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Hammett equation

Hammett equation is an equation relating the structure to the reactivity of side-chain derivatives of aromatic compounds. It arises from a comparison between rate constants for various reactions with the rate of hydrolysis of benzyl chloride on the one hand and a comparison between equilibrium constants (such as the dissociation constant of benzoic acid) on the other hand. The Hammett equation can be written in the form log(k/k₀) = plog (K/Ko), where log (K/K₀) refers to comparing dissociation constants to the dissociation constant, K_o, of benzoic acid in water at 25°C, and log (k/k₀) refers to comparing rates of reaction to the rate, k₀, of hydrolysis of benzyl chloride. The term log (K/K₀) = a is called the substituent constant, since the nature of the substituent affects the strength of the benzoic acid. If s is positive, the substituent is electron attracting, while if s is negative the substituent is electron donating, p is a reaction constant, which is determined for a given reaction by the slope of a graph of log (k/k₀) against s. The numerical value of p depends on temperature and the type of solvent.

The Hammett equation applies to meta- and parasubstituents (provided that resonance interaction from the substituents does not occur) but not to ortho-substituents.

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