Ester

In chemistry, an ester is a compound made from an acid, either organic or inorganic, in which the hydrogen atom (H) of at least one acidic hydroxyl group (−OH) is replaced by an organyl group (R′). Esters contain a distinctive functional group. Compounds where oxygen is replaced by other chalcogens also belong to the ester category. Some authors also call organyl derivatives of other acids esters, such as amides, but the IUPAC does not.

Glycerides are fatty acid esters of glycerol. They are important in biology because they are one of the main classes of lipids and make up most of animal fats and vegetable oils. Lactones are cyclic carboxylic esters. They add to the smell of fruits, butter, cheese, vegetables like celery, and other foods.

Esters can be formed from oxoacids, such as esters of acetic acid, sulfuric acid, and xanthic acid. They can also come from acids without oxygen, such as thiocyanic acid and trithiocarbonic acid. One way an ester forms is through a substitution reaction between a carboxylic acid and an alcohol.

Organyl esters of carboxylic acids usually have a pleasant smell. Those with low molecular weight are often used as fragrances and are found in essential oils and pheromones. They work as strong solvents for many plastics, plasticizers, resins, and lacquers, and they are one of the largest classes of synthetic lubricants sold commercially. Polyesters are important plastics whose monomers are linked by ester moieties. Esters of phosphoric acid form the backbone of DNA molecules, while esters of nitric acid, such as nitroglycerin, are known for their explosive properties.

Some compounds have an acidic hydrogen replaced by a different group rather than an organyl. According to some authors, these are also esters, especially when the first carbon atom of that organyl group is replaced by another group 14 elements (Si, Ge, Sn, Pb). For example, trimethylsilyl acetate is a trimethylsilyl ester of acetic acid, dibutyltin dilaurate is a dibutylstannylene ester of lauric acid, and the Phillips catalyst is a trimethoxysilyl ester of chromic acid (H₂CrO₄).

Nomenclature

The word ester was coined in 1848 by German chemist Leopold Gmelin, probably as a short form of the German Essigäther, "acetic ether".

IUPAC nomenclature

Main article: IUPAC nomenclature of organic chemistry § Esters

Esters made from an alcohol and an acid are named after the parent alcohol and the parent acid. Esters from the simplest carboxylic acids often have traditional names, such as formate and acetate. Esters from more complex carboxylic acids usually use the systematic name, based on the acid's name with the ending -oate. Cyclic esters are called lactones. Some rarer esters are orthoesters. Esters can also be made from inorganic acids, such as sulfuric acid, nitric acid, and phosphoric acid.

Structure and bonding

Esters made from carboxylic acids and alcohols have a carbonyl group, C=O, which is a divalent group at a C atom. This gives angles of 120° around the group. Unlike amides, carboxylic acid esters are flexible, because parts of the molecule can rotate easily around their C–O–C bonds. This makes them less rigid, with a lower melting point, and more volatile, with a lower boiling point, than the matching amides. The alpha-hydrogens on these esters have a p_K__a of about 25. Alpha-hydrogens are hydrogen atoms attached to the carbon next to the carbonyl group.

Many carboxylic acid esters can take different shapes, called conformational isomerism. Most of them prefer the S-cis (or Z) shape instead of the S-trans (or E) shape. This is due to hyperconjugation and dipole minimization effects. The preferred shape can depend on the groups attached to the molecule and on the solvent, if one is present. Lactones with small rings are limited to the s-trans (E) shape because of their ring structure.

Physical properties and characterization

Esters made from carboxylic acids and alcohols are more polar than ethers but less polar than alcohols. They can accept hydrogen bonds but cannot give them, unlike their parent alcohols. This lets esters dissolve a little in water. Because they cannot give hydrogen bonds, esters do not stick to each other, so they are more volatile than carboxylic acids of similar molecular weight.

Esters are usually identified by gas chromatography, which takes advantage of how easily they turn into vapour. IR spectra of esters show a strong, sharp band, and its position changes depending on what is attached nearby. For example, a benzene ring or a double bond next to it moves the band a little.

Applications and occurrence

Esters are common in nature and are widely used in industry. In nature, fats are generally triesters made from glycerol and fatty acids. Esters give many fruits their smell, including apples, durians, pears, bananas, pineapples, and strawberries.

Several billion kilograms of polyesters are made in industry each year. Important examples include polyethylene terephthalate, acrylate esters, and cellulose acetate.

Preparation

Esterification is the general name for a chemical reaction in which two reactants, usually an alcohol and an acid, form an ester as the reaction product. Esters are common in living things and in many materials, and they often have a pleasant, fruity smell. This is why they are widely used in the fragrance and flavor industry. Ester bonds are also found in many polymers. The classic method is the Fischer esterification, which heats a carboxylic acid with an alcohol. Sulfuric acid is a typical catalyst, and a dehydrating agent helps remove water so more ester forms. The Steglich esterification is a gentler method, and it is popular in peptide synthesis.

Other methods include the Mitsunobu reaction and the use of diazomethane, which can make methyl esters for analysis by gas chromatography. Alcohols can also react with acyl chlorides and acid anhydrides to make esters, and transesterification changes one ester into another. Chemists also make esters by adding acids to alkenes or alkynes, or from aldehydes, along with several other specialized methods.

Reactions

Esters are less reactive than acid halides and anhydrides. Like other reactive acyl compounds, they can react with ammonia and amines to make amides, but this is not often used. Esters can be changed into other esters through transesterification, which uses an alcohol. Using a large amount of the reactant alcohol, or removing the leftover alcohol by distillation, pushes the reaction forward, following Le Chatelier's principle. Acid-catalyzed hydrolysis of esters is also reversible and is the reverse of Fischer esterification.

Basic hydrolysis, called saponification, is not reversible. It uses up a full amount of hydroxide and makes an alcohol and a salt. Saponification of fatty acids is used to make soap.

Esters are relatively resistant to reduction compared to ketones and aldehydes. Catalytic hydrogenation, using a catalyst such as copper chromite, changes esters of fatty acids into fatty alcohols. Before this method was developed, the Bouveault–Blanc reduction was used, but it is now mostly out of use. Strong reagents such as lithium aluminium hydride turn esters into alcohols, while sodium borohydride is slow in this reaction and DIBAH turns esters into aldehydes. Esters also react with Grignard reagents and organolithium compounds, and they can form enolates that take part in the Claisen condensation, which can be used to make β-keto esters.

Esters can also be used as protecting groups for carboxylic acids. This is useful in making peptides, which are chains of amino acids, because it stops them from reacting with themselves.

List of ester odorants

Many esters have pleasant, fruit-like smells, and many are found naturally in the essential oils of plants. Because of this, they are often used in artificial flavorings and fragrances that copy those smells.