Q2(b). Cyclopropylmethyl cation; acidity of two 1,3-diones; a diazo ester heated in cyclohexane
(i) Comment upon the structure and stability of the cyclopropylmethyl carbocation. (ii) Which one of the above compounds (cyclohexane-1,3-dione and a bicyclic 1,3-dione with a bridgehead C–H between the two C=O) is more acidic and why? (iii) Write the product(s) of heating tert-butyl diazoacetate in cyclohexane. [structures given in the paper]
Answer in brief
(i) A bisected, delocalised cation more stable than benzyl; (ii) cyclohexane-1,3-dione (pKa ≈ 5.3) is far more acidic, the bridgehead enolate being forbidden by Bredt's rule; (iii) tert-butyl cyclohexylacetate + N₂ by carbene C–H insertion.
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Coverscyclopropylmethyl carbocationnon-classical carbocationBredt's rule1,3-dione aciditycarbene C–H insertiondiazoacetate
All parts of Question 2
- 2(a) Elimination from trans- and cis-1-chloro-2-methylcyclohexane20 marks
- 2(b) Cyclopropylmethyl cation; acidity of two 1,3-diones; a diazo ester heated in cyclohexane15 marks
- 2(c)(i) 1-Phenylethanol with SOCl₂ and pyridine10 marks
- 2(c)(ii) Cyclohexanone enolate with methyl iodide and with trimethylsilyl chloride5 marks