Halogenoalkanes

sN2 - 1

Halogenoalkanes are hydrocarbons which contain at least one halogen and only single carbon – carbon bonds. They are used for a large number of applications such as  flame retardants, fire extinguishants, refrigerants, propellants, solvents, and pharmaceuticals.

Unfortunately, one set of halogenoalkanes – Chlorofluorocarbons (CFC’s) – have had a detrimental effect on the ozone layer. Depleting it to the point at which there is almost no ozone left at the poles. This led to the Montreal protocol being agreed in 1989 which has led to the almost complete reduction in use of CFC’s. Because of this there have been signs of repair to the ozone layer, it will still take a very large number of years before it is fully repaired.

Forming Halogenoalkanes

Halogenoalkanes can be formed using a variety of methods.

  1. From Alkanes
    Alkanes can undergo radical substitution to form halogenoalkanes.
    CH4 + X2 –> CH3X + HX
    See the page on alkanes for more details
  2. From alkenes
    Addition of halogens to an alkene results in the halogen being added across the double bond.
    C2H4 + X2 –> C2H4X2
    You can find out more about this reaction by visiting the alkenes page

Reactions of the Halogenoalkanes.

All Halogenoalkanes undergo nucleophilic substitution.

CH3X + NaOH –> CH3OH + NaX

This has a three step reaction mechanism. For example:

Step one involves a hydroxide anion acting as a nucleophile ‘attacking’ the δ+ C

sN2 - 1

As the hydroxide ion moves towards the carbon the halogen leaves, this sets up a transition state.

sN2 - 2

As the halogen leaves completely the alcohol is formed.

sN2 - 3

 

This reaction is known as an sN2 reaction.

Reactivity of The Halogenoalkanes

As we move from C-F to C-I the bond becomes less polar. We would expect the C-F bond therefore to be the most reactive. However, the bond enthalpy of the C-X bond also decreases as you go from C-F to C-I. This means that the reactivity increases as you descend the group.

Because of this we sometimes describe the halogens as good leaving groups (Br & I being the best)