UPSC Civil Services (Mains), Chemistry optional · 2021 · Paper II

Q3(c)(ii). 3-Iodo-2,2-dimethylbutane with silver nitrate in hot ethanol

10 marks
Topic: Substitution reactions
Answered

Question

Complete the reaction of 3-iodo-2,2-dimethylbutane with AgNO₃ in ethanol on heating, along with mechanism. [structure given in the paper]

Answer in brief

Ag⁺ removes iodide; the secondary cation undergoes a 1,2-methyl shift to the tertiary cation, giving 2,3-dimethylbut-2-ene (major) with 2,3-dimethylbut-1-ene and 2-ethoxy-2,3-dimethylbutane.

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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 →

CoversWagner–Meerwein rearrangement1,2-methyl shiftsilver nitrateSN1/E1carbocation rearrangement

All parts of Question 3

3(a)(i) 1-(Hydroxydiphenylmethyl)cyclopentanol with conc. H₂SO₄10 marks
3(a)(ii) 1-Acetyl-1-chloro-4-methylcyclohexane with ethoxide10 marks
3(b) Cyclopropenyl cation; acidity of indene and cyclopentadiene; tropolones and sydnones15 marks
3(c)(i) meso-2,3-Dibromobutane with iodide and with ethoxide5 marks
3(c)(ii) 3-Iodo-2,2-dimethylbutane with silver nitrate in hot ethanol10 marks