Steric and Polar Effects—Taft Parameters

These parameters show that the rate of the hydrolysis reaction is faster with hydrogen than methyl, and slows as the R group incrementally increases in size to f-butyl (that is, Es becomes more negative). Phenyl, however, comes out surprisingly large in this analysis (Es = -2.55). In Chapter 2 we noted that efforts to characterize the steric size of a group will be strongly context dependent, and this is an example. In an alternative measure of the steric size of substituents—the cyclohexane A value (Table 2.14)—phenyl is much smaller than f-butyl. 

Randacdo, L., Geremia, S., Zangrando, E., and Ebert, C. Quantitative Rationalization of Solution and Solid State Properties in Cobaloximes, RCo(DH)2L, as a Function of the Electronic and Steric Properties of R. Inorg. Chem., 33,4641-4650 (1994). 

---