The intermediate nium ion, also referred to as the Wheland intermediate, benefits from resonance stabilization even through it has lost the very imporant stabilization associated with the six electrons of the aromatic system. The second elimination step regenerates the aromatic ring with all of its associated stabilization and is therefore a very rapid and usually irreversible process. Thus, such substitutions are under kinetic control with the initial electrophilic addition to form the arenium ion being the rate determining step. An exception to this general imeversibility of aromatic substitutions is sulfonation, but generally substitutions present on the aromatic nucleus can exert a strong controlling influence on both the reactivity of the substrate and the orientation of substitution.