In the following reaction, intermediate [A] is formed : CHCl₃ + ᵗBuOK ——> [A] (a) Write the structures of the major products M…

(a) Products M, N, O, P and Q from 4-bromonitrobenzene (15 marks)

• M: 4-bromoaniline, 4-BrC₆H₄NH₂, by reduction of the nitro group with Sn/HCl (the C-Br bond is untouched).
• N: 4-bromobenzonitrile, 4-BrC₆H₄CN. M is diazotised with NaNO₂/HCl at 0-5 °C to the diazonium salt, and CuCN then replaces N₂⁺ by CN (Sandmeyer reaction).
• O: 4-nitrobenzonitrile, 4-O₂NC₆H₄CN, by nucleophilic aromatic substitution (SNAr). The para nitro group stabilises the Meisenheimer anion, so cyanide displaces bromide.
• P: 4-nitrobenzamide, 4-O₂NC₆H₄CONH₂. SOCl₂ acts only on a carboxylic acid, so the nitrile O is first hydrolysed (aqueous acid, heat) to 4-nitrobenzoic acid; SOCl₂ converts this to the acid chloride, and NH₃ then gives the amide (nucleophilic acyl substitution).
• Q: 4-nitroaniline, 4-O₂NC₆H₄NH₂, by the Hofmann rearrangement of the amide with Br₂/NaOH and heat, followed by H₂O. The isocyanate formed from the N-bromoamide is hydrolysed, and CO₂ is lost, so the product has one carbon fewer.

(b)(i) Benzaldehyde with concentrated NaOD in D₂O (5 marks)

Benzaldehyde has no α-hydrogen, so it undergoes the Cannizzaro reaction, a disproportionation into benzyl alcohol and benzoate: 2 PhCHO + NaOD → PhCH₂OD (PhCH₂OH after work-up) + PhCO₂Na (PhCO₂H on acidification).

Mechanism: (1) OD⁻ adds to the carbonyl carbon of one molecule to give the tetrahedral alkoxide PhCH(OD)O⁻ (in strong base its OD group is also ionised, giving the dianion). (2) This intermediate transfers its C-H hydride directly to the carbonyl carbon of a second benzaldehyde molecule, giving benzoate (from the first) and the alkoxide PhCH₂O⁻ (from the second). (3) The alkoxide picks up D⁺ from D₂O, giving PhCH₂OD.

The hydride comes from the substrate, in an intermolecular hydride transfer, not from the solvent, so the benzylic carbon of the alcohol carries two H (PhCH₂-), and no deuterium is incorporated at carbon; the only D is on oxygen.

(b)(ii) E2 elimination in the two 2-bromo-1,3-dimethylcyclohexanes (5 marks)

E2 needs the leaving group and a β-hydrogen anti-periplanar, which in a cyclohexane chair means both axial (trans-diaxial). The only β-hydrogens of C2 are on C1 and C3, each of which also carries a methyl.

Z (all-cis, methyls and Br on the same face): in the chair where Br is axial, the cis neighbouring methyls are equatorial (cis-1,2 is axial/equatorial), which is also the more stable chair. The H on C1 and C3 are then axial and anti-periplanar to Br, so E2 occurs readily, giving 1,3-dimethylcyclohexene (Zaitsev, trisubstituted).

Y (Br trans to both methyls): Br is axial only in the chair in which both methyls are also axial (trans-1,2 is diaxial or diequatorial). Then the H atoms on C1 and C3 are equatorial, not anti to Br, and no trans-diaxial hydrogen is available (the other chair, all-equatorial, has Br equatorial). So Y cannot undergo E2 (and the all-axial chair is strained by the 1,3-diaxial Me/Me interaction). Only Z eliminates.

(b)(iii) Conjugate additions to M and N (5 marks)

The nucleophile can add to the carbonyl carbon (1,2-addition, fast and kinetic, but reversible for cyanide) or to the β-carbon (1,4-addition, conjugate or Michael addition, giving the more stable, thermodynamic product). Soft nucleophiles favour 1,4-addition.

M: but-3-en-2-one with NaCN/HCN gives 1,4-addition, 4-oxopentanenitrile, CH₃COCH₂CH₂CN. The 1,2-cyanohydrin forms reversibly, and the conjugate adduct accumulates because it is stable and the C-C bond is not easily reversed.

N: mesityl oxide with PhSH gives the conjugate (thia-Michael) adduct, 4-methyl-4-(phenylthio)pentan-2-one, CH₃COCH₂C(CH₃)₂SPh. The thiol (with traces of thiolate) is a soft nucleophile, so it attacks the β-carbon, even though the β-carbon carries two methyl groups.

(c)(i) Intermediate [A] (2 marks)

[A] is dichlorocarbene, :CCl₂. The base tert-butoxide removes a proton from CHCl₃ to give ⁻CCl₃, which loses Cl⁻ (α-elimination) to give :CCl₂.

(c)(ii) Dichlorocarbene with cis- and trans-2-butene (5 marks)

The addition is a concerted syn addition of the singlet carbene to the double bond, so the geometry of the alkene is kept.

Reaction 1: cis-2-butene gives cis-1,1-dichloro-2,3-dimethylcyclopropane, with both methyls on the same face (a meso compound).

Reaction 2: trans-2-butene gives trans-1,1-dichloro-2,3-dimethylcyclopropane, with the methyls on opposite faces (formed as a racemic pair of enantiomers, 2R,3R and 2S,3S).

(c)(iii) Stereospecific or stereoselective (3 marks)

Both additions are stereospecific. A stereospecific reaction is one in which stereoisomeric reactants give stereoisomerically different products, so the stereochemistry of the product is fixed by that of the reactant: here cis-2-butene gives only the cis-cyclopropane and trans-2-butene only the trans. (A stereoselective reaction, in contrast, is one in which one stereoisomer is formed in preference from a given reactant, and every stereospecific reaction is also stereoselective.)

(c)(iv) Phenol with NaOH, dichlorocarbene and H⁺ (5 marks)

This is the Reimer-Tiemann reaction. NaOH converts phenol to phenoxide, whose ring is strongly nucleophilic at the ortho and para positions, and the electrophilic :CCl₂ (from CHCl₃ and base) is attacked at the ortho position. The dichloromethyl intermediate (ArCHCl₂) is hydrolysed in base to the aldehyde, and acidification (H⁺) liberates the phenol. The major product is salicylaldehyde (2-hydroxybenzaldehyde); a little p-hydroxybenzaldehyde is the minor product.

(c)(v) Dichlorocarbene with the bicyclic diene (5 marks)

The hydrocarbon has two non-conjugated double bonds: a disubstituted one in the six-membered ring and a tetrasubstituted one at the ring fusion. Dichlorocarbene is an electrophile, so it attacks the more electron-rich, more highly alkylated double bond faster. The major product is therefore the dichlorocyclopropane at the tetrasubstituted ring-fusion double bond (cis-fused, by syn addition), while the disubstituted double bond survives. With only one equivalent of carbene, the reaction stops at this mono-adduct.