Q6(c). Nylon-6 from cyclohexanone: Beckmann rearrangement

Introduction. Nylon-6 is a polyamide, -[NH-(CH₂)₅-CO]_(n)⁻, made from a single monomer, caprolactam (azepan-2-one, the cyclic amide of 6-aminohexanoic acid). Caprolactam is made from cyclohexanone in two steps: oxime formation and the acid-catalysed Beckmann rearrangement. The lactam is then polymerised by ring-opening polymerisation.

Route

Scheme: Cyclohexanone → oxime → caprolactam → nylon-6.

Step 1: cyclohexanone oxime

Hydroxylamine (as its hydrochloride, buffered with sodium acetate) adds to the C=O; the carbinolamine loses water to give cyclohexanone oxime, C₆H₁₀=NOH. The reaction is fastest near pH 4–5, where the C=O is activated but NH₂OH is not fully protonated.

Step 2: Beckmann rearrangement (mechanism)

Scheme: Ring expansion from six to seven atoms: N is inserted into the ring.

1. Activation. The acid protonates the oxime OH (or converts it into a sulfate ester), making a good leaving group.
2. Migration (rate-determining). The ring C–C bond anti-periplanar to the leaving group moves from carbon to nitrogen as water leaves, in one concerted step. The migrating carbon keeps its configuration. The ring is expanded from six to seven atoms and a cyclic nitrilium ion is formed.
3. Hydration. Water attacks the nitrilium carbon; loss of H⁺ gives the imidic acid (iminol), -C(OH)=N⁻.
4. Tautomerisation. The imidic acid tautomerises to the more stable amide: ε-caprolactam, C₆H₁₁NO.

Cyclohexanone oxime is symmetrical, so whichever ring bond is anti to OH, the same lactam forms.

Step 3: ring-opening polymerisation to nylon-6

Caprolactam is heated to about 250°C with a little water. Some lactam is hydrolysed to 6-aminohexanoic acid; its -NH₂ attacks the C=O of another lactam (nucleophilic acyl substitution), opening the ring and lengthening the chain at each step. No small molecule is lost overall (each unit keeps all its atoms), and molten polymer is spun into fibre.

n C₆H₁₁NO -[NH-(CH₂)₅-CO]_(n)⁻

Conclusion. Cyclohexanone + NH₂OH → cyclohexanone oxime H₂SO₄ ε-caprolactam (monomer of nylon-6) by the Beckmann rearrangement (anti-periplanar alkyl migration to N, nitrilium ion, hydration, tautomerisation). Ring-opening polymerisation of caprolactam at about 250°C gives nylon-6.

Application. Nylon-6 (Perlon) is used for fibres, ropes, tyre cord and engineering plastics; the Beckmann process is the main industrial source of caprolactam, with ammonium sulfate as a by-product used as fertiliser.