Chlorobenzene, for example, can be boiled with sodium hydroxide for days without producing a detectable amount of phenol (or sodium phenoxide). Similarly, when vinyl chloride is heated with sodium hydroxide, no substitution occurs:
We can understand this lack of reactivity on the basis of several factors. The benzene ring of an aryl halide prevents back-side attack in an SN2 reaction:
Phenyl cations are very unstable; thus SN1 reactions do not occur. The carbon–halogen bonds of aryl (and vinylic) halides are shorter and stronger than those of alkyl, allylic, and benzylic halides. Stronger carbon–halogen bonds mean that bond breaking by either an SN1 or SN2 mechanism will require more energy.
Two effects make the carbon–halogen bonds of aryl and vinylic halides shorter and stronger: (1) The carbon of either type of halide is sp2 hybridized, and therefore the electrons of the carbon orbital are closer to the nucleus than those of an sp3-hybridized carbon. (2) Resonance of the type shown here strengthens the carbon–halogen bond by giving it double-bond character:
Having said all this, we shall find in the next two subsections that aryl halides can be remarkably reactive toward nucleophiles if they bear certain substituents or when we allow them to react under the proper conditions.