Tafts polar a and steric s parameters

Taft [53] utilised as a model of steric and polar inductive effects the hydrolysis of 

The standard substituent is by convention taken as methyl whereas for a, it is hydrogen. Ingold [54] indicated that steric effects in acid and base hydrolysis of a given ester should be identical this is supported by modern ideas on ester hydrolysis where both mechanisms involve similar transition states in their rate-limiting steps (Eqns. 66 and 67). 

The transition states, which will resemble A and B in Eqns. 66 and 67 are solvated and thus quite close in steric requirements. Eqns. 68 and 69 may be defined and lead to Eqn. 70 because OH the steric requirements, are assumed identical. 

In order to define a on essentially the same scale as a we define (p5h Ph) as 2.48 the difference between Pqh and Ph for the hydrolysis of ethyl benzoates. 

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