Q.Write a short note on Hoffmann elimination.

Nitrous acid reacts differently with the three classes of aromatic amines; only the secondary amine, N-methylaniline, gives the diagnostic yellow oily liquid (an N-nitrosamine) — this is a classic distinguishing test for 1°, 2°, and 3° amines.

Reaction of amines with $HNO_2$ (generated in situ from $NaNO_2 + HCl$, at $0$–$5°C$):

• Primary aromatic amine (aniline, option d): forms a benzenediazonium chloride, $C_6H_5N_2^+Cl^-$, a colourless/pale-yellow crystalline solid in cold solution — not an oily liquid.
• Secondary aromatic amine (N-methylaniline, option a): the N-H hydrogen is replaced by the nitroso group ($-N=O$), giving N-methyl-N-nitrosoaniline, $C_6H_5N(CH_3)-N=O$ — this N-nitrosamine is a characteristic yellow oily liquid. …

Aniline undergoes N-acylation with acetic anhydride: the amine nitrogen's lone pair attacks the anhydride's carbonyl carbon, displacing acetate, to give the amide acetanilide.

Aniline, $C_6H_5NH_2$, has a nucleophilic nitrogen lone pair that attacks the electrophilic carbonyl carbon of acetic anhydride, $(CH_3CO)_2O$, in a nucleophilic acyl substitution reaction. One acetyl group is transferred to nitrogen (displacing acetate/acetic acid as the leaving group), converting the -NH$_2$ into an amide -NHCOCH$_3$ group: $C_6H_5NH_2 + (CH_3CO)_2O \rightarrow C_6H_5NHCOCH_3\ (\text{acetanilide}) + CH_3COOH$ The product, N-phenylacetamide, is commonly called acetanilide, and this specific N-acetylation reaction is also used industrially/in the lab to protect (and mildly deactivate) the -NH$_2$ group before performing further electrophilic aromatic sub …

Acylation of a primary amine by an acid anhydride produces an amide.

An acid anhydride reacts with a primary amine by nucleophilic acyl substitution: the amine's lone pair attacks the carbonyl carbon of the anhydride, displacing a carboxylate/carboxylic acid and forming an N-substituted amide: …

Converting a primary amine to a quaternary ammonium salt by exhaustive methylation consumes 3 moles of CH₃I per mole of amine.

Methylamine, $CH_3NH_2$, has one methyl group and two N–H bonds. Exhaustive (excess) alkylation with methyl iodide replaces both remaining N–H hydrogens and then alkylates the resulting tertiary amine's lone pair to form the quaternary ammonium salt: …

Vigorous oxidation of aniline with acidified potassium dichromate ($K_2Cr_2O_7/H^+$) oxidises the ring, converting aniline into p-benzoquinone.

Aniline's strongly activating $-NH_2$ group makes the ring very susceptible to oxidative attack. Acidified dichromate (a strong oxidising agent) oxidises aniline not at a side chain (there is none) but at the ring itself, removing the amino nitrogen as part of the process and installing two carbonyl groups at the para positions to give p-benzoquinone ($C_6H_4O_2$). Benzaldehyde (a) and benzoic acid (d) would arise from oxidation of a methyl/aldehyde side chain (a …

Primary amine + $CHCl_3$/alcoholic KOH is the carbylamine reaction, giving a foul-smelling isocyanide.

This is the carbylamine (isocyanide) test, characteristic of primary amines only (both aliphatic and aromatic): the amine, chloroform and alcoholic KOH react to form an isocyanide (carbylamine) with a distinctive foul/offensive smell, along with KCl and water:

$RNH_2 + CHCl_3 + 3KOH \rightarrow RNC + 3KCl + 3H_2O$

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The carbylamine reaction (foul-smelling isocyanide) is given only by primary amines.

When a primary amine reacts with chloroform ($CHCl_3$) in the presence of alcoholic KOH, it forms an isocyanide (carbylamine), which has an extremely unpleasant, foul odour — this is the carbylamine test, specific to primary amines only (both aliphatic and aromatic).

$RNH_2 + CHCl_3 + 3KOH \rightarrow RNC + 3KCl + 3H_2O$

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