UPSC Civil Services (Mains), Chemistry optional · 2024 · Paper II
Q1(c). (S)-2-Butanol to (S)-2-butyl chloride
Question
How is the following conversion brought about? (S)-2-butanol → (S)-2-butyl chloride. Explain its mechanism.
Answer in brief
Reagent: SOCl₂ alone (ether, no pyridine). Mechanism: SNi, with retention of configuration.
Key points
- Step 1: ROH attacks S; HCl lost; alkyl chlorosulfite forms. C–O bond untouched.
- Step 2 (RDS): C–O ionises to an intimate ion pair, R⁺ ⁻OS(O)Cl.
- Step 3: SO₂ leaves; Cl⁻ collapses on the same face before the ions separate.
- Result: retention; (S)-alcohol gives (S)-chloride. By-products SO₂ and HCl are gases.
- Contrast: pyridine releases Cl⁻, which attacks from the back (SN2): (R)-chloride, inversion.
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.
CoversSNi mechanismthionyl chlorideretention of configurationchlorosulfiteintimate ion pair
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