Bromination of an alkene by N-bromosuccinimide (NBS) in the presence of light or peroxide is a radical reaction and produces an allylic bromide. For the following bromination of 3-methylcyclopentene, select the allylic bromides from the set at the right that would be products of the reaction.
The Correct Answer and Explanation is :
In the reaction of 3-methylcyclopentene with N-bromosuccinimide (NBS) in the presence of light or peroxide, a radical mechanism takes place. The reaction proceeds via the formation of a bromine radical (Br•), which abstracts a hydrogen atom from the allylic positions of the alkene. This generates an allylic radical, which can then combine with the bromine radical to form the product.
To understand the products, let’s break down the structure of 3-methylcyclopentene. The double bond in 3-methylcyclopentene is between the 2 and 3 carbons of the cyclopentene ring. The 3-methyl group is attached to carbon 3, which is next to the double bond. The allylic positions that can undergo abstraction of a hydrogen atom (to form the allylic radical) are the carbons adjacent to the double bond: C1 (on the left of the double bond) and C4 (on the right of the double bond).
When the bromine radical abstracts a hydrogen atom from one of these positions, it generates an allylic radical. There are two possible positions for the formation of the allylic radical:
- C1 radical: If the hydrogen is abstracted from C1, the resulting allylic radical would be stabilized by resonance with the double bond.
- C4 radical: If the hydrogen is abstracted from C4, the resulting allylic radical would also be stabilized by resonance with the double bond.
Both C1 and C4 radicals are equally stabilized due to the conjugation with the double bond, so both are viable sites for the bromination to occur.
The final products are the two possible allylic bromides:
- Bromination at C1: The bromine atom attaches to C1, producing 1-bromo-3-methylcyclopentene.
- Bromination at C4: The bromine atom attaches to C4, producing 4-bromo-3-methylcyclopentene.
These are the two possible products of the reaction, as the reaction is a radical process that allows for the formation of bromine at either of the two allylic positions (C1 or C4).