Ester

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

Esters are chemical compounds derived by reacting an oxoacid (one containing an oxo group, X=O) with a hydroxyl compound such as an alcohol or phenol.^[1] Esters are usually derived from an inorganic acid or organic acid in which at least one -OH (hydroxyl) group is replaced by an -O-alkyl (alkoxy) group, and most commonly from carboxylic acids and alcohols. Basically, esters are formed by condensing an acid with an alcohol.

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.

Nomenclature

The names for esters are derived from the names of the parent alcohol and the carboxylic acid. For esters derived from the simplest carboxylic acids, the traditional names are used, such as formate, acetate, propionate, and butyrate. For esters from more complex carboxylic acids, the systematic name for the acid is used, followed by the suffix -oate. For example, hexyl octanoate, also called hexyl caprylate, has the formula CH₃(CH₂)₆CO₂(CH₂)₅CH₃.

The chemical formulas of esters are typically written in the format of RCO₂R', where R and R' are the organic parts of the carboxylic acid and alcohol respectively. For example butyl acetate, derived from butanol and acetic acid would be written CH₃CO₂C₄H₉. Alternative presentations are common including BuOAc and CH₃COOC₄H₉.

Cyclic esters are called lactones. One example of many is valerolactone.

Orthoesters

An uncommon class of organic esters are the orthoesters, which have the formula RC(OR')₃. Triethylorthoformate (HC(OC₂H₅)₃) is derived, in terms of its name (but not its synthesis) from orthoformic acid (HC(OH)₃) and ethanol.

"Inorganic esters"

A phosphoric acid ester

Ester is a general term for the product derived from the condensation of an oxo acid and an alcohol. Thus, the nomenclature extends to inorganic acids, especially phosphoric acid, sulfuric acid, and nitric acid. Cyclic esters are called lactones. The preparation of an ester is known generally as an esterification reaction. For example, triphenyl phosphate is the ester derived from phosphoric acid and phenol. Organic carbonates, such as ethylene carbonate are derived from carbonic acid and ethylene glycol. The term boronic ester is widely used but these compounds are not formally esters because they lack a B=O bond.

Structure and bonding

Esters contain a carbonyl center, which gives rise to 120° C-C-O and O-C-O angles. Unlike amides, esters are structurally flexible functional groups because rotation about the C-O-C bonds has a low barrier. Their flexibility and low polarity is manifested in their physical properties; they tend to be less rigid (lower melting point) and more volatile (lower boiling point) than the corresponding amides.

Physical properties and characterization

Esters are more polar than ethers but less polar than alcohols. They 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 confers some water-solubility. Because of their lack of hydrogen-bond-donating ability, esters do not self-associate. Consequently esters are more volatile than carboxylic acids of similar molecular weight.

Characterization and analysis

Esters are usually identified by gas chromatography, taking advantage of their volatility. IR spectra for esters feature an intense sharp band in the range 1730-1750 cm^-1 assigned to ν_C=O. This peak changes depending on the functional groups attached to the carbonyl. For example, a benzene ring or double bond in conjugation with the carbonyl will bring the wavenumber down about 30 cm^-1.

Applications and occurrence

Esters are widespread in nature and are widely used in industry. In nature, fats are generally triesters derived from glycerol and fatty acids. Esters are responsible for the aroma of many fruits, including apples, pears, bananas, pineapples, and strawberries.^[2] Several billion kilograms of polyesters are produced industrially annually, important products being polyethylene terephthalate, acrylate esters, and cellulose acetate.^[3]

Preparation

Fischer esterification

The classic synthesis is the Fischer esterification, which involves treating a carboxylic acid with an alcohol in the presence of a dehydrating agent:

R¹COOH + R²OH R¹COOR² + H₂O

Strong acids, typically sulfuric acid, catalyze this reaction. Many other acids are also used. Esterification is highly reversible. The yield of the product may be improved using le Chatelier's principle:

• using the alcohol in large excess (i.e. as a solvent),
• using a dehydrating agent. Sulfuric acid not only catalyzes the reaction but sequesters water (a reaction product). Other drying agents like molecular sieves can also be used.
• removal of water by physical means such as distillation as a low-boiling azeotropes with toluene, in conjunction with a Dean-Stark apparatus.

Alcoholysis of acyl chlorides and acid anhydrides

Alcohols react with acyl chlorides or acid anhydrides to give esters:

RCOCl + R'OH → RCOOR' + HCl

(RCO)₂O + R'OH → RCO₂R' + RCO₂H

These reactions are irreversible, simplifying work-up. Since, acyl chlorides and acid anhydrides react also with water, anhydrous conditions are preferred. The analogous acylations of amines to give amides are less sensitive because amines are stronger nucleophiles and react more rapidly.

Steglich esterification

The Steglich esterification is method of forming esters under mild conditions. The method 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 methods

• Transesterifications between other esters. This method is widely used industrially.
• 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 react primarily at one of two locations, the carbonyl at the carbon adjacent the carbonyl group. The carbonyl is weakly electrophilic and is attacked by strong nucleophilies (amines, alkoxides, hydride sources, organolithium compounds, etc). The C-H bonds adjacent to the carbonyl are weakly acidic but undergo deprotonation with strong bases. This process that usually initiates condensation reactions.

Nucleophilic substitutions

Esters undergo hydrolysis under acid and basic conditions. Under acid conditions, the reaction is the reverse reaction of the Fischer esterification. Under basic conditions, hydroxide acts as a nucleophile, while an alkoxide is the leaving group. This reaction, saponification, is the basis of soap making.

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, whereas sodium borohydride does so more slowly. DIBAH reduces esters to aldehydes, while forcing conditions are required for hydrogenation.^[4] In the Bouveault-Blanc reduction, sodium in the presence of proton sources is the reducing agent. Such reactions can also be conducted by hydrogenation.

RCO₂R' + 2 H₂ → RCH₂OH + R'OH

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 condensation. 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.

Protecting groups

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.

External links

• An introduction to esters
• Molecule of the month: Ethyl acetate and other esters
• An example of esters commercial application

References

Appendix of ester odorants

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

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 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