Substituent constants Hammett equation

having obtained value k0, series of equilibrium constants (k) determined based on same process, variation of para substituent—for instance, p-hydroxybenzoic acid (r=oh, r =h) or p-aminobenzoic acid (r=nh2, r =h). these values, combined in hammett equation k0 , remembering ρ = 1, give para substituent constants compiled in table 1 amine, methoxy, ethoxy, dimethylamino, methyl, fluorine, bromine, chlorine, iodine, nitro , cyano substituents. repeating process meta-substituents afford meta substituent constants. treatment not include ortho-substituents, introduce steric effects.

the σ values displayed in table above reveal substituent effects. ρ = 1, group of substituents increasing positive values—notably cyano , nitro—cause equilibrium constant increase compared hydrogen reference, meaning acidity of carboxylic acid (depicted on left of equation) has increased. these substituents stabilize negative charge on carboxylate oxygen atom electron-withdrawing inductive effect (-i) , negative mesomeric effect (-m).

the next set of substituents halogens, substituent effect still positive more modest. reason while inductive effect still negative, mesomeric effect positive, causing partial cancellation. data show these substituents, meta effect larger para effect, due fact mesomeric effect reduced in meta substituent. meta substituents carbon atom bearing negative charge further away carboxylic acid group (structure 2b).

this effect depicted in scheme 3, where, in para substituted arene 1a, 1 resonance structure 1b quinoid positive charge on x substituent, releasing electrons , destabilizing y substituent. destabilizing effect not possible when x has meta orientation.

other substituents, methoxy , ethoxy, can have opposite signs substituent constant result of opposing inductive , mesomeric effect. alkyl , aryl substituents methyl electron-releasing in both respects.

of course, when sign reaction constant negative (next section), substituents likewise negative substituent constant increase equilibrium constants.