The preorganization step in organic reaction mechanisms. Charge-transfer complexes as precursors to electrophilic aromatic substitutions.

Metastable (pre-reactive) intermediates, as commonplace transients in simple bimolecular reactions, are usually unobserved (and ignored)-though they provide vital mechanistic insight. Thus, the preequilibrium (charge-transfer) complexes of various aromatic donors with rather typical electron acceptors such as Br(2), NO(+), and NO(2)(+) are examined quantitatively (via their molecular and electronic structures) to reveal surprisingly unorthodox aspects of what is conventionally referred to in organic chemistry textbooks as electrophilic aromatic bromination, nitrosation, and nitration, respectively.