Nucleophilic Aromatic Substitution
82.2 Rate of Reaction
The better a leaving group, the faster a nucleophilic reaction will occur. This is demon-
strated by comparisons of the kinetics between halogenalkanes, where the bromides disso-
ciate more quickly than the chlorides, but the iodides dissociate more rapidly than either of
the other two. This is because the bond between the halogen and its nearest carbon must be
broken at some point for a nucleophilic substitution to take place. A bond between iodine
and carbon is far more polarizable than a bond between carbon and chlorine, for example,
due to iodine’s relatively large size and relatively large number of ionizable electrons. The
fact that water is a far better leaving group than hydroxide also has the important conse-
quence that the rate of a reaction in which hydroxide leaves is increased dramatically by the
presence of an acid, for hydroxide is then protonated to water, a much weaker nucleophile.
82.3 Types of Reactions.
There are three nucleophilic substitution mechanisms commonly encountered with aromatic
systems, the SNAr (addition-elimination) mechanism, the benzyne mechanism and the free
radical SRN1 mechanism. The most important of these is the SNAr mechanism, where
electron withdrawing groups activate the ring towards nucleophilic attack, for example if
there are nitro functional groups positioned ortho or para to the halide leaving group. It is
not generally necessary to discuss these types in detail within the context of an introductory
organic chemistry course.