The maximum number of possible isomers (including stereoisomers) which may be formed on mono-bromination of 1-methylcyclohex-1-ene using and UV light is ____.
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Answer and worked solution
Official answer: 9
Free-radical bromination with /hν. The official key accepts 9, 13 or 14, depending on which products are counted.
Direct substitution at each sp³ C-H (9 products):
- 3-, 4-, 5- and 6-bromo-1-methylcyclohexene: each has a new stereocentre, so ;
- 1-(bromomethyl)cyclohexene: achiral, 1.
Products from allylic radicals reacting at their other end:
- From the C3 radical: 3-bromo-3-methylcyclohexene, 2 enantiomers.
- From the C6 radical: 3-bromo-2-methylcyclohexene, 2 enantiomers.
These raise the count to 13. Also counting the exocyclic alkene from the radical (1-bromo-2-methylenecyclohexane) is what leads to the larger counts.