Reactions: Carbylamine, Acylation, Alkylation

CHEReactions Of Amines

This cluster covers a confirmatory test (carbylamine) and how amines behave toward acylating and Friedel-Crafts conditions. The recurring theme is that only $1^\circ$ amines give the carbylamine reaction, and that aniline's basic N interferes with Lewis-acid catalysed reactions.

Carbylamine (isocyanide) test

C_6H_5NH_2 + CHCl_3 + 3\,KOH \xrightarrow{\Delta} C_6H_5NC + 3\,KCl + 3\,H_2O

only

1^\circ

amines give the foul-smelling isocyanide

C_6H_5NC

Acylation (acetylation) of aniline

C_6H_5NH_2 + (CH_3CO)_2O \rightarrow C_6H_5NHCOCH_3 + CH_3COOH

product is acetanilide;

CH_3COCl

also works

Carbylamine (isocyanide) test: a $1^\circ$ amine $+ CHCl_3 + alc.\,KOH$ (heat) gives a foul-smelling isocyanide, e.g. $C_6H_5NH_2 \xrightarrow{CHCl_3,\,KOH} C_6H_5NC$ .
Only $1^\circ$ amines respond to the carbylamine test; $2^\circ$ and $3^\circ$ amines give no offensive smell, so it distinguishes ethylamine from diethylamine.
Acylation: aniline $+ (CH_3CO)_2O$ (or $CH_3COCl$ ) $\rightarrow$ acetanilide ( $C_6H_5NHCOCH_3$ ); the $\text{-}NH_2$ is converted to an amide.
Acylation works on $1^\circ$ and $2^\circ$ amines (which have an N-H to replace); $3^\circ$ amines have no N-H and are not acylated.
Acetylation before nitration protects aniline: the $\text{-}NHCOCH_3$ group is less activating, prevents oxidation and over-substitution, and directs nitration mainly to the $para$ position; the acetyl group is later hydrolysed off.
Alkylation: amines react with alkyl halides to give a higher-order amine, but successive alkylation gives a mixture of $1^\circ$ , $2^\circ$ , $3^\circ$ amines and the quaternary ammonium salt, so it is poorly controlled.
Aniline does not undergo Friedel-Crafts alkylation/acylation: its basic N forms a salt (acid-base adduct) with the Lewis acid $AlCl_3$ , deactivating both the catalyst and the ring.
Acylation reduces the electron density on N, which is why an acylated amine is a poorer nucleophile/base than the free amine.
Pyridine is often added during acylation to neutralise the $HCl$ liberated and drive the reaction forward.
Distinguishing memory hook: carbylamine = $1^\circ$ only; acylation = any N-H amine ( $1^\circ$ / $2^\circ$ ); Friedel-Crafts on aniline = fails.

Where the marks go

Saying $2^\circ$ or $3^\circ$ amines give a positive carbylamine test — only $1^\circ$ amines (and not amides) form the isocyanide.
Writing the carbylamine product as a cyanide $R\text{-}CN$ instead of an isocyanide $R\text{-}NC$ — the foul smell comes from the isocyanide.
Forgetting that aniline's Friedel-Crafts failure is due to salt formation with $AlCl_3$ , not just ring deactivation — both the catalyst and ring are deactivated.
Attempting to acylate a $3^\circ$ amine — it has no N-H, so no amide forms.
Omitting the hydrolysis step that removes the acetyl group after using acetanilide as a protected intermediate in nitration.

How the board asks it

Distinguishcarbylamine test responds to $1^\circ$ amines only
How will you distinguish between ethylamine and diethylamine by a simple chemical test? Give the reagents used and the observation.
Predict the productacylation of aniline with acetic anhydride
Write the equation for the reaction of aniline with acetic anhydride and name the organic product formed.
Conversionacetylation as protection before nitration
How will you convert aniline into $p$ -nitroaniline? Explain why direct nitration of aniline is unsatisfactory.
Give reasonsbasic N of aniline forms a salt with the Lewis acid $AlCl_3$
Account for the fact that aniline does not undergo the Friedel-Crafts reaction.
Give reasonssuccessive alkylation gives a mixture of $1^\circ$ , $2^\circ$ , $3^\circ$ amines and a quaternary salt
Give a reason why the reaction of ethylamine with excess ethyl bromide is not a controlled method for preparing a pure secondary amine.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.