Why halogenation in a polar protic solvent leads to the formation of halohydrin

As a student of organic chemistry, you will study many reaction mechanisms. You will also be faced with an endless list of chemical reagents and solvents. Understanding the nature of each solvent will help you understand the steps of the mechanism and ultimately the product of the reaction. In this article, I’ll help you understand why trying the halogenation reaction in a polar protic solvent like water results in a halohydrin formation.

First, a brief overview of halohydrin. Halo = halogen and hydrine = OH

Halohydrin is a molecule that contains both a halogen and a hydroxyl group, located on neighboring carbon atoms. This is the result of an electrophilic alkene addition reaction.

The halogenation reaction, on the other hand, results in a vicinal dihalide, with 2 halogen atoms attached to the carbon atoms of the pi bond above.

The halogenation reaction begins when nucleophilic pi electrons reach and attack a neutral dihalide such as bromine or chlorine.

Halogens don’t like to attack, and the halogen will retaliate by using one of its lone electron pairs to attack the carbon atom. The second halogen breaks down to form a negative halide in solution. The attacked halogen is now attached to the two sp2 carbon atoms above. This heterocyclic structure is called cyclic bromonium when Br is involved, and cyclic chloronium when Cl is involved.

This is the point where the solvent makes a difference. When carried out in an inert solvent like CH2Cl2 or CCl4, the solvent “ignores” the reaction and lets the halogen proceed to the next step. The negative halide in solution will approach the cyclic halogen and attack one of the partially positive carbon atoms. This breaks the bridge and results in a halogen attached to each of the pi-attached carbon atoms.

However, when carried out in a polar protic solvent, we no longer have a “passive environment”. Water is polar due to the uneven distribution of charge between the pi bond of hydrogen and oxygen. This leaves the oxygen atom partially negative and highly nucleophilic. This also leaves the H atom partially positive.

Partially positive hydrogen atoms will be strongly attracted to the negative halide in solution. They will surround the essentially caged halogen. This prevents the halogen from attacking the cyclic halide in the alkene reaction intermediate.

With the halogen out of the way, water as the weakest nucleophile takes advantage and attacks the molecule.

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