Why don primary halogenoalkanes use the SN1 mechanism?
If a primary halogenoalkane did use this mechanism, the first step would be, for example: A primary carbocation would be formed, and this is much more energetically unstable than the tertiary one formed from tertiary halogenoalkanes – and therefore much more difficult to produce. This instability means that there will be a very high activation energy for the reaction involving a primary halogenoalkane. The activation energy is much less if it undergoes an SN2 reaction – and so that’s what it does instead.
