Alkene Reactions: Bromohydrin Formation using Br2 and H2O, followed by Epoxide formation using NaOH

Bromine (Br2) or N-bromosuccinate (NBS) in the presence of alkenes and water, react together form bromohydrins. In the following step, the bromohydrin is reacted with a strong base, such as sodium hydroxide (NaOH) to form an epoxide:

This reaction is often written in one combined reagent however, it takes place in two steps. The first step is the bromohydrin formation using Br2 and H2O. After bromohydrin formation, if the stereochemistry of the OH and Br group are different (anti-configuration), then a reaction with NaOH can occur resulting in epoxide formation. The first example shows unspecified stereochemistry, the second example shows the required stereochemistry, and the third example shows a bromohydrin with syn configuration, which would not react.:

The reaction mechanism is depicted below:

In the first step, electrons from the alkene bond react with the first bromine to make a bond, and electrons from the first bromine atom react with the other carbon to form another bond, forming a ring structure as an intermediate. The bromine-bromine bond is broken with electrons from the bond going to the second bromine atom.

In the second step, water attacks the backside of the intermediate structure on the more substituted carbon, causing the bond between the more substituted carbon and bromine to break, transferring the electrons to bromine.

In the third step, another water molecule enters the reaction and the electrons from the oxygen atom in the water molecule react with one of the two hydrogens bonded to the oxygen on the alkane chain.

In the fourth step, the OH- group from NaOH attacks the proton on the OH group attached to the bromohydrin molecule, sending the free electrons to the oxygen atom.

In the fifth step, the negatively charged O atom attacks the carbon atom bound to to the bromine group, kicking the bromine group off and forming the epoxide

The reagents used for this reaction are Br2 in a solution of H2O followed by treatment with a strong base such as NaOH or KOH.