Chemistry
The Hammett Equation: Quantifying Substituent Electronic Effects
Quick fact
The Hammett equation uses a single substituent constant (σ) derived from the ionization of benzoic acid to predict rate or equilibrium changes for hundreds of reactions, even though those reactions may be completely different in nature.
Why this is interesting
Why does adding a nitro group to a benzene ring make an acid stronger, while adding a methoxy group makes it weaker? The Hammett equation provides a surprising way to predict and quantify these effects.