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

Q2(b)(ii). Both glycosyl bromides give the β-glycoside

5 marks
Topic: Substitution reactions
Answered

Question

The reaction of the compounds (A) and (B) with AgClO₄ in MeOH gives the same product (C). Explain. [structures of the glycosyl bromides (A) and (B) and product (C) given in the paper]

Answer in brief

Ag⁺ removes Br⁻ from either anomer to give the same oxocarbenium ion. The C-2 acetoxy group then bridges C-1 on the α-face, so methanol enters only from the β-face: both give (C).

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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 →

Coversneighbouring group participationglycosylationoxocarbenium ionacyloxonium ionβ-glycosideAgClO₄

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