# What happens when H-X adds to the double bond of an alkene?

Jan 10, 2015

The end result will be that one of the two bonds opens up. $H$ adds to one side, and $X$ to the other. This is called mono-halogenation.

($X$ stands for $F$, $C l$, $B r$ or $I$)

This is an example of an addition reaction, because the whole $H X$ molecule is added tot the alkene. There are no other reaction products.

The mechanism is as follows:
$H - X$ is always very polar, the $X$-side drawing the electrons more to its side of the molecule. The extra electron pair in the double bond matches up with the (positive) $H$ and then the $X$ has 'no other choice' but matching up with the other side of the now open bond.

Example :
Ethene + hydrogen bromide

$\left({H}_{2} C = C {H}_{2}\right) + \left(H - B r\right) \to \left({H}_{3} C - C {H}_{2} B r\right)$

Extra :
Contrary to mono-halogenation of alkAnes, this reaction occurs under rather normal conditions.