Hammette Equation

To give the numerical values of the effect of structure on reactivity the first try was taken by Hammett e and known as Hammette equation .
The basic equation is:

relating the equilibrium constant, K, for a given equilibrium reaction with substituent R and the reference K₀ constant when R is a hydrogen atom to the substituent constant σ which depends only on the specific substituent R and the reaction constant ρ which depends only on the type of reaction but not on the substituent used.

The equation also holds for reaction rates k of a series of reactions with substituted benzene derivatives: