ester

A carboxylic acid ester. R and R' denote any alkyl or aryl group

A phosphoric acid ester

Esters are chemical compounds derived formally from an oxoacid (one containing an oxo group, X=O), and a hydroxyl compound such as an alcohol or phenol.^[1] Esters consist of an inorganic acid or organic acid in which at least one -OH (hydroxyl) group is replaced by an -O-alkyl (alkoxy) group. They are analogous to salts, using organic alcohols instead of metallic hydroxides.

Esters are ubiquitous. Many naturally occurring fats and oils are the fatty acid esters of glycerol. Esters with low molecular weight are commonly used as fragrances and found in essential oils and pheromones. Phosphoesters form the backbone of DNA molecules. Nitrate esters, such as nitroglycerin, are known for their explosive properties, while polyesters are important plastics, with monomers linked by ester moieties.

Some acids that are commonly esterified are carboxylic acids, phosphoric acid, sulfuric acid, nitric acid, and boric acid. Cyclic esters are called lactones. The preparation of an ester is known generally as an esterification reaction.

This article will deal primarily with the esters derived from carboxylic acids and alcohols or phenols, the most common type of esters.

Nomenclature

Since most esters, or carbonate, are derived from carboxylic acids, a specific nomenclature is used for them. For esters derived from the simplest carboxylic acids, the traditional name for the acid constituent is generally retained, such as formate, acetate, propionate, and butyrate.^[2] For esters from more complex carboxylic acids, the systematic name for the acid is used, followed by the suffix -oate. For example, methyl formate is the ester of methanol and methanoic acid (formic acid): the simplest ester. It could also be called methyl methanoate.^[3]

Esters of aromatic acids are also encountered, including benzoates such as methyl benzoate, and phthalates, with substitution allowed in the name.

The chemical formulas of esters are typically in the format of R-COO-R', in which the alkyl group (R') is mentioned first, and the carboxylate group (R) is mentioned last.^[4] For example the ester: butyl ethanoate - derived from butanol (C₄H₉OH) and ethanoic acid (CH₃COOH) would have the formula: CH₃COOC₄H₉. Sometimes the formula may be 'broken up' to show the structure, in this case: CH₃COO[CH₂]₃CH₃.

Oligoesters

Look up monoester in Wiktionary, the free dictionary.

Look up diester in Wiktionary, the free dictionary.

Look up triester in Wiktionary, the free dictionary.

The term oligoester refers to any ester polymer containing a small number of component esters. As an example, chemically, fats are generally diesters of glycerol and fatty acids. Most of the mass of a fat/triester is in the 3 fatty acids.

Tetraesters can be found as part of membrane-spanning lipids in bacteria from the order Thermotogales.^[5]

Pentaesters have been used as indicators^[6] or in isotopic labelling^[7] compounds.

Hexaesters such as calix[6]arene have been used in optodes as sensing devices for optical determination of potassium ion concentration in pH-buffer solutions.^[8]

Heptaesters have been found in Euphorbia species.^[9]

Octaesters can be inclusions of ester moieties within cavitand cavities.^[10]

The number of esters can be up to ten as in oligo-(R)-3-hydroxybutyrate^[11].

Structure and physical properties

Esters contain the C=O moiety. As a result, their IR spectra contain a strong, sharp absorption between 1600-1800 due to the C=O stretch.

Esters participate in hydrogen bonds as hydrogen-bond acceptors, but cannot act as hydrogen-bond donors, unlike their parent alcohols. This ability to participate in hydrogen bonding makes them more water-soluble than their parent hydrocarbons. However, the limitations on their hydrogen bonding also make them more hydrophobic than either their parent alcohols or their parent acids. Their lack of hydrogen-bond-donating ability means that ester molecules cannot hydrogen-bond to each other, which, in general, makes esters more volatile than a carboxylic acid of similar molecular weight. This property makes them very useful in organic analytical chemistry: Unknown organic acids with low volatility can often be esterified into a volatile ester, which can then be analyzed using gas chromatography, gas liquid chromatography, or mass spectrometry.

Many esters have distinctive fruit-like odors, which has led to their commonplace use in artificial flavorings and fragrances. For example:

Ester Name	Structure	Odor or occurrence
Allyl hexanoate		pineapple
Benzyl acetate		pear, strawberry, jasmine
Bornyl acetate		pine tree flavor
Butyl butyrate		pineapple
Ethyl acetate		nail polish remover, model paint, model airplane glue
Ethyl butyrate		banana, pineapple, strawberry
Ethyl hexanoate		pineapple,waxy-green banana
Ethyl cinnamate		cinnamon
Ethyl formate		lemon, rum, strawberry
Ethyl heptanoate		apricot, cherry, grape, raspberry
Ethyl isovalerate		apple
Ethyl lactate		butter, cream
Ethyl nonanoate		grape
Ethyl pentanoate		apple
Geranyl acetate		geranium
Geranyl butyrate		cherry
Geranyl pentanoate		apple
Isobutyl acetate		cherry, raspberry, strawberry
Isobutyl formate		raspberry
Isoamyl acetate		pear, banana (flavoring in Pear drops)
Isopropyl acetate		fruity
Linalyl acetate		lavender, sage
Linalyl butyrate		peach
Linalyl formate		apple, peach
Methyl acetate		glue
Methyl anthranilate		grape, jasmine
Methyl benzoate		fruity, ylang ylang, feijoa
Methyl benzyl acetate		cherry
Methyl butyrate (methyl butanoate)		pineapple, apple, strawberry
Methyl cinnamate		strawberry
Methyl pentanoate (methyl valerate)		flowery
Methyl phenylacetate		honey
Methyl salicylate (oil of wintergreen)		Modern root beer, wintergreen, Germolene and Ralgex ointments (UK)
Nonyl caprylate		orange
Octyl acetate		fruity-orange
Octyl butyrate		parsnip
Amyl acetate (pentyl acetate)		apple, banana
Pentyl butyrate (amyl butyrate)		apricot, pear, pineapple
Pentyl hexanoate (amyl caproate)		apple, pineapple
Pentyl pentanoate (amyl valerate)		apple
Propyl ethanoate		pear
Propyl isobutyrate		rum
Terpenyl butyrate		cherry

Applications

The applications of esters are vast, and often depend on the particular ester in consideration. As a class, esters serve as protecting groups for carboxylic acids. Protecting a carboxylic acid is useful in peptide synthesis, to prevent self-reactions of the bifunctional amino acids. Methyl and ethyl esters are commonly available for many amino acids; the t-butyl ester tends to be more expensive. However, t-butyl esters are particularly useful because under strongly acidic conditions, the t-butyl esters undergo elimination to give the carboxylic acid and isobutene, simplifying work-up.

Preparation

Fischer esterification

The most classic method is the Fischer esterification: refluxing a carboxylic acid in an alcohol, which acts as both solvent and reactant:

R¹COOH + R²OH R¹COOR² + H₂O

A strong acid, traditionally sulfuric acid, is used as a catalyst. It protonates the -OH group of the carboxylic acid, making it a better leaving group. Lewis acids may also be used.

Since the reaction is an equilibrium, simply reacting one mole of acid with one mole of alcohol will give a mixture of starting materials and products. The yield of the product may be improved using le Chatelier's principle:

• using the alcohol as a solvent (i.e. in large excess) will help push the equilibrium to the right
• where sulfuric acid is used, it acts both as the acid catalyst, and as a dehydrating agent. By sequestering water (a reaction product), the equilibrium is pushed to the right. The use of a solvent which forms low-boiling azeotropes with water, such as toluene, in conjunction with a Dean-Stark apparatus has a similar effect, as is the use of other drying agents like molecular sieves.
• since low molecular weight esters typically have lower boiling points than their parent carboxylic acids and alcohols because they are not able to form hydrogen bonds, the ester products may be distilled out of the reaction vessel as they are formed. By removing the ester product in a reactive distillation, the equilibrium once again lies to the right.

Reaction with acyl chlorides and acid anhydrides

Alcohols react with an acyl chloride or acid anhydride to give esters:

R¹COCl + R²OH → R¹COOR² + HCl

(R¹CO)₂O + R²OH → R¹COOR² + R¹COOH

These reactions are irreversible, simplifying the product mixture. No acid catalysts are necessary, as the chloride and carboxylate are good leaving groups. However, acyl chlorides and acid anhydrides react with water. As alcohols are poorer nucleophiles than amines, anhydrous conditions are preferred, compared with the analogous reactions to generate amides.

Steglich esterification

The Steglich esterification is method of forming esters under mild conditions. It is especially popular in peptide synthesis, where the substrates are sensitive to harsh conditions like high heat. DCC (dicyclohexylcarbodiimide) is used to activate the carboxylic acid to further reaction. DMAP (4-dimethylaminopyridine) is used catalytically as an acyl-transfer agent.

Other reactions

• Transesterifications between other esters
• Favorskii rearrangement of α-haloketones in presence of base
• Nucleophilic displacement of alkyl halides with carboxylic acid salts
• Baeyer-Villiger oxidation of ketones with peroxides
• Pinner reaction of nitriles with an alcohol

Reactions

Esters may react primarily at two locations: at the carboxyl, and at the carbon adjacent the carbonxyl group.

Nucleophilic substitutions

Esters undergo hydrolysis under acid and basic conditions. Under acid conditions, the reaction is the reverse reaction of the Fisher esterification. Under basic conditions, hydroxide acts as a nucleophile, while an alkoxide is the leaving group.

The alkoxide group may also be displaced by stronger nucleophiles such as ammonia or primary or secondary amines to give amides.

Reduction

Esters are relatively resistant to reduction. Lithium aluminium hydride is able to reduce esters to two primary alcohols, while sodium borohydride does so more slowly. DIBAH reduces esters to aldehydes, while forcing conditions are required for hydrogenation.^[12]

Reactions adjacent the carboxyl group

Similar to carbonyl compounds, the hydrogen atoms on the carbon adjacent ("α to") the carboxyl group are acidic. They may be removed by relatively strong bases, such as an alkoxide salt. Deprotonating the alpha position gives a nucleophile, which may further react, e.g. the Claisen condensation and its intramolecular equivalent, the Dieckmann rearrangement.

This property is exploited in the malonic ester synthesis, where the diester of malonic acid reacts with an electrophile (e.g. alkyl halide), and is subsequently decarboxylated. This is a two carbon homologation reaction.

Other reactions

• Phenyl esters react to hydroxyarylketones in the Fries rearrangement.
• Specific esters are functionalized with an α-hydroxyl group in the Chan rearrangement.
• Esters are converted to isocyanates through intermediate hydroxamic acids in the Lossen rearrangement.
• Esters with β-hydrogen atoms can be converted to alkenes in ester pyrolysis.

External links

• An introduction to esters
• Molecule of the month: Ethyl acetate and other esters
• Making an Ester A simple guide to naming and making esters, as well as the chemistry behind it.
• An example of esters commercial application

References