UPSC Civil Services (Mains), Chemistry optional · 2023 · Paper II
Q1(a)(ii). Hydrogen exchange in cyclopropyl and cyclopropenyl ketones
Question
Rate of hydrogen exchange reaction in the above compound (A) is 6000 times faster than that of (B). Explain. [structures of the cyclopropyl ketone (A) and cyclopropenyl ketone (B) given in the paper]
Answer in brief
Exchange needs the α-carbanion; from (B) it is a 4π antiaromatic cyclopropenyl anion: 6000 times slower.
Key points
- Exchange rate = rate of carbanion formation; deprotonation is the slow step.
- (B): lone pair joins ring π bond: cyclic 4π, antiaromatic; (A) unaffected.
- Allylic C–H of (B) should exchange faster; observed slower: 4n destabilisation.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Covershydrogen–deuterium exchangecyclopropenyl anionantiaromaticitycarbanion stabilityenolate
All parts of Question 1
- 1(a)(i) Acidity of cyclopentadiene5 marks
- 1(a)(ii) Hydrogen exchange in cyclopropyl and cyclopropenyl ketones5 marks
- 1(b)(i) IUPAC name with stereodescriptors5 marks
- 1(b)(ii) Dimerisation ability of three triarylmethyl-type radicals5 marks
- 1(c) Methyl iodide with sodium azide: DMF against methanol10 marks
- 1(d) Photochemical electrocyclic closure of two cyclonona-1,3-dienes10 marks
- 1(e)(i) 2-Chloro-6-methylcyclohexanone with methoxide5 marks
- 1(e)(ii) Name reaction that gives nitrogen as a by-product5 marks