UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper III
Q6(b). Bromine addition: styrene reacts faster than propene
Question
The rate of addition of bromine to styrene (Ph–CH=CH₂) is greater than propene (CH₃–CH=CH₂). Provide a suitable explanation with mechanisms for both.
Answer in brief
Styrene is faster: its slow step gives a resonance-stabilised benzylic cation.
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Coversbromination of alkenesbromonium ionbenzylic carbocationelectrophilic additionstyrene
All parts of Question 6
- 6(a) Singlet and triplet carbenes; fluorenylidene with cis-but-2-ene4 marks
- 6(b) Bromine addition: styrene reacts faster than propene4 marks
- 6(c) Addition of HCl to an α,β-unsaturated acid4 marks
- 6(d) Michael addition of a cyclic 1,3-dione to cyclohex-2-enone4 marks
- 6(e) Hydroboration–oxidation of 1-methylcyclohexene4 marks
- 6(f) Cope rearrangement of a 1,5-diene4 marks
- 6(g) Diels–Alder reaction: product and reactants4 marks
- 6(h) IR distinction: hydrogen bonding and ring strain4 marks
- 6(i) McLafferty rearrangement: 2-pentanone and 3-pentanone4 marks
- 6(j) NMR assignment of 4-methylpent-1-yne4 marks