UPSC Civil Services (Mains), Chemistry optional · 2021 · Paper II

Q2(c)(i). 1-Phenylethanol with SOCl₂ and pyridine

10 marks
Topic: Substitution reactions
Answered

Question

Indicating the stereochemistry of the product, complete the reaction of (S)-1-phenylethanol with SOCl₂ in pyridine, with mechanism. [structure given in the paper]

Answer in brief

(R)-1-Chloro-1-phenylethane forms with inversion of configuration by SN2 attack of chloride on the alkyl chlorosulfite; SO₂ and pyridinium chloride are by-products.

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Topic notes: nucleophilic substitution (SN1, SN2, SNi, neighbouring group participation, solvent effects, rearrangement), with every Civil Services, IFoS and Geo-Scientist PYQ →

Topic notes: stereochemistry and conformational analysis (R/S, E/Z, Fischer and Newman projections, cyclohexane chair), with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversthionyl chloridepyridineSNi mechanismSN2 inversionalkyl chlorosulfite1-phenylethanol

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