If you are not sure what a haloalkane is then have a look on the nomenclature page.

Nucleophillic Substitution

Haloalkanes contain a polar bond on the functional group, because the halogen is electronegative so becomes δ^- and the carbon is positive. This means it is susceptible to nucleophillic attack by ions.

The stength of the C - Halogen bond also influences the rate of substitution. The C-F bond recquires a quite a bit (484 kJmol^-1) of energy to break, however, the C-Br bond is more easy to break so the rate of reaction is faster.

The mechanism below uses the notation :Nu^- to refer to the nucleophile which could either be OH^- or NC^-.

mechanism of nucleophillic substitution in Bromoalkanes

The mechanism below shows what happens with ammonia and a haloalkane.

nucleophillic substitution of haloalkane by ammonia

The bromine ion and NH₄ join to make NH₄Br, and the final product is an amine

Elimination

In the first mechanism the OH ion replaces the Br and forms an alcohol. However, the OH^- can also act as a base, where an elimination reaction occurs, as outlined below.

elimination of a haloalkane

So how do you decide whether substitution or elimination is going to happen. There are several factors that will determine what will happen. Have a look at the table below to find them out.

Haloalkane Structure	Primary haloalkanes prefer substitution Secondary halalkanes will do both at the same rate Tertiary haloalkanes prefer elimination.
Base Strength	Elimination is more likely as the strength of the base increases.
Temperature	Higher reaction temperatures make elimination happen more.

If you are not sure about the differences between primary, secondary and tertiary then look at the page alcohols as the same rules apply.