Chemical Reactions of Amines

(1) ALKYLATION: an amine reacts with an alkyl halide (e.g. CH3Br) in successive substitutions, climbing 1 degree to 2 degree to 3 degree amine and finally to a quaternary ammonium salt as more equivalents react. (2) ACYLATION: primary/secondary aliphatic or aromatic amines react with acetyl chloride or acetic anhydride, in the presence of pyridine (to mop up the HCl by-product), to form an N-alkyl acetamide, e.g. ethylamine + acetyl chloride gives N-ethylacetamide + HCl. Aniline specifically reacting with BENZOYL CHLORIDE (in NaOH) to give N-phenylbenzamide is called the SCHOTTEN-BAUMANN REACTION -- a nucleophilic acyl substitution. (3) REACTION WITH NITROUS ACID (made in situ from NaNO2 + HCl) distinguishes all three amine classes: PRIMARY aliphatic amines give an unstable alkyl diazonium salt that immediately loses N2 to give the alcohol (ethylamine gives ethanol + N2 via unstable ethyl diazonium chloride); PRIMARY aromatic amines at low temperature (273-278 K) give a diazonium salt that IS reasonably stable for a short time (this specific step is called DIAZOTISATION, feeding directly into the diazonium-salt chemistry of the next section); SECONDARY amines (alkyl or aryl) give a yellow, water-insoluble N-nitrosoamine oil -- this colour test is LIEBERMANN'S NITROSO TEST; TERTIARY aliphatic amines give a water-soluble trialkylammonium nitrite salt, while tertiary AROMATIC amines (e.g. N,N-dimethylaniline) react at the ring instead, giving a p-nitroso compound. (4) CARBYLAMINE REACTION: primary amines (aliphatic or aromatic) react with chloroform and alcoholic KOH to give a foul-smelling isocyanide (carbylamine) -- this carbylamine test is used specifically to IDENTIFY primary amines, since secondary and tertiary amines give no such reaction. (5) MUSTARD OIL REACTION (the Hofmann mustard-oil test, also a primary-amine identification test): a primary amine + carbon disulphide (CS2) gives an N-alkyldithiocarbamic acid, which with HgCl2 gives an alkyl isothiocyanate (mustard-oil smell); aniline + CS2 similarly gives S-diphenylthiourea, which on boiling with concentrated HCl gives phenyl isothiocyanate. (6) ELECTROPHILIC SUBSTITUTION IN ANILINE: the -NH2 group is a strong activating, ortho/para-directing group (its nitrogen lone pair conjugates into the ring). BROMINATION with Br2/water gives 2,4,6-tribromoaniline directly (a white precipitate) without needing a catalyst; to isolate the MONO-bromo product instead, aniline is first acetylated (reducing -NH2's activating power by delocalising the lone pair into the new carbonyl), brominated to p-bromoacetanilide, then hydrolysed back to p-bromoaniline. NITRATION of free aniline gives a mixture of o-/p-nitroaniline plus some m-nitroaniline (via the protonated, meta-directing anilinium ion formed in the strongly acidic medium) along with dark oxidation 'tar …