(a) Write the structures of the products formed in the following reactions : 2×5=10 (i) (Cyclohex-2-en-1-one) —NaBH4/CeCl3…

(a) Products of five reductions (10 marks)

Each reagent reduces a different functional group, which is the point of the question. The structures are drawn below.

(i) NaBH₄/CeCl₃ in MeOH (Luche reduction): the cerium salt makes the carbonyl a harder, more electrophilic centre, so hydride adds 1,2 to the C=O rather than 1,4. The product is the allylic alcohol, cyclohex-2-en-1-ol, and the C=C survives. (2 marks)

(ii) Na/NH₃(l): the enone accepts an electron to give a radical anion; a second electron and protonation reduce the C=C bond (the carbonyl carbon is not reduced), giving an enolate that on work-up gives the saturated ketone, cyclohexanone. (2 marks)

(iii) B₂H₆ then H⁺: borane reduces a carboxylic acid faster than an ester, because it coordinates the electron-rich carbonyl of the acid and forms a triacyloxyborane. The CO₂H becomes CH₂OH and the ester is untouched: ethyl 5-hydroxy-3-methylpentanoate, EtO₂C-CH₂-CH(CH₃)-CH₂-CH₂OH. (On acid work-up it can cyclise to 4-methyltetrahydropyran-2-one.) (2 marks)

(iv) LiAlH₄ then H⁺: this hydride reduces both acid and ester, so both become primary alcohols: 3-methylpentane-1,5-diol, HOCH₂-CH₂-CH(CH₃)-CH₂-CH₂OH, plus ethanol from the ester. (2 marks)

(v) BH₃-THF: borane reduces the amide (lactam) faster than the ester, because the amide oxygen is the most basic and coordinates borane best. The lactam C=O becomes CH₂, giving the pyrrolidine, methyl 1-benzylpyrrolidine-3-carboxylate, with the ester intact. (2 marks)

(b)(i) Preparation of nylon 6 and nylon 6,6 (5 marks)

Nylon 6 is made from a single monomer, caprolactam (ε-caprolactam, the cyclic amide of 6-aminohexanoic acid), by ring-opening polymerisation. The lactam is heated at about 250-270 °C with a little water, which hydrolyses some rings to 6-aminohexanoic acid; its amine group then attacks other lactam rings and opens them, chain after chain, without loss of water. Repeat unit: -[NH-(CH₂)₅-CO]-.

Nylon 6,6 is made from two six-carbon monomers, adipic acid (hexanedioic acid) and hexamethylenediamine (hexane-1,6-diamine), by condensation polymerisation with loss of water. They are first combined into the 1:1 salt (nylon salt) to fix the stoichiometry, which is then heated to about 250-280 °C under pressure, and the water is removed. Repeat unit: -[NH-(CH₂)₆-NH-CO-(CH₂)₄-CO]-.

(b)(ii) Combustion of polyethylene (5 marks)

Polyethylene is (C₂H₄)ₙ, and the repeat unit -CH₂CH₂- has a mass of 28 g/mol.

Moles of repeat units in 20 g = 20/28 = 0.714 mol.

C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

Each repeat unit gives 2 CO₂, so the moles of CO₂ = 2 × 0.714 = 1.43 mol (about 63 g). The check by carbon: 20 g of CH₂ units at 14 g/mol is 1.43 mol of C, and each C gives one CO₂.

(b)(iii) Polystyrene versus polyethylene (5 marks)

Polyethylene has a bare -CH₂-CH₂- chain with no bulky groups. The chains are flexible, they pack closely, and the polymer is partly crystalline. Its glass transition temperature (about -100 °C or lower) is far below room temperature, so at room temperature the amorphous regions are rubbery, and the crystalline regions act as tie points, so it bends and stretches.

Polystyrene has a bulky pendant phenyl group on every second carbon. These groups hinder rotation about the backbone bonds, which stiffens the chains, and they interlock through π-π interactions but also prevent regular packing, so the usual atactic polymer is completely amorphous. The glass transition temperature is about 100 °C, well above room temperature, so at room temperature the chains are frozen in a glassy state. Under stress they cannot slide past each other, so the material cracks instead of deforming: it is hard but brittle.

(b)(iv) Teflon versus PVC (5 marks)

PTFE, -(CF₂-CF₂)ₙ-, is inert for three reasons. The C-F bond is one of the strongest single bonds (about 485 kJ/mol), much stronger than C-Cl (about 330 kJ/mol), and the C-C backbone is also strengthened. The fluorine atoms are larger than hydrogen and form a tight, uniform sheath round the carbon chain, which blocks the approach of reagents, and the high electronegativity of fluorine makes the sheath non-polarisable and unreactive. There is no hydrogen at all, so there is no route for elimination.

PVC, -(CH₂-CHCl)ₙ-, has weaker C-Cl bonds and hydrogens on the carbon next to the C-Cl carbon (β-hydrogens). On heating (from about 100-150 °C), HCl is eliminated (dehydrochlorination), and the reaction, once started, proceeds along the chain (a zip reaction), leaving conjugated polyene sequences that discolour the polymer. The HCl released catalyses further loss, which is why PVC needs heat stabilisers.

(c)(i) Natural rubber versus gutta-percha (10 marks)

Natural rubber (from Hevea latex) is soft, sticky and highly elastic: it stretches several hundred per cent and springs back. It is amorphous, with a low glass transition temperature (about -70 °C), and dissolves or swells in hydrocarbon solvents. Gutta-percha (from Palaquium) is hard, tough and inelastic. It is crystalline (about 60 °C melting point, softening in warm water), and is used for golf-ball covers and root-canal fillings.

Both polymers are 1,4-polyisoprene, made of isoprene (2-methylbuta-1,3-diene) units joined head to tail, and they differ only in the geometry of the double bond in each repeat unit. In natural rubber it is cis (Z): the two CH₂ groups are on the same side of the double bond, so the chain has a bend at each unit. The kinked chains coil, pack poorly and cannot crystallise, and the weak intermolecular forces let them uncoil under stress and return, giving elasticity. In gutta-percha the double bond is trans (E): the chain is nearly straight and regular, so chains line up and pack closely and crystallise. The stronger intermolecular attraction makes the material hard, rigid and non-elastic.

(c)(ii) Nucleic acids (10 marks)

Nucleic acids are linear biopolymers of nucleotides that store and transfer genetic information. Each nucleotide has three parts: a pentose sugar, a nitrogen base and a phosphate group. Sugar plus base, joined by an N-glycosidic bond at C1′ of the sugar, is a nucleoside (for example adenosine); a nucleoside with a phosphate ester on its 5′-OH is a nucleotide (for example AMP). The bases are the purines adenine and guanine, and the pyrimidines cytosine, thymine (DNA) and uracil (RNA).

There are two classes. DNA contains 2′-deoxyribose and the bases A, G, C and T; it is usually a double-stranded helix that carries the genetic information. RNA contains ribose and the bases A, G, C and U; it is generally single-stranded (mRNA, tRNA, rRNA) and is involved in protein synthesis.

The primary structure is the sequence of bases along the polynucleotide chain. Nucleotides are joined by phosphodiester bonds: the phosphate group links the 3′-OH of one sugar to the 5′-OH of the next, giving an alternating sugar-phosphate backbone with a free 5′ end at one end and a free 3′-OH at the other. The bases hang from the backbone and the sequence is written from the 5′ to the 3′ end.