Q4(d). Cyclohexanone to nylon-6; phenylhexatriene to biphenyl

(i) Cyclohexanone to nylon-6

Scheme: The ring N–C(=O) bond of caprolactam opens and the units link head to tail.

1. Cyclohexanone + hydroxylamine (NH₂OH.HCl, buffer) gives cyclohexanone oxime.
2. Conc. H₂SO₄ (oleum) brings about the Beckmann rearrangement: the ring C–C bond anti to the leaving O migrates to N, so the ring expands by one atom to the seven-membered lactam, ε-caprolactam.
3. Caprolactam is heated strongly with a little water: some lactam is hydrolysed to 6-aminohexanoic acid, whose -NH₂ opens further lactam rings (ring-opening polymerisation, a chain-growth step without loss of water).
4. Product: nylon-6, [-NH-(CH₂)₅-CO⁻]ₙ, a polyamide with one kind of repeating unit (six carbons).

(ii) 1-Phenylhexa-1,3,5-triene to biphenyl

Scheme: Electrocyclic ring closure, then dehydrogenation (aromatisation).

1. The printed triene has the central C=C Z, drawn in the s-cis,s-cis form needed for ring closure. On heating, the hexatriene undergoes a thermal 6π electrocyclic ring closure: the HOMO (ψ₃) has the same phase at the two termini only after a disrotatory motion, so the closure is allowed thermally (Woodward–Hoffmann).
2. The new σ bond links the phenyl-bearing carbon and the terminal CH₂: product 5-phenylcyclohexa-1,3-diene.
3. Dehydrogenation with DDQ (or chloranil; or Pd/C, or sulfur on heating) removes two H atoms and the ring becomes aromatic: biphenyl, C₆H₅-C₆H₅. The gain of aromatic stabilisation makes this step easy.

Conclusion. (i) Cyclohexanone → oxime → caprolactam (Beckmann) → nylon-6 (ring-opening polymerisation). (ii) Thermal disrotatory 6π closure to 5-phenylcyclohexa-1,3-diene, then dehydrogenation to biphenyl.

Application. Nylon-6 is used for tyre cord, fishing lines and textiles; electrocyclisation–aromatisation sequences are a general route to substituted biaryls and to the vitamin D triene system (in reverse, by photochemical ring opening).