Question

when isopropylbenzene (cumene) is treated with nbs and irradiated with uv light, only one product is obtained. propose a mechanism and explain why only one product is formed.

Answer 1

In the reaction of isopropylbenzene with NBS under UV light, a radical bromination occurs, producing bromoisopropylbenzene as the sole product due to the formation of the most stable secondary free radical intermediate.

Step-by-step explanation:

Mechanism for the Reaction of Isopropylbenzene with NBS

When isopropylbenzene (cumene) is treated with NBS (N-Bromosuccinimide) and irradiated with UV light, a radical bromination reaction occurs. The reaction is selective due to the formation of the most stable free radical intermediate. Here is a step-by-step explanation:

UV light initiates the reaction by homolytically cleaving the N-Br bond in NBS, forming a bromine radical.

The bromine radical abstracts a hydrogen atom from the isopropyl group of cumene, leading to the formation of the most stable (secondary) free radical adjacent to the phenyl group. This is because secondary radicals are more stable than primary radicals.

The cumene radical reacts with a new bromine molecule to form the final product, which is bromoisopropylbenzene.

The bromine atom from this reaction can then go on to react with another NBS molecule, perpetuating the chain reaction.

Only one product is formed because the radical intermediate forms at the position which leads to the greatest stability, which in this case is the secondary carbon of the isopropyl group. This carbon is more substituted and adjacent to the aromatic ring, which provides additional stability through resonance.

Answer 2

When isopropylbenzene (cumene) is treated with NBS and irradiated with UV light, only one product is obtained due to the radical halogenation mechanism. The reaction proceeds through the formation of a bromine-substituted intermediate and ultimately gives isopropylbromobenzene. The selectivity of the reaction ensures that the bromine atom adds to the most substituted carbon atom in the benzene ring.

Step-by-step explanation:

When isopropylbenzene (cumene) is treated with NBS (N-bromosuccinimide) and irradiated with UV light, only one product is obtained. This is because the reaction follows a radical halogenation mechanism.

First, NBS generates a bromine radical in the presence of UV light. This bromine radical then reacts with isopropylbenzene to form a bromine-substituted intermediate. Finally, this intermediate undergoes a rearrangement and loses a hydrogen atom to give the final product, which is isopropylbromobenzene.

The reason only one product is obtained is because isopropylbenzene has only one position where bromine can be added. Additionally, the radical halogenation reaction is highly selective, meaning it prefers to add a bromine atom to the most substituted carbon atom in the benzene ring.