UPSC Civil Services (Mains), Chemistry optional · 2023 · Paper II

Q2(b)(iii). Ketene, hν, then cis-but-2-ene

10 marks
Topic: Reactive intermediates
Answered

Question

Write the major product of the above reaction showing proper stereochemistry and explain the mechanism. [ketene: (i) hν; (ii) cis-but-2-ene; scheme given in the paper]

Answer in brief

Product: cis-1,2-dimethylcyclopropane (CO is lost). Photolysis of ketene gives singlet methylene, which adds to the alkene in one concerted step with retention of the cis geometry.

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Topic notes: reactive intermediates (carbocations, carbanions, radicals, carbenes, nitrenes, benzyne), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coverssinglet carbenemethyleneketene photolysiscyclopropanationSkell rule

All parts of Question 2

2(a)(i) Aromatic, antiaromatic or non-aromatic by the Hückel rule5 marks
2(a)(ii) Primary and secondary kinetic isotope effects5 marks
2(a)(iii) Enolates of butan-2-one: NaOMe versus LDA5 marks
2(b)(i) Hydrolysis of bis(2-chloroethyl) sulfide and of 1,5-dichloropentane5 marks
2(b)(ii) Both glycosyl bromides give the β-glycoside5 marks
2(b)(iii) Ketene, hν, then cis-but-2-ene10 marks
2(c)(i) Acetolysis of 2-(cyclopent-3-enyl)ethyl tosylate10 marks
2(c)(ii) Indole: electrophilic substitution at C-3, not C-25 marks