## Quick Reference

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*/*k*_{0})=ρlog(*K*/*K*_{0}), where log(*K*/*K*_{0}) refers to comparing dissociation constants to the dissociation constant, K_{0}, of benzoic acid in water at 25°C, and log(*k*/*k*_{0}) refers to comparing rates of reaction to the rate, *k*_{0}, of hydrolysis of benzyl chloride. The term log(*K*/*K*_{0})=σ is called the substituent constant, since the nature of the substituent affects the strength of the benzoic acid. If σ is positive, the substituent is electron attracting, while if σ is negative the substituent is electron donating. ρ is a reaction constant, which is determined for a given reaction by the slope of a graph of log(*k*/*k*_{0}) against σ. The numerical value of ρ depends on temperature and the type of solvent.

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

*Subjects:*
Chemistry.

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