotisation and coupling — aniline with HNO₂ at 273–278 K gives the diazonium salt, the gateway to a large set of aromatic conversions.
• Carbylamine reaction — a foul-smelling isocyanide, given only by primary amines.

Distinguishing 1°, 2° and 3° amines

The Hinsberg test is the systematic answer: with benzenesulfonyl chloride, a primary amine gives a product soluble in alkali, a secondary amine gives one insoluble in alkali, and a tertiary amine does not react at all. The carbylamine test is quicker but identifies only primary amines. Both are worth knowing, because questions ask for either.

FAQs

Why is a secondary amine more basic than a tertiary amine in water?

Because basicity in solution depends on solvation as well as electron density. The tertiary ammonium ion has fewer N–H bonds available for hydrogen bonding and is more sterically crowded, so it is less well stabilised by water despite the greater inductive push.

Why can the Gabriel synthesis not make aniline?

The synthesis relies on nucleophilic substitution at an sp³ carbon by the phthalimide anion. Aryl halides do not undergo that substitution under these conditions, so no aromatic amine is formed.

What does the carbylamine reaction identify?

Primary amines only, aliphatic or aromatic. The foul-smelling isocyanide produced makes it a distinctive qualitative test, but it says nothing about secondary or tertiary amines.

Is aniline more or less basic than ammonia?

Less. The lone pair on nitrogen is delocalised into the aromatic ring and is therefore less available for protonation, which makes aniline a markedly weaker base than ammonia.