(a) In the reaction R—COO⁻ + Br—CN → R—CN + Br⁻ + CO₂↑ What is the origin of —CN group in the product ? Explain by using isotopic…

(a) Origin of the –CN group by isotopic labelling Carboxylate oxygen attacks the electrophilic cyano carbon of Br–C≡N and displaces bromide, giving an acyl cyanate, R–CO–O–C≡N. This rearranges (O→N acyl shift) to acyl isocyanate R–CO–N=C=O, which loses CO₂ to give R–C≡N. Two carbons are available for the nitrile carbon, the carboxyl carbon and the cyano carbon of BrCN, so a labelling experiment is designed. Use R–¹⁴COO⁻ with ordinary BrCN (or R–COO⁻ with Br–¹⁴CN), isolate the nitrile and the evolved CO₂ (as BaCO₃), and compare radioactivity. If the nitrile carries ¹⁴C and the CO₂ does not, the nitrile carbon came from the carboxylate and nitrogen from BrCN; if the CO₂ is active, the CN came intact from BrCN (direct substitution route). The mechanism above predicts the first outcome, to be confirmed experimentally.

(b) Benzyltrimethylammonium salt with NaNH₂ NaNH₂ forms ammonium ylides in equilibrium: the more stable benzylic ylide PhC⁻H–N⁺Me₃ and the less stable N-methyl ylide PhCH₂–N⁺Me₂–CH₂⁻. At low temperature (liquid NH₃, about −33 °C), the N-methyl ylide undergoes Sommelet–Hauser rearrangement ([2,3]-sigmatropic shift): its carbanion carbon bonds to the ortho ring carbon while the N–benzyl bond breaks, the ring is temporarily dearomatised, and a 1,3-proton shift restores aromaticity, forming o-methylbenzyl-dimethylamine, 2-CH₃–C₆H₄–CH₂–NMe₂. It is kinetically favoured and has a low barrier. At high temperature, [1,2]-Stevens rearrangement through a radical-pair mechanism predominates: a methyl group migrates from N to the carbanionic carbon, forming N,N-dimethyl-1-phenylethylamine, PhCH(CH₃)NMe₂. No β-hydrogen is available, so Hofmann elimination does not occur.

(c) Dehydration of 2-cyclohexylpropan-2-ol with BF₃·OEt₂ The Lewis acid coordinates the oxygen, which leaves as BF₃–OH⁻, giving the tertiary carbocation C₆H₁₁–C⁺(CH₃)₂ (E1 mechanism). Loss of a proton from a methyl group gives A, 2-cyclohexylpropene (disubstituted, exocyclic isopropenyl); loss of the ring C–H proton gives B, isopropylidenecyclohexane (tetrasubstituted alkene). B is the major product, as it is the more substituted and more stable alkene (Zaitsev rule), and proton loss from the ring is statistically and thermodynamically favoured. A hydride shift from the ring carbon to the cation, giving the other tertiary cation, can also lead to a small amount of 1-isopropylcyclohexene.

(d) Reactivity of benzaldehydes toward nucleophiles with BF₃·OEt₂ BF₃ coordinates the carbonyl oxygen, polarises C=O and makes the carbon more electrophilic, so both aldehydes react faster than uncomplexed. (i) p-CF₃C₆H₄CHO: CF₃ is a strongly electron-withdrawing group (–I) (σp = +0.54), which raises the positive charge on the carbonyl carbon, so nucleophilic attack is easiest. It is the more reactive electrophile. (ii) p-CH₃C₆H₄CHO: CH₃ donates electrons (+I and hyperconjugation, σp = −0.17), which reduces the carbonyl carbon’s electrophilicity and slows addition. Competing effect: the more basic oxygen of the tolualdehyde binds BF₃ more strongly, but this only partly offsets the intrinsic electrophilicity difference. Overall, nucleophile addition is faster for the p-CF₃ compound; the order is p-CF₃ > p-CH₃.

(e) Completing the transformations (i) The (2E,4Z,6E)-triene undergoes thermal 6π disrotatory electrocyclisation to cis-5,6-dimethylcyclohexa-1,3-diene, then thermal 4π conrotatory ring-opening gives the (2E,4Z,6Z)-triene (one terminal alkene geometry inverted), as drawn. (ii) The cis-fused bicyclic diene opens by a thermal conrotatory 4π ring-opening of the central σ-bond to a nine-membered (E,Z,Z)-triene, which closes by disrotatory 6π closure to the trans-fused diene (H’s on opposite faces). Woodward–Hoffmann rules decide the stereochemistry.