What happens when Br2 reacts with cyclohexene in the absence...

What happens when $\mathrm{Br}_{2}$ reacts with cyclohexene in the absence of a radical initiator? Include in your answer discussion of the mechanism, evidence for proposed intermediates, and an explanation of the stereochemistry of the product.

Key Concepts

Mechanistic Evidence

Experimental observations such as the stereochemistry of the final product and kinetic studies provide indirect evidence for the formation of the bromonium ion. These studies demonstrate that the intermediate cannot rotate freely, leading to a predictable anti-addition pattern which is consistent with a stepwise mechanism involving a cyclic bromonium ion rather than a planar carbocation or free radical pathway.

Anti-Addition Stereochemistry

The cyclic bromonium ion intermediate forces the subsequent nucleophilic attack to occur from the opposite side of the leaving group, resulting in anti-addition. This means that the two substituents are added on opposite faces of the former double bond, leading to a stereospecific product configuration. The observed stereochemistry provides strong evidence supporting the cyclic intermediate mechanism.

Electrophilic Addition Reaction

This reaction type involves an electrophile attacking a nucleophilic double bond to form a new bond, typically leading to the addition of substituents across the double bond. In the absence of radical initiators, the reaction proceeds via polar mechanisms rather than radical intermediates, with the ? electrons of the alkene serving as the nucleophile that attacks the electrophilic species.

Bromonium Ion Intermediate

In the reaction with a diatomic electrophile, the initial step involves the formation of a cyclic bromonium ion intermediate. This intermediate is characterized by a three-membered ring structure where the positively charged bromine is bonded to both carbons of the original double bond, temporarily bridging them. The formation of this intermediate explains the regioselectivity and stereospecificity observed in the reaction.

Transcript

Please give Ace some feedback