Halogen Addition Reaction - Reaction Mechanism

Reaction Mechanism

The reaction mechanism for an alkene bromination can be described as follows. In the first step of the reaction, a bromine molecule approaches the electron-rich alkene carbon–carbon double bond. The bromine atom closer to the bond takes on a partial positive charge as its electrons are repelled by the electrons of the double bond.

The atom is electrophilic at this time and is attacked by the pi electrons of the alkene . It forms for an instant a single sigma bond to both of the carbon atoms involved. The bonding of bromine is special in this intermediate, due to its relatively large size compared to carbon, the bromide ion is capable to interacting with both carbons which once shared the π-bond, making a three-membered ring. The bromide ion acquires a positive formal charge. At this moment the halogen ion is called a "bromonium ion" or "chloronium ion", respectively.

When the first bromine atom attacks the carbon–carbon pi-bond, it leaves behind one of its electrons with the other bromine that it was bonded to in Br₂. That other atom is now a negative bromide anion and is attracted to the slight positive charge on the carbon atoms. It is blocked from nucleophilic attack on one side of the carbon chain by the first bromine atom and can only attack on the other side. As it attacks and forms a bond with one of the carbons, the bond between the first bromine atom and the other carbon atoms breaks, leaving each carbon atom with a halogen substituent.

In this way the two halogens add in an anti addition fashion, and when the alkene is part of a cycle the dibromide adopts the trans configuration. For maximum overlap of the C-Br σ* antibonding molecular orbital (the LUMO, shown to the right in red) and the nucleophile (X−) lone pair (the HOMO, shown to the right below in green), X− must attack the bromonium ion from behind, at carbon.

This reaction mechanism was proposed by Roberts and Kimball in 1937. With it they explained the observed stereospecific trans-additions in brominations of maleic acid and fumaric acid. Maleic acid with a cis-double bond forms the dibromide as a mixture of enantiomers:

while the trans-isomer fumaric acid forms a single meso compound:

The reaction is even stereospecific in alkenes with two bulky tert-butyl groups in a cis-position as in the compound cis-di-tert-butylethylene. Despite the steric repulsion present in the chloronium ion, the only product formed is the anti-adduct.