Question

Provide a structure based explanation for the selectivity of the radical monobromination of 4-methylcyclopentane. The selectivity is unexpected as bromination does not occur preferentially at the tertiary carbon in this reaction. It may be useful to note that reacting 4-methylcyclopentane with a strong base results in deprotonation at the same position the bromine is attached.

Answer 1

Answer :

There is a small mistake in the question. The compound should be 4-methylcyclopentene instead of 4-methylcyclopentane. The structure of 4-methylcyclopentene is as follows

Now if we brominate the compound the product will be

instead of the following

This is called allylic bromination. The Br^. radical prefers allylic position to the double bond over classical tertiary position. This preference originates due to the better charge delocalisation in case of allylic position.

Deprotonation of allylic position in presence of base is also due to the same reason; stability of negative charge via delocalization.

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