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Alkene Dihalide Addition AKA Halogenation

This post introduces the addition of two halides (e.g. bromine, chlorine, iodine) to an alkene, a reaction commonly known as Dihalide Addition.

The Dihalide Addition Mechanism

the alkene attacks one halogen, which both breaks off the second halogen and attacks the alkene
Concerted three-arrow mechanism between an alkene and a dihalide

It can be a bit tricky at first, but the halogenation mechanism is pretty easy when you get the hang of it. In the first step, three things occur simultaneously. I like to think of them in this order, though:

  • First, the alkene attacks one of the halogens

  • Second, the bond between the halogens breaks

  • Third, the halogen we attacked earlier turns around and attacks the alkene.

The halonium-ion intermediate AKA the bridged-ion intermediate
The halonium-ion intermediate AKA the bridged-ion intermediate

That creates this weird thing here: the halonium-ion AKA bridged-ion intermediate, which consists of a halogen with two bonds, one to each carbon, and a positive charge distributed across the three atoms in the ring. Some professors show at least one partial bond between the carbons and halogen.

This intermediate means there are no rearrangements.

halide breaks halonium ion, resulting in opposite wedge and dash configuration
The halide does a backside attack and breaks open the three-member ring

In the second step, the halide that was kicked off earlier now does a backside attack on the more-substituted carbon, inverting stereochemistry at the attachment site and breaking one of the halonium ion's bonds to carbon. Make a bond, break a bond.

Dihalide addition products are a 50% mixture of dihalides on neighboring carbons and on opposite wedge and dash
The products of alkene dihalide addition is a racemic mixture of anti, vicinal dihalides

The final product is a racemic mixture of anti, vicinal dihalides.There are now two halogens attached to adjacent (i.e. vicinal) carbons. They are on opposite wedge and dash because that’s what happens when you break a three-member ring via nucleophilic attack.

The Complete Alkene Dihalide Addition mechanism
The entire alkene halogenation mechanism, with the formation of both enantiomers explicitly drawn

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