Tafts Steric Factor Es

An attempt has been made to quantify the steric features of various substituents Q.e., fimetional moieties) by the help of Taft s steric factor (Es). 

In fact, there are three predominant constants, namely  

Hammett Substitution Constant (o). It is a measure of either the electron-withdrawing or electron-donating capability of a substituent i.e., the functional moiety). Hammet substitution constant may be determined conveniently by actual measurement of the dissociation of a series of benzoic acid substituted derivatives vis-a-vis the dissociation of pure benzoic acid itself. 

However, benzoic acid being a weak-acid gets partially ionized in an aqueous medium (H2O) as depicted under  

An equilibrium is established between the two distinct species i.e., the ionized and non-ionized forms. Thus, the relative proportion of the said two species is usually termed as the dissociation or equilibrium constant and invariable designated by (wherein the subscript H represents/signifies that there is no substituents normally attached to the aromatic nucleus i.e., the phenyl ring). 

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Application of the multipaxaineter extrathexmodynamic technique to this series, where and R were alkyl groups, reveeded a good relationship between the A inhibitory pot cy (isorelative activity to DSG) and the substitu t partition constant (rr) and Taft steric factor (Es) in the position (equation 1, where the figures in parentheses are we Student t vedues for the coefficients of the equation and n, r, s, F and p have their usual statistical meaning). 

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