engleski

A Practical Guide for Estimating Rates of Heterolysis Reactions

Chemists are well-trained to recognize the factors which control relative reactivities within series of compounds. Thus, it is well-known, how rates of ionization of R–X are affected by the stabilization of the carbocations, the nature of the leaving groups X–, or by the solvent ionizing power. On the other hand, when asked to estimate the half-life of the ionization of a certain substrate in a certain solvent, most chemists resign. It is this question, however, which is relevant in daily laboratory practice. Can a certain substrate R–X be handled in alcoholic or aqueous solution without being solvolyzed? Can a biologically active tertiary amine or azole be released by ionization of a quaternary ammonium ion? A semiquantitative answer to these questions is given by the correlation equation log k25°C = sf(Nf + Ef), in which carbocations R+ are characterized by the electrofugality parameter Ef, and leaving groups X– in a certain solvent are characterized by the nucleofugality parameter Nf and the nucleofuge-specific sensitivity parameter sf. As sf is typically around 1 (0.8<sf< 1.2), ionization half-lives of around 1 hour at 25°C can be expected when Ef + Nf = –4.

nucleophilic substitution; electrofugality; nucleofugality; solvent effect; leaving group; benzhydryl; kinetics

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