Chemical Reactions

Once a haloalkane or haloarene has been made and characterised, the question that matters most for organic chemistry is what it can be converted into. The C—X bond is the reactive centre of the molecule, and its behaviour falls into three broad categories that organise everything covered in this section.

Because the halogen is more electronegative than carbon, the carbon–halogen bond is polarised, leaving the carbon end short of electron density. This partial positive carbon becomes the target for any electron-rich species that comes along, so haloalkanes are broken down into these categories of reaction:

1. Nucleophilic substitution — an electron-rich nucleophile is attracted to the electron-poor carbon, forms a new bond to it, and the halogen departs as a halide ion, $\mathrm{X^-}$. The C—X bond breaks heterolytically, with both electrons of the bond going to the more electronegative halogen.
2. Elimination reactions — instead of a nucleophile attaching to carbon, a base removes a hydrogen from the carbon next door while the halogen leaves from its own carbon, and a new carbon–carbon double bond is formed in the process (dehydrohalogenation). …