a lowering of the rate of a chemical reaction caused by the blocking of the reactive site of a molecule by adjacent atoms or groups of atoms. For example, the esterification of di-ortho-substituted benzoic acids (I,a) is quite difficult, as is the hydrolysis of their esters (I,b):

For di-ortho-substituted phenylacetic acids (II), where the COOH group is somewhat removed from the blocking substituents (X and Y), both esterification and hydrolysis of their esters proceed without hindrance.