Ester

The name "ester" is derived from "vinegar-ether", the original term for ethyl acetate. An ester is the result of a chemical reaction in which an alcohol (or a phenol) and an organic or inorganic acid come together to form a condensate when water escapes: R1-COOH + HO-R2 = R1-COO-R2 + H2O.

The hydrolytic cleavage of an ester again produces alcohols and acids. An enzymatic cleavage of ester compounds is carried out by esterases, enzymes (hydrolases) which hydrolytically cleave esters to alcohol and acid (examples are lipases, phosphatases, sulfatases).

A distinction is made:

• Esters of mineral acids (examples are phosphoric acid esters with the alcohol function of sugars e.g. ribose or deoxyribose or sulphuric acid esters) - are widely used in nature e.g. carrageenan; heparin.
• Esters of organic acids (carboxylic acid esters). Examples are:
  • Fats: mono-, di- or tri-esters of glycerol =glycerol and higher fatty acids
  • Waxes: esters of a long chain acid such as palmitic acid and long chain1 alcohols.

Further examples of biologically and technically important ester compounds:

• Internal esters: Internal esters of hydroxycarboxylic acids, called lactones, are esters of hydroxycarboxylic acids in which the alcohol and acid parts are located in a single molecule.
• Polyester compounds: Polyhydric alcohols (>1 OH group e.g. diols/triols) can combine with carboxylic acid with 2 -COOH groups to form long-chain polymers (polyesters). Polyesters are used as plastics (e.g. PET).
• Fruit esters: Numerous esters are used as flavouring agents. Ethanoic acid 2-butyl ester (CH3-COO-C4H9- apple smell), ethanoic acid 2-methyl-1-propyl ester (CH3-COO-C3H7- banana smell) ethanoic acid 2-hexyl ester (CH3-COO-C6H13- strawberry smell).
• Esters as plasticizers: Esters are used as plasticizers, e.g. diethylhexylphthalate as plasticizer for polyvinylchloride (PVC)
• Examples of esters in medicine: The best known medical ester compound is acetylsalicylic acid, an ester of acetic acid.

1. Ammon HPT et al (2014) Hunnius Pharmaceutical Dictionary. De Gruyter Publishing House p.615