CHARGE DEMANDS OF ELECTRON-WITHDRAWING GROUPS - EVIDENCE FOR A SATURATION EFFECT

Estimates of charge demands qx(the fraction of ? charge withdrawn by the electronegative functionalities X in X2CH-) are obtained through a charge-?(13C) relationship and are shown to be additive for predicting the 13C shift of the carbanionic carbon atom in XYCH-: the facts that such values are less than those for PhCH-X and that the combined effect of the two phenyl groups in Ph2C-X is almost the same as that of one phenyl in PhCH-X in delocalizing the charge are attributed to saturation phenomena.