AbstractInstances of high reactivity (as signaled by a positive Brönsted deviation) by nucleophiles bearing one or more unshared pairs of electrons on an atom adjacent to the nucleophilic center (the alpha effect) are surveyed in the context of possible explanations for this phenomenon. No single cause appears to account satisfactorily for all the data. However, four factors (ground‐state destabilization of the nucleophile, transition‐state stabilization, solvent effect differences for alpha and nonalpha nucleophiles, and product stability) may be involved in contributory roles. The response to proton basicity of a substrate is probably not related to its susceptibility to the alpha effect. Carbon electrophiles seem to be receptive to the alpha effect in the order digonal>trigonal>tetrahedral. The inconsistent behavior of alpha nucleophiles makes the prediction of alpha effects rather risky and confirms the complicated nature of nucleophilic substitut