UPSC Civil Services (Mains), Chemistry optional · 2023 · Paper II
Q2(c)(ii). Indole: electrophilic substitution at C-3, not C-2
Question
Write the preferred position (C-2 or C-3) in electrophilic substitution of indole. Explain your observation with the help of resonance structures.
Answer in brief
C-3: its σ-complex is stabilised by nitrogen with the benzene sextet intact; C-2 attack breaks it.
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.
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.
Coversindoleelectrophilic aromatic substitutionC-3 substitutionσ-complexresonance structures
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