Prediction of Taft's σ* parameter for alkyl groups and alkyl groups containing polar substituents

X-1, X-2, and X-3 are polar groups, hydrogens or alkyl groups bound to a carbon atom in a group (CXXX3)-X-1-X-2. The value of sigma*((CXXX3)-X-1-X-2) has been found to be equal to the sum of the contributions of all groups X-1, X-2, and X-3. The contribution of a polar group X to the value of sigma*((CXXX3)-X-1-X-2) is b x Sigma sigma*(X) and that of an alkyl group R is a x Sigma[sigma*(R) - sigma*(H)] where a = 0.202 and b = 0.326. The additivity of the contributions of the groups is good, giving sigma* values usually as good as those obtained by experiments.