The oxygen of a carbonyl group undergoes equilibrium protonation in strong acid to give an oxonium ion, which greatly increases the electrophilicity of the carbonyl carbon, allowing weak nucleophiles to add to form "tetrahedral addition intermediates". In this example, equilibrium protonation of the carbonyl oxygen occurs, followed by attack of the solvent (alcohol) on the carbonyl carbon of a carboxylic acid. The resulting addition intermediate rapidly loses a proton to form the neutral intermediate. Equilibrium protonation of this intermediate on one of the hydroxyl groups converts that hydroxyl group into a "good leaving group". Loss of water forms an intermediate oxonium ion which rapidly loses a proton to give the final product, a carboxylic acid ester. Note that all of these steps are in equilibrium, and the position of the equilibrium (ester hydrolysis or ester formation) is governed by the reaction conditions; i.e., excess water leads to hydrolysis, excess alcohol leads to esterification. This reaction is generally referred to as Fischer Esterification.