An ester is an organic compound containing a functional group in which a carbonyl carbon is bonded to an oxygen atom that is itself bonded to another carbon containing group. A common general representation is R C equals O O R prime. Esters can be considered derivatives of carboxylic acids in which the hydrogen of the acid hydroxyl group has been replaced by an organic group.
Simple esters are often associated with pleasant fruity or floral odours, although odour is not a defining feature and many esters have little or no strong smell. Esters are widespread in nature and industry and are found in fats, oils, waxes, fragrances, flavour compounds, solvents, polymers and pharmaceuticals. Their structure gives them a combination of carbonyl and ether like characteristics, producing distinctive polarity, reactivity and physical properties.
Esters are commonly produced through reactions between carboxylic acids and alcohols. In Fischer esterification, a carboxylic acid reacts with an alcohol in the presence of an acid catalyst to form an ester and water. The reaction is reversible and therefore the equilibrium can be shifted towards ester formation by using an excess of one reactant or removing water.
The mechanism involves proton transfers, nucleophilic attack by the alcohol oxygen on the carbonyl carbon, formation of a tetrahedral intermediate and elimination of water before regeneration of the catalyst. Other synthetic routes include reactions of acid chlorides or acid anhydrides with alcohols, which can be more favourable under suitable conditions. Esters can therefore be prepared through several complementary pathways depending on the desired molecule and laboratory or industrial requirements.
Esters undergo hydrolysis, which breaks the ester linkage and regenerates an alcohol and a carboxylic acid or carboxylate depending on the conditions. Acid catalysed hydrolysis is reversible and is effectively the reverse of Fischer esterification. Base promoted hydrolysis, commonly called saponification when applied to suitable fats or oils, produces an alcohol and a carboxylate salt and is effectively irreversible under many ordinary conditions because the carboxylate ion is less susceptible to the reverse esterification process.
Esters can also undergo transesterification, in which the alcohol component is exchanged for another alcohol. This reaction is important in biodiesel production and in lipid chemistry. Strong reducing agents can convert esters into alcohols, while other reagents can transform them into amides, ketones or related functional groups.
Esters are named by identifying the alcohol derived portion first and the acid derived portion second. For example, ethyl ethanoate consists of an ethyl group derived from ethanol and an ethanoate group derived from ethanoic acid. Methyl propanoate contains a methyl group and a propanoate group. The ester functional group should be distinguished from an ether, which has an oxygen atom bonded to two carbon groups but lacks the adjacent carbonyl group.
Esters also differ from carboxylic acids because the oxygen attached to the carbonyl carbon is bonded to carbon rather than hydrogen. Lactones are cyclic esters formed when an ester linkage occurs within a ring. Polyester refers to polymers containing many ester linkages, illustrating how a functional group can occur repeatedly within a macromolecular structure.
Many naturally occurring lipids contain ester linkages. Triglycerides, also called triacylglycerols, consist of glycerol esterified with three fatty acids. The ester bonds connect the glycerol backbone to the fatty acid chains. Hydrolysis of these bonds is central to lipid digestion and metabolism. Phospholipids also contain ester linkages in many common structural classes, although some lipid families contain ether or other linkages instead. Waxes are generally esters formed between long chain fatty acids and long chain alcohols. These naturally occurring esters contribute to biological membranes, energy storage, protective coatings and surface properties in organisms. The ester functional group therefore has major biological significance beyond its importance as a synthetic chemical group.
Esters have characteristic physical properties resulting from their molecular structure. They contain polar carbonyl and ether oxygen atoms and can act as hydrogen bond acceptors, but ordinary esters lack an OH group and therefore cannot donate hydrogen bonds to themselves. Consequently, many small esters have lower boiling points than carboxylic acids or alcohols of comparable molecular mass. Smaller esters can have appreciable water solubility, but solubility generally decreases as the hydrocarbon portions become larger. Volatility varies greatly with molecular size and structure. The familiar fruity odours of some esters arise because volatile ester molecules interact with olfactory receptors, but smell is highly structure dependent and should not be treated as a general chemical definition.
Esters are important industrial materials and intermediates. They are used as solvents in coatings, inks, adhesives and cleaning products, and some are used as fragrance and flavour ingredients. Esters are also important in polymer chemistry, where repeated ester linkages produce polyesters such as polyethylene terephthalate. Biodiesel commonly consists of fatty acid methyl esters or fatty acid ethyl esters produced by transesterification of oils or fats.
Plasticisers can contain ester groups, and many pharmaceutical molecules include ester functionalities that affect absorption, stability and metabolism. In medicinal chemistry, ester groups may act as temporary prodrug functionalities because enzymes can hydrolyse them under physiological conditions. Their practical value therefore arises from both their predictable chemistry and their ability to tune molecular physical and biological properties.
Ester chemistry illustrates how functional groups can connect structure, reactivity and material properties. Steric effects around the carbonyl carbon influence hydrolysis rates, while electron withdrawing or donating substituents can change carbonyl electrophilicity. Conjugation can alter the electronic structure of an ester and affect its spectroscopic properties. Infrared spectroscopy commonly identifies esters through a strong carbonyl stretching absorption and characteristic carbon oxygen absorptions, while nuclear magnetic resonance spectroscopy provides detailed information about the carbon and hydrogen environments. In organic synthesis, ester groups can serve as temporary protecting groups, reactive intermediates or final functional groups. Their reversible hydrolysis and transesterification chemistry makes them especially useful for designing molecules and materials whose structures can be chemically modified under controlled conditions.
