Why Halogenation in a Polar Protic Solvent Leads to a Halohydrin Forma…

As an organic chemistry student you will study many reaction mechanisms. You will also be faced with an endless list of chemical reagents and solvents. Understanding the character of each solvent will help you understand the mechanism steps and ultimately the product of the reaction. In this article I will help you understand why attempting the Halogenation Reaction in a polar protic solvent like water results in a Halohydrin Formation.

First a quick background on the Halohydrin. Halo = halogen and Hydrin = OH

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

The halogenation reaction however results in a vicinal dihalide, with 2 halogen atoms attached to the former pi bond carbon atoms.

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

Halogens do not take kindly to attack, and the halogen will retaliate using one of its lone electron pairs to attack the carbon atom. The second halogen breaks off to form a negative halide in solution. The attacked halogen is now bound to both of the former sp2 carbon atoms. This heterocyclic structure is called a cyclic bromonium when Br is involved, and a cyclic chloronium when a Cl is involved.

This is the point in which the solvent makes a difference. When carried out in an inert solvent such as CH2Cl2 or CCl4, the solvent ‘ignores’ the reaction and lets the halogen proceed with the next step. The negative halide in solution will approach the cyclic halogen and attack one of the slightly positive carbon atoms. This breaks the bridge resulting in a halogen attached to each of the former pi bound carbon atoms.

However, when carried out in a polar protic solvent, we no longer have a ‘passive ecosystem’. Water is polar due to the rough dispensing of charge between the hydrogen and oxygen pi bond. This leaves the oxygen atom slightly negative and highly nucleophilic. This also leaves the H atom slightly positive.

slightly positive hydrogen atoms will be strongly attracted to the negative halide in solution. They will surround the halogen essentially caging it. This blocks the halogen from attacking the cyclic halide on the alkene reaction intermediate.

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

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