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

Q2(a)(iii). Enolates of butan-2-one: NaOMe versus LDA

5 marks
Topic: Reaction mechanisms
Answered

Question

Identify (A) and (B) in the above reactions and explain the mechanism. [butan-2-one treated with NaOMe/MeOH to give (A) and with LDA/THF, −78 °C to give (B); scheme given in the paper]

Answer in brief

(A) is the thermodynamic enolate, sodium but-2-en-2-olate (C-3 proton removed). (B) is the kinetic enolate, lithium but-1-en-2-olate (C-1 proton removed).

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Topic notes: methods of studying mechanisms (isotopic labelling, crossover, trapping, kinetic isotope effect, kinetic and thermodynamic control), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coverskinetic enolatethermodynamic enolateLDAsodium methoxidebutan-2-one

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