(a) (i) After polymerization of p-hydroxybenzoic acid, IR analysis shows 0·2% unreacted —COOH. Calculate the molecular weight of…

(a)(i)

Molecular weight from 0·2 % residual –COOH. *p*-Hydroxybenzoic acid (HBA) undergoes step‑growth condensation to a polyester, and every linear chain that is not cyclic retains one terminal –COOH group, which is exactly the end‑group the IR spectrum detects. If 0·2 % of the –COOH groups remain unreacted, the extent of reaction is p = 0·998, and the Carothers equation gives X̄ₙ = 1/(1 − p) = 1/0·002 = 500. The repeat unit –O–C₆H₄–CO– has M₀ = 120 g mol⁻¹, so M̄ₙ ≈ 500 × 120 = 6·0 × 10⁴ g mol⁻¹. This places the material firmly in the high‑molecular‑weight condensation‑polyester class, well above the fibre‑forming threshold.

(a)(ii)

Polymer structure. The product is the *para*‑linked aromatic polyester poly(4‑oxybenzoyl) (poly(p‑hydroxybenzoic acid), sold as Ekonol; HBA is also the main unit of liquid-crystalline Vectra copolyesters) with the repeating unit [–O–C₆H₄–CO–]ₙ, joined by ester (–CO–O–) linkages formed with loss of water as the condensation by‑product.

(a)(iii)

Precursor of natural rubber. The monomer (precursor) is isoprene, 2‑methylbuta‑1,3‑diene, CH₂=C(CH₃)–CH=CH₂. Natural rubber is cis‑1,4‑polyisoprene, in which the monomers add through the 1,4‑positions so that the residual C=C lies inside the chain: –[CH₂–C(CH₃)=CH–CH₂]–ₙ, with the two chain bonds on the same side of the double bond (cis configuration). The cis geometry is what gives the elastomer its coiled, elastic chain conformation.

(b)(i)

Nylon 6 vs Nylon 6,6.

• Monomers and repeat unit: Nylon 6 is made from a single monomer, caprolactam, and has one C₆ unit, –[NH–(CH₂)₅–CO]–ₙ; Nylon 6,6 is made from hexamethylenediamine and adipic acid, –[NH–(CH₂)₆–NH–CO–(CH₂)₄–CO]–ₙ.
• Route: Nylon 6 by ring-opening polymerisation of caprolactam (water-initiated or anionic); Nylon 6,6 by step-growth condensation of the nylon salt.
• Melting point: Nylon 6 about 220 °C; Nylon 6,6 about 265 °C.
• Mechanical and thermal behaviour: Nylon 6,6 has somewhat higher strength, stiffness and heat resistance; Nylon 6 has slightly lower crystallinity, better impact strength and surface finish.
• Moisture and dye uptake: slightly higher for Nylon 6, which also dyes more easily; Nylon 6,6 absorbs slightly less water.
• Uses: both are used for fibres (textiles, tyre cord, carpets) and engineering plastics; Nylon 6,6 is preferred where higher service temperature is needed.

Thus Nylon 6,6, with its more symmetric, better‑packing diad, is the stronger, higher‑melting fibre, while Nylon 6 is easier to process from one monomer.

(b)(ii)

Preparation of syndiotactic polystyrene. Syndiotactic PS (first made by Ishihara at Idemitsu in 1986) is made by coordinate (chain‑growth) polymerisation of styrene with a homogeneous single‑site catalyst — e.g. CpTiCl₃/MAO or Cp*Ti(OMe)₃/MAO (half-sandwich titanium catalysts) — under inert atmosphere at moderate temperature. Under chain-end control, each incoming styrene is inserted with the opposite face to the preceding unit, so the phenyl groups alternate sides of the backbone, giving the syndiotactic (r, racemo) placement. This contrasts with free‑radical polymerisation (atactic PS) and with TiCl₃/AlR₃ Ziegler catalysts (isotactic PS).

(b)(iii)

α‑Helix: features and stabilisation. The α‑helix is right‑handed, with 3·6 residues per turn and a rise (elongation) of 1·5 Å per residue (pitch ≈ 5·4 Å). It is stabilised principally by intra‑chain C=O(i)···H–N(i+4) hydrogen bonds running parallel to the helix axis, and by the outward orientation of the side chains, which packs favourably and shields the polar backbone. Proline residues and contiguous like‑charges disrupt the helix.

(c)(i)

Same azide from the two dichloro‑bicyclic sulfides. Compounds 1 and 2 are the two diastereomeric dichloro‑bicyclic thioethers (fused bicyclic sulfides) shown, differing in the relative orientation of the C–Cl bonds. The common product is the bicyclic sulfide azide in which N₃ has replaced the chlorine at the carbon β to sulfur (the assisted centre), the other C–Cl being retained. In 1 that C–Cl is antiperiplanar to the C–S bond, so the sulfur lone pair participates (neighbouring‑group / anchimeric assistance) to form a bridged three‑membered sulfonium (thiiranium) ion; this stabilised transition state lowers the activation energy, so N₃⁻ reacts at room temperature, then opens the sulfonium (second inversion → net retention). In 2 the same C–Cl is oriented syn/endo, not antiperiplanar to C–S, so sulfur cannot assist and displacement must proceed by a slow, high‑energy direct path, requiring 200 °C. Because the two isomers carry that Cl on opposite faces, the retention product from 1 and the inversion product from 2 place N₃ on the same face, hence the identical azide; steric/electronic effects and sulfonium stabilisation account for the rate difference.

(c)(ii)

Products P and Q from 1‑cyclohexylpropan‑1‑one. The substrate is the ethyl ketone C₆H₁₁–CO–CH₂–CH₃, with two distinct α‑sites: the less hindered ethyl α‑CH₂ and the more substituted ring α‑CH. Q (LDA/THF, 1 equiv, low T, then D₂O): LDA is strong, bulky and the enolate formation is irreversible, so the kinetic (less substituted) enolate forms at the ethyl CH₂; quenching with D₂O gives Q = C₆H₁₁–CO–CHD–CH₃ (D on the ethyl α‑carbon only). P (excess NaOH, then D₂O): hydroxide enolisation is reversible and thermodynamic, so every α‑hydrogen exchanges at both sites, giving P = (1‑D‑cyclohexyl)–CO–CD₂–CH₃ (D at the ring α‑CH and at both H of the ethyl α‑CH₂, i.e. three D). Thus Q reflects kinetic control at the less hindered site, while P reflects complete thermodynamic α‑deuteration.