Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Addition Reactions Involving Alkenes and Alkynes
8.5 Addition of Halogens and Water to Alkenes (Halohydrin Formation)

An alkene in the presence of a bromine and water will result in the addition of Br-OH across the double bond, and it is observed from experiments that this type of reaction occurs in a stereospecific manner, as shown in Reaction (8-46) involving an alkene in the presence of bromine and water to give the trans product, which is known as a bromohydrin.

(8-46)

In the first step of the mechanism for Reaction (8-46), the nucleophilic alkene attacks the bromine molecule as shown in Reaction (8-47) to form the bromonium ion and a bromide anion.

(8-47)

In the next step of the mechanism, the more nucleophilic water attacks the bromonium electrophilic ion instead of the bromide anion as shown in Reaction (8-48).

(8-48)

Note that the water attached the more tertiary-like carbon of the bromonium ion. The final step of the mechanism involves the loss of a proton, which is abstracted by the bromide anion as shown in Reaction (8-49).

(8-49)

For the reactions that lead to the formation of bromohydrins, rearrangement is typically not observed owing to the stability of the bromonium ion. These reactions are stereospecific resulting in trans-additions since attack of the bridged intermediate by water takes from the opposite side of the bromonium ion as shown in Reaction (8-50).

(8-50)

These reactions are also regiospecific since the water will bond to the carbon of the bromonium ion that is more carbocationic-like. That is, if carbocations were to be formed, the order of stability of carbocations will dictate and water will prefer to bond to a tertiary type more stable carbon, compared to a secondary type carbon. This concept is illustrated in Reaction (8-51), and the bromohydrin shown will not form.

(8-51)

Problem 8.8

Give the major organic product that results from the addition of bromine and water in the presence of dimethylsulfoxide (DMSO) as solvent to the alkenes shown below. Show stereochemistry where appropriate.