Taft Equation - Steric Substituent Constants, E_s

Steric Substituent Constants, E_s

Although the acid catalyzed and base catalyzed hydrolysis of esters gives transition states for the rate determining steps that have differing charge densities, their structures differ only by two hydrogen atoms. Taft thus assumed that steric effects would influence both reaction mechanisms equally. Due to this, the steric substituent constant E_s was determined from solely the acid catalyzed reaction, as this would not include polar effects. E_s was defined as:

where k_s is the rate of the studied reaction and k_CH3 is the rate of the reference reaction (R = methyl). δ is a reaction constant that describes the susceptibility of a reaction series to steric effects. For the definition reaction series δ was set to 1 and E_s for the reference reaction was set to zero. This equation is combined with the equation for σ* to give the full Taft equation.

From comparing the E_s values for methyl, ethyl, isopropyl, and tert-butyl, it is seen that the value increases with increasing steric bulk. However, because context will have an effect on steric interactions some E_s values can be larger or smaller than expected. For example, the value for phenyl is much larger than that for tert-butyl. When comparing these groups using another measure of steric bulk, axial strain values, the tert-butyl group is larger.