Alpha effect

The α - effect describes the enhanced nucleophilicity of a molecule by the presence of a nearby lone pair. The molecule does not necessarily indicate an increased basicity when compared to a comparable molecule without the related electron donor. The α - effect can be explained by various theoretical explanations, which to date it is not clear which approach best explains the effect.

The effect was first observed by Jencks Carriuolo and in a series of experiments on the kinetics of the reaction of the p- nitrophenyl ester with a variety of nucleophiles. Nucleophilic, such as the fluoride anion, pyridine or ethylenediamine responded, as expected according to their indicated by the pKa basicity. Other ions, such as hydroxylamine, hypochlorite anion or the hydroperoxide anion, but reacted significantly faster than expected.

1962 designated Edwards and Pearson this effect " α " effect. They suggested that this effect could be caused by a stabilization of the transition state (TS ): On entering the TS is the free electron pair moves from the nucleus away, thus creating a partial positive charge, which can be stabilized by a related pair of free electrons, as it occurs for example in carbocations.

The α - effect is also unpredictable depending on the solvent: it can be stronger or weaker, or even pass through maxima.