• Substituents can have a profound influence on the dissociation of benzoic acid.
• Thus substituents can have influence on the acidity of benzoic acid.
• Hammett equation attempts to quantify this influence of substituents.
• To derive a relation only the para subtituents are considered.
• Ortho substituents complicate due to steric influence as well.
• Meta substituents are not in through resonance with the carbonyl group of benzoic acid.
• Electron withdrawing group increases dissociation while electron donating decrease dissociation of benzoic acid.
• The influence is studied at a specific temperature (25⁰C) and a specific solvent like water.

The relation may be derived as follows,

If ΔG^o is the free energy change for the dissociation of substituted benzoic acid it must be a combination of free energy change of unsubstituted benzoic acid and the influence due to the substituent which can be represented as an equation

Thus the relation can be expressed as