Elimination and addition reactions. Part 31. Polar effects on the ionisation of sulphones, nitriles, and ketones

Abstract

Rates of ionisation in ethanolic sodium ethoxide of a series of sulphones, PhSO₂·CH₂·CH₂·Z, have been measured by radioisotopic and n.m.r. techniques. The derived Taft plot is rectilinear over a range of reactivity values of 10^9.5 and shows that the ionisation of sulphones is very senstive to polar effects (ρ*= 4.89). Taft plots for the ionisation of nitriles and of ketones have also been constructed, and comparison with earlier work on nitro compounds, fluorenes, and esters shows that the sensitivity of the ionisation rates of carbon acids (G–CH) is in the order G = PhSO₂ > CN > PhCO > fluorenyl > NO₂ CO₂Et. This order is discussed in terms of the degree of delocalisation of negative charge in the derived carbanion. Deviations from the Taft plots for Sulphones, nitrites, and ketones are seen when the polar group Z is PhSO₂ or CN. In the sulphone series, deviations from the Taft plot are seen when phenyl groups are inserted adjacent to the group Z. The nature of these deviations is discussed briefly and comparisons are drawn with other results.