Ester

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For other uses, see Ester (disambiguation).
A carboxylate ester. R and R' denote any alkyl or aryl group

In chemistry, esters are chemical compounds derived from an inorganic acid or organic acid in which at least one -OH (hydroxyl) group is replaced by an -O-alkyl (alkoxy) group.[1] Esters are generally derived from a carboxylic acid and an alcohol. Esters comprise most naturally occurring fats and oils, which are 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[edit]

Etymology[edit]

The word 'ester' was coined in 1848 by German chemist Leopold Gmelin,[2] probably as a contraction of the German Essigäther - acetic ether.

IUPAC nomenclature of Esters[edit]

Ester names are derived from the parent alcohol and the parent acid, where the latter may be organic or inorganic. Esters derived from the simplest carboxylic acids are commonly named according to the more traditional, so-called "trivial names" e.g. as formate, acetate, propionate, and butyrate, as opposed to the IUPAC nomenclature methanoate, ethanoate, propanoate and butanoate. Esters derived from more complex carboxylic acids are, on the other hand, more frequently named using the systematic IUPAC name, based on the name for the acid followed by the suffix -oate. For example the ester hexyl octanoate, also known under the trivial name hexyl caprylate, has the formula CH3(CH2)6CO2(CH2)5CH3.

Ethyl acetate derived from an alcohol (blue) and an acyl group (yellow) derived from a carboxylic acid.

The chemical formulas of organic esters usually take the form RCO2R', where R and R' are the hydrocarbon parts of the carboxylic acid and thealcohol, respectively. For example butyl acetate (systematically butyl ethanoate), derived from butanol and acetic acid (systematically ethanoic acid) would be written CH3CO2C4H9. Alternative presentations are common including BuOAc and CH3COOC4H9.

Cyclic esters are called lactones, regardless of whether they are derived from an organic or an inorganic acid. One example of a (organic) lactone is gamma-valerolactone.

Orthoesters[edit]

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[edit]

A phosphoric acid ester

Esters can also be derived from an inorganic acid and an alcohol. Thus, the nomenclature extends to inorganic oxo acids, e.g. phosphoric acid, sulfuric acid, nitric acid and boric acid. For example, triphenyl phosphate is the ester derived from phosphoric acid and phenol. Organic carbonates are derived from carbonic acid; for example, ethylene carbonate is derived from carbonic acid and ethylene glycol.

Structure and bonding[edit]

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.[3] The pKa of the alpha-hydrogens on esters is around 25.[4]

Physical properties and characterization[edit]

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.[3]

Characterization and analysis[edit]

Esters are generally 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 νC=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[edit]

Esters are widespread in nature and are widely used in industry. In nature, fats are, in general, triesters derived from glycerol and fatty acids.[5] Esters are responsible for the aroma of many fruits, including apples, durians, pears, bananas, pineapples, and strawberries.[6] Several billion kilograms of polyesters are produced industrially annually, important products being polyethylene terephthalate, acrylate esters, and cellulose acetate.[7]

Representative triglyceride found in a linseed oil, a triester (triglyceride) derived of linoleic acid, alpha-linolenic acid, and oleic acid.

Preparation[edit]

Esterification is the general name for a chemical reaction in which two reactants (typically an alcohol and an acid) form an ester as the reaction product. Esters are common in organic chemistry and biological materials, and often have a characteristic pleasant, fruity odor. This leads to their extensive use in the fragrance and flavor industry. Ester bonds are also found in many polymers.

Esterification of carboxylic acids[edit]

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

RCO2H + R'OH is in equilibrium with RCO2R' + H2O

The equilibrium constant for such reactions is about 5 for typical esters, e.g., ethyl acetate.[8] The reaction is slow in the absence of a catalyst. Sulfuric acid is a typical catalyst for this reaction. Many other acids are also used such as polymeric sulfonic acids. Since esterification is highly reversible, the yield of the ester can 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 such as molecular sieves are also effective.
  • removal of water by physical means such as distillation as a low-boiling azeotropes with toluene, in conjunction with a Dean-Stark apparatus.

Reagents are known that drive the dehydration of mixtures of alcohols and carboxylic acids. One example is the Steglich esterification, which is a method of forming esters under mild conditions. The method is 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 as an acyl-transfer catalyst.[9]

Steglich-1.svg

Another method for the dehydration of mixtures of alcohols and carboxylic acids is the Mitsunobu reaction:

RCO2H + R'OH + P(C6H5)3 + R2N2 → RCO2R' + OP(C6H5)3 + R2N2H2

Carboxylic acids can be esterified using diazomethane:

RCO2H + CH2N2 → RCO2CH3 + N2

Using this diazomethane, mixtures of carboxylic acids can be converted to their methyl esters in near quantitative yields, e.g., for analysis by gas chromatography. The method is useful in specialized organic synthetic operations but is considered too expensive for large scale applications.

Alcoholysis of acyl chlorides and acid anhydrides[edit]

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

RCOCl + R'OH → RCO2R' + HCl
(RCO)2O + R'OH → RCO2R' + RCO2H

The reactions are irreversible simplifying work-up. Since acyl chlorides and acid anhydrides also react 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 than does water. This method is employed only for laboratory-scale procedures, as it is expensive.

Alkylation of carboxylate salts[edit]

Although not widely employed for esterifications, salts of carboxylate anions can be alkylating agent with alkyl halides to give esters. In the case that an alkyl chloride is used, an iodide salt can catalyze the reaction (Finkelstein reaction). The carboxylate salt is often generated in situ. In difficult cases, the silver carboxylate may be used, since the silver ion coordinates to the halide aiding its departure and improving the reaction rate. This reaction can suffer from anion availability problems and, therefore, can benefit from the addition of phase transfer catalysts or highly polar aprotic solvents such as DMF.

Transesterification[edit]

Transesterification, which involves changing one ester into another one, is widely practiced:

RCO2R' + CH3OH → RCO2CH3 + R'OH

Like the hydrolysation, transesterification is catalysed by acids and bases. The reaction is widely used for degrading triglycerides, e.g. in the production of fatty acid esters and alcohols. Poly(ethylene terephthalate) is produced by the transesterification of dimethyl terephthalate and ethylene glycol:[7]

(C6H4)(CO2CH3)2 + 2 C2H4(OH)2 → 1/n {(C6H4)(CO2)2(C2H4)}n + 2 CH3OH

Carbonylation[edit]

Alkenes undergo "hydroesterification" in the presence of metal carbonyl catalysts. Esters of propionic acid are produced commercially by this method:

C2H4 + ROH + CO → C2H5CO2R

The carbonylation of methanol yields methyl formate, which is the main commercial source of formic acid. The reaction is catalyzed by sodium methoxide:

CH3OH + CO → CH3O2CH

Addition of carboxylic acids to alkenes[edit]

In the presence of palladium-based catalysts, ethylene, acetic acid, and oxygen react to give vinyl acetate:

C2H4 + CH3CO2H + 1/2 O2 → C2H3O2CCH3 + H2O

Direct routes to this same ester are not possible because vinyl alcohol is unstable.

Other methods[edit]

Reactions[edit]

Esters react with nucleophiles at the carbonyl carbon. The carbonyl is weakly electrophilic but 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 is the one that usually initiates condensation reactions. The carbonyl oxygen is weakly basic (less so than in amides) but forms adducts.

Addition of nucleophiles at carbonyl[edit]

Esterification is a reversible reaction. Esters undergo hydrolysis under acid and basic conditions. Under acidic 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.

Ester saponification (basic hydrolysis)

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

RCO2R' + NH2R" → RCONHR" + R'OH

This reaction is not usually reversible. Hydrazines and hydroxylamine can be used in place of amines. Esters can be converted to isocyanates through intermediate hydroxamic acids in the Lossen rearrangement.

Sources of carbon nucleophiles, e.g., Grignard reagents and organolithium compounds, add readily to the carbonyl.

Reduction[edit]

Compared to ketones and aldehydes, esters are relatively resistant to reduction. The introduction of catalytic hydrogenation in the early part of the 20th century was a breakthrough; esters of fatty acids are hydrogenated to fatty alcohols.

RCO2R' + 2 H2 → RCH2OH + R'OH

A typical catalyst is copper chromite. Prior to the development of catalytic hydrogenation, esters were reduced on a large scale using the Bouveault-Blanc reduction. This method, which is largely obsolete, uses sodium in the presence of proton sources.

Especially for fine chemical syntheses, lithium aluminium hydride is used to reduce esters to two primary alcohols. The related reagent sodium borohydride is slow in this reaction. DIBAH reduces esters to aldehydes.[13]

Claisen condensation and related reactions[edit]

As for aldehydes, the hydrogen atoms on the carbon adjacent ("α to") the carboxyl group in esters are sufficiently acidic to undergo deprotonation, which in turn leads to a variety of useful reactions. Deprotonation requires relatively strong bases, such as alkoxides. Deprotonation gives a nucleophilic enolate, which can further react, e.g., the Claisen condensation and its intramolecular equivalent, the Dieckmann condensation. This conversion is exploited in the malonic ester synthesis, wherein the diester of malonic acid reacts with an electrophile (e.g., alkyl halide), and is subsequently decarboxylated. Another variation is the Fráter–Seebach alkylation.

Other reactions[edit]

Protecting groups[edit]

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 isobutylene, simplifying work-up.

List of ester odorants[edit]

Many esters have distinctive fruit-like odors, and many occur naturally in the essential oils of plants. This has also led to their commonplace use in artificial flavorings and fragrances when those odors aim to be mimicked.

Ester Name Formula Odor or occurrence
Allyl hexanoate Prop-2-enyl hexanoate.svg pineapple
Benzyl acetate Benzyl acetate-structure.svg pear, strawberry, jasmine
Bornyl acetate Bornyl acetate.svg pine
Butyl acetate Butylacetat.svg apple, honey
Butyl butyrate Butyl butyrate2.svg pineapple
Butyl propanoate pear drops
Ethyl acetate Ethyl acetate2.svg nail polish remover, model paint, model airplane glue
Ethyl butyrate Ethyl butyrate2.svg banana, pineapple, strawberry
Ethyl hexanoate Ethyl-hexanoate.svg pineapple, waxy-green banana
Ethyl cinnamate Ethyl-cinnamate.svg cinnamon
Ethyl formate Ethyl-formate.svg lemon, rum, strawberry
Ethyl heptanoate Ethyl-heptanoate.svg apricot, cherry, grape, raspberry
Ethyl isovalerate Ethyl-isovalerate.svg apple
Ethyl lactate Ethyl lactate.svg butter, cream
Ethyl nonanoate Ethyl-nonanoate.svg grape
Ethyl pentanoate Ethyl valerate.svg apple
Geranyl acetate Geranyl-acetate.svg geranium
Geranyl butyrate Geranyl butyrate.svg cherry
Geranyl pentanoate Geranyl pentanoate.svg apple
Isobutyl acetate Isobutyl-acetate.svg cherry, raspberry, strawberry
Isobutyl formate Isobutyl formate.svg raspberry
Isoamyl acetate Isoamyl acetate.svg pear, banana (flavoring in Pear drops)
Isopropyl acetate Isopropyl acetate.svg fruity
Linalyl acetate Linalyl acetate.svg lavender, sage
Linalyl butyrate Linalyl butyrate.svg peach
Linalyl formate Linalyl formate.svg apple, peach
Methyl acetate Methyl-acetate.svg glue
Methyl anthranilate Methyl anthranilate.svg grape, jasmine
Methyl benzoate Methyl benzoate.svg fruity, ylang ylang, feijoa
Methyl butyrate (methyl butanoate) Buttersauremethylester.svg pineapple, apple, strawberry
Methyl cinnamate Methyl cinnamate.svg strawberry
Methyl pentanoate (methyl valerate) Methyl pentanoate.svg flowery
Methyl phenylacetate Methyl phenylacetate.svg honey
Methyl salicylate (oil of wintergreen) Methyl salicylate.svg Modern root beer, wintergreen, Germolene and Ralgex ointments (UK)
Nonyl caprylate Nonyl caprylate.svg orange
Octyl acetate Octyl acetate.svg fruity-orange
Octyl butyrate Octyl butyrate.svg parsnip
Amyl acetate (pentyl acetate) Amyl acetate.svg apple, banana
Pentyl butyrate (amyl butyrate) Pentyl butyrate.svg apricot, pear, pineapple
Pentyl hexanoate (amyl caproate) Pentyl hexanoate.svg apple, pineapple
Pentyl pentanoate (amyl valerate) Pentyl pentanoate.svg apple
Propyl acetate Propyl acetate.svg pear
Propyl hexanoate Propyl-hexanoate.svg blackberry, pineapple, cheese, wine
Propyl isobutyrate Propyl isobutyrate.svg rum
Terpenyl butyrate Terpenyl butyrate.svg cherry

See also[edit]

References[edit]

  1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "esters".
  2. ^ Leopold Gmelin, Handbuch der Chemie, vol. 4: Handbuch der organischen Chemie (vol. 1) (Heidelberg, Baden (Germany): Karl Winter, 1848), page 182.
    Original text:

    b. Ester oder sauerstoffsäure Aetherarten.
    Ethers du troisième genre.

    Viele mineralische und organische Sauerstoffsäuren treten mit einer Alkohol-Art unter Ausscheidung von Wasser zu neutralen flüchtigen ätherischen Verbindungen zusammen, welche man als gepaarte Verbindungen von Alkohol und Säuren-Wasser oder, nach der Radicaltheorie, als Salze betrachten kann, in welchen eine Säure mit einem Aether verbunden ist.

    Translation:

    b. Ester or oxy-acid ethers.
    Ethers of the third type.

    Many mineral and organic acids containing oxygen combine with an alcohol upon elimination of water to [form] neutral, volatile ether compounds, which one can view as coupled compounds of alcohol and acid-water, or, according to the theory of radicals, as salts in which an acid is bonded with an ether.

  3. ^ a b March, J. “Advanced Organic Chemistry” 4th Ed. J. Wiley and Sons, 1992: New York. ISBN 0-471-60180-2.
  4. ^ Chemistry of Enols and Enolates - Acidity of alpha-hydrogens
  5. ^ Isolation of triglyceride from nutmeg: G. D. Beal "Trimyristen" Organic Syntheses, Coll. Vol. 1, p.538 (1941). Link
  6. ^ McGee, Harold. On Food and Cooking. 2003, Scribner, New York.
  7. ^ a b Wilhelm Riemenschneider1 and Hermann M. Bolt "Esters, Organic" Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a09_565.pub2
  8. ^ Roger J. Williams, Alton Gabriel, Roy C. Andrews “The Relation Between the Hydrolysis Equilibrium Constant of Esters and the Strengths of the Corresponding Acids” J. Am. Chem. Soc., 1928, volume 50, 1267. doi:10.1021/ja01392a005
  9. ^ B. Neises and W. Steglich, Esterification of Carboxylic Acids with Dicyclohexylcarbodiimide/4-Dimethylaminopyridine: tert-Butyl ethyl fumarate, Org. Synth. ; Coll. Vol. 7: 93 
  10. ^ Ignatyev, Igor; Charlie Van Doorslaer; Pascal G.N. Mertens; Koen Binnemans; Dirk. E. de Vos (2011). "Synthesis of glucose esters from cellulose in ionic liquids". Holzforschung 66 (4): 417–425. doi:10.1515/hf.2011.161. 
  11. ^ Neumeister, Joachim; Keul, Helmut; Pratap Saxena, Mahendra; Griesbaum, Karl (1978). "Ozone Cleavage of Olefins with Formation of Ester Fragments". Angewandte Chemie International Edition in English 17 (12): 939–940. doi:10.1002/anie.197809392. 
  12. ^ Makhova, Irina V.; Elinson, Michail N.; Nikishin, Gennady I. (1991). "Electrochemical oxidation of ketones in methanol in the presence of alkali metal bromides". Tetrahedron 47 (4-5): 895–905. doi:10.1016/S0040-4020(01)87078-2. 
  13. ^ W. Reusch. "Carboxyl Derivative Reactivity". Virtual Textbook of Organic Chemistry. 

External links[edit]