Evaluation of the steric substituent effect by Omega(s): reinvestigation of the reaction dependency of the steric substituent constant

Using the OMEGAS program, two sets [Omega (s)(0.5,0.5) and Omega (s)(0.9,0. 5)] of steric substituents were calculated by assuming different sizes of e ffective radii for the reaction center atom. The former set reproduces the steric substituent effect on the nucleophilic reactions of carboxylic acid derivatives better; the latter set is far better for the reactions of amine s and alcohol derivatives. Our results clearly show that the effective size of the reaction center atom is another important factor in characterizing the steric effect in addition to the bulkiness of the substituent. This can be rationalized by the fact that such a general steric constant cannot be used to evaluate all sorts of steric effects using a single set of substitu ent constants. Copyright (C) 2001 John Wiley & Sons, Ltd.