Esterifikation

Esterifikation, or esterification, is a chemical reaction that forms esters from an alcohol and a carboxylic acid, typically with the elimination of water. The most common variant is the Fischer esterification, an acid-catalyzed, reversible process in which a carboxylic acid reacts with an alcohol to yield an ester and water.

In acid-catalyzed esterification, the carbonyl oxygen of the acid is protonated to increase electrophilicity. The alcohol attacks the carbonyl carbon, forming a tetrahedral intermediate. After proton transfers and loss of water, an ester is produced, and the acid catalyst is regenerated. Because the reaction is reversible, the position of equilibrium depends on the relative concentrations and conditions. Removing water, using an excess of alcohol, or carrying out the reaction in an azeotropic solvent or with a Dean–Stark trap shifts the equilibrium toward ester formation. Typical laboratory conditions involve heat and a catalytic acid such as sulfuric acid or p-toluenesulfonic acid.

Variants include transesterification, where one ester exchanges its alkoxy group with another alcohol, and intramolecular esterification (lactonization). Enzymatic esterification uses lipases in non-aqueous media and can proceed under mild conditions. Industrially, esters are prepared for solvents, fragrances, and plasticizers; biodiesel is produced by transesterifying fats or oils with methanol or ethanol to give fatty acid methyl or ethyl esters.

Example: acetic acid plus ethanol in the presence of sulfuric acid affords ethyl acetate and water. Safety considerations include handling corrosive acids and flammable solvents; methanol-containing biodiesel poses toxicity risks.