IN some of their applications, the current generalised theories of benzene substitution discard steric factors too lightly. For the purpose of a priori consideration, it can be assumed that the effective reagent in a substitution does not occupy a smaller volume than the group which is eventually installed in place of hydrogen.
"Atomic Radii", An. Rep. Chem. Soc., p. 402; 1931.
Bradfield and Jones, J. Chem. Soc., 1009; 1928.