UPSC Civil Services (Mains), Chemistry optional · 2024 · Paper II
Q1(b). Intermediate in the amination of bromobenzene
Question
What is the intermediate formed during the following reaction? [bromobenzene with :NH₂⁻, Δ; scheme given in the paper] Explain any one experimental proof for the formation of the intermediate.
Answer in brief
The intermediate is benzyne (1,2-dehydrobenzene), formed by elimination and consumed by addition.
Key points
- Elimination: NH₂⁻ removes the ortho-H; Br⁻ leaves.
- Benzyne: extra π bond from two sp² orbitals in the ring plane; weak, strained.
- Addition: NH₂⁻ attacks either aryne carbon; NH₃ protonates the carbanion.
- Proof: ¹⁴C labelling (Roberts). The label is shared equally between C-1 and C-2 of aniline.
- Only a symmetrical C≡C intermediate makes C-1 and C-2 equivalent.
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.
Coversbenzyneelimination–additioncine substitution¹⁴C labellingRoberts experimentaryne
All parts of Question 1
- 1(a)(i) Aromatic, non-aromatic or antiaromatic5 marks
- 1(a)(ii) Diphenylcyclopropenone resists nucleophilic addition5 marks
- 1(b) Intermediate in the amination of bromobenzene10 marks
- 1(c) (S)-2-Butanol to (S)-2-butyl chloride10 marks
- 1(d) Name and reagent for five conversions10 marks
- 1(e) Electrocyclic steps of cyclononatriene10 marks