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Alkene Reactions: Bromohydrin Formation using Br2 and H2O

Bromine (Br2) or N-bromosuccinate (NBS) in the presence of alkenes and water, react together form bromohydrins:

Alkene Reactions: Bromohydrin Formation using Br2 and H2O image1.png

Bromine reacts first with the alkene molecule, and ultimately resides on the least substituted carbon on the double bond (anti-Markovnikov). The water molecule (eventually OH) bonds with the most substituted carbon on the double bond (Markovnikov). The two functional groups are added in an anti addition where bromine and OH are added to the opposite side of each other, giving 2 products that are enantiomers:

Anti Addition

Alkene Reactions: Bromohydrin Formation using Br2 and H2O image2.png

Partial anti Addition

Alkene Reactions: Bromohydrin Formation using Br2 and H2O image1.png

No stereocenters

Alkene Reactions: Bromohydrin Formation using Br2 and H2O image3.png

The reaction mechanism is depicted below:

Alkene Reactions: Bromohydrin Formation using Br2 and H2O image4.png

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.

The reagents used for this reaction are Br2 or NBS, in a solution of H2O or ethanol (EtOH / CH3CH2OH)..