Hammett Substituent Constants

Benzoic acid ionizes into C6H5CO2− and H+ only partially in water. However, this ionization is rendered facile by electron-withdrawing substituents and more difficult by electron-rich substituents on the aromatic ring. The effect on ionization is also dependent on the position of the substituent on the ring in respect of the carboxylic acid group. While substituents on the meta and para positions exert only electronic effects on ionization, substituents on the ortho position cause steric effects as well. The Hammett constant σR for a given substituent R is given by the equation: σR = log (KR/KH), where KR and KH are the dissociation constants of the substituted and unsubstituted acids, respectively. Obviously, the constant \(\sigma\) for a given substituent is accurate only for the molecular structure from which it has been derived and also its position with respect to the ionizing group. An electron-withdrawing substituent will have a positive \(\sigma\) value and an electron-donating substituent a negative σ value. The σ constant for substituents on meta and para positions of the aromatic ring are designated as σm and σp, respectively. The constant σ is derived from the ionization of benzoic acids wherein the negative charge of the carboxylate ion is not in conjugation with the ring and/or the substituent through the ring. The Hammett equation, therefore, need not apply to instances in which the substituent comes into direct resonance interaction with the reaction site. When there is resonance between a reaction site that becomes electron-rich and an electron-withdrawing substituent, σ− constant is used. The standard reactions for the evaluation of σ− constants are the ionization of p-substituted phenols and p-substituted anilinium ions. Likewise, when there is resonance between a reaction site that becomes electron-deficient and an electron-donating substituent, σ+ constant is used. The standard reaction for the evaluation of σ+ constant is the solvolysis of p-substituted tert-cumyl chlorides. The application of σ constants is primarily in the study of reaction pathways.