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

Q2(b)(i). Hydrolysis of bis(2-chloroethyl) sulfide and of 1,5-dichloropentane

5 marks
Topic: Substitution reactions
Answered

Question

The rate of hydrolysis (k₁ and k₂) of the reaction (A) is much faster than that of (B). Explain. [(A) bis(2-chloroethyl) sulfide and (B) 1,5-dichloropentane with H₂O; structures given in the paper]

Answer in brief

In (A) sulfur is a neighbouring group: it displaces Cl⁻ internally to give a cyclic episulfonium ion, which water opens at once. (B) has only the slow direct SN2 by water.

Part of VKC Basic

Read the key points and the full model answer with VKC Basic

VKC Basic (₹999 for 6 months) opens every solved answer, the notes, the handbook and the daily answer archive. Every Mains test series includes it.

See what is included Already a member? Sign in

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.

The handwritten model answer pages open with the VKC plan.

Read the whole 2023 solution 2023 question paper

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

Coversneighbouring group participationanchimeric assistanceepisulfonium ionmustard gashydrolysis

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