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ester

 
Dictionary: es·ter   (ĕs'tər) pronunciation
 
n.

Any of a class of organic compounds corresponding to the inorganic salts and formed from an organic acid and an alcohol.

[German, short for Essigäther : Essig, vinegar (from Middle High German ezzich, from Old High German ezzīh, from Latin acētum) + Äther, ether (from Latin aethēr; see ether).]


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The product of a condensation reaction (esterification) in which a molecule of an acid unites with a molecule of alcohol with elimination of a molecule of water as shown in the following reaction.


At one time it was thought that esterification was analogous to neutralization, and esters are still named as though they are “alkyl salts” of carboxylic acids.

Esters are generally insoluble in water and have boiling points slightly higher than hydrocarbons of similar molecular weight.

Ethyl and butyl acetates are volatile industrial solvents, used particularly in the formulation of lacquers. Higher-boiling esters such as butyl phthalate are used as softening agents (plasticizers) in the compounding of plastics. The natural waxes of biological origin are largely simple esters. For example, a principal component of beeswax is myricyl palmitate. See also Solvent.

Esters of cellulose (cellulose triacetate) are used in photographic film, as a textile fiber (acetate rayon), and several have become important as thermoplastic materials. Cellulose nitrate, called celluloid pyroxylin, forms celluloid, dynamite cotton, and gun cotton. Cordite and ballistite are made from gun cotton. Dimethyl and diethyl sulfates (esters of sulfuric acid) are excellent agents for alkylating organic molecules that contain labile hydrogen atoms, for example, starch and cellulose.

Esters of unsaturated acids, for example, acrylic or methacrylic acid, are reactive and polymerize rapidly, yielding resins; thus, methyl methacrylate yields a polymethyl methacrylate resin (Lucite). Analogously, esters of unsaturated alcohols are reactive and readily react with themselves; thus, vinyl acetate polymerizes to polyvinyl acetate.

Many low-molecular-weight esters have characteristic, fruit-like odors: banana (isoamyl acetate), rum (isobutyl propionate), and pineapple (butyl butyrate). These esters are used to some extent in compounding synthetic flavors and perfumes. See also Alcohol; Carboxylic acid; Fat and oil; Polyester resins.


 
Dental Dictionary: ester
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(es′tur)
n

Any compound formed from alcohol and an acid.

 

Any of a class of organic compounds that can react with water (see hydrolysis) to produce an alcohol and an organic or inorganic acid. They are formed by the reverse process, esterification, in which acid reacts with alcohol to form an ester and water. Esters of carboxylic acids, the most common esters, contain the acid's carbonyl group (-C=O; see functional group); the carbon's fourth bond is with the alcohol's oxygen atom. Hydrolysis of esters in the presence of an alkali (saponification) is used to make soaps from fats and oils. Carboxylic acid esters of low molecular weight are colourless, volatile liquids with pleasant odours; they give flavour and fragrance to fruits and flowers and are used as synthetic flavours and fragrances. Others, such as ethyl acetate and butyl acetate, are used as solvents for lacquers, paints, and varnishes. Certain polymers are esters, including Lucite (polymethyl methacrylate) and Dacron (polyethylene terephthalate). Esters of alcohols and inorganic acids include nitrate esters (e.g., nitroglycerin), which are explosive; phosphate esters, including such biologically important compounds as nucleic acids; and others that are used as flame retardants, solvents, plasticizers, gasoline and oil additives, and insecticides.

For more information on ester, visit Britannica.com.

 
ester, any one of a group of organic compounds with general formula RCO2R′ (where R and R′ are alkyl groups or aryl groups) that are formed by the reaction between an alcohol and an acid. For example, when ethanol and acetic acid react, ethyl acetate (an ester) and water are formed; the reaction is called esterification. Ethyl acetate is used as a solvent. Methyl acetate, formed by the reaction between methanol and acetic acid, is a sweet-smelling liquid used in making perfumes, extracts, and lacquers. Esters react with water (hydrolysis) under basic conditions to form an alcohol and an acid. When heated with a hydroxide certain esters decompose to yield soap and glycerin; the process is called saponification. Common fats and oils are mixtures of various esters, such as stearin, palmitin, and linolein, formed from the alcohol glycerol and fatty acids. Naturally occurring esters of organic acids in fruits and flowers give them their distinctive odors. Esters perform important functions in the animal body; e.g., the ester acetylcholine is a chemical transmitter of nerve stimuli.


 

Chemical combination of an alcohol and an acid. In domestic animals the most common linkage is glycerol with fatty acid to form glycerides.

 
Wikipedia: Ester
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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.

Contents

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]

Image:Ethylethanoate.png

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

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 is in equilibrium with 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

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.

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.

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

External links

References

  1. ^ International Union of Pure and Applied Chemistry. "esters". Compendium of Chemical Terminology Internet edition.
  2. ^ IUPAC parent groups using traditional names
  3. ^ IUPAC naming of esters
  4. ^ http://www.acdlabs.com/iupac/nomenclature/93/r93_511.htm
  5. ^ Damsté JS, Rijpstra WI, Hopmans EC, Schouten S, Balk M, Stams AJ (December 2007). "Structural characterization of diabolic acid-based tetraester, tetraether and mixed ether/ester, membrane-spanning lipids of bacteria from the order Thermotogales". Arch Microbiol. 188 (6): 629–41. doi:10.1007/s00203-007-0284-z. PMID 17643227. 
  6. ^ Najafi A, Fawcett HD, Hutchison N (August 1986). "Sarcoplasmic reticulum interacts with the Ca(2+) indicator precursor fura-2-am". Biochem Biophys Res Commun. 138 (3): 1153–62. doi:10.1016/S0006-291X(86)80403-X. PMID 3755905. 
  7. ^ Highsmith S, Bloebaum P, Snowdowne KW (1986). "Comparison of two methods of labeling proteins with 111In". Int J Rad Appl Instrum B. 13 (4): 345–6. PMID 3539883. 
  8. ^ Chan WH, Lee AW, Kwong DW, Tam WL, Wang KM (April 1996). "Potassium ion-selective optodes based on the calix[6]arene hexaester and application in human serum assay". Analyst. 121 (4): 531–4. doi:10.1039/an9962100531. PMID 8633794. 
  9. ^ Evanics F, Hohmann J, Rédei D, Vasas A, Günther G, Dombi G (October 2001). "[New diterpene polyesters isolated from Hungarian Euphorbia species] [Article in Hungarian]". Acta Pharm Hung. 71 (3): 289–92. PMID 11961895. 
  10. ^ Dueno EE, Bisht KS (April 2004). "Intramolecular inclusion in novel octaester cavitands". Chem Commun (Camb). (8): 954–5. doi:10.1039/b316498e. PMID 15069491. 
  11. ^ Xian M, Fuerst MM, Shabalin Y, Reusch RN (November 2007). "Sorting signal of Escherichia coli OmpA is modified by oligo-(R)-3-hydroxybutyrate". Biochim Biophys Acta. 1768 (11): 2660–6. doi:10.1016/j.bbamem.2007.06.019. PMID 17659252. 
  12. ^ W. Reusch. "Carboxyl Derivative Reactivity". Virtual Textbook of Organic Chemistry. http://www.cem.msu.edu/~reusch/VirtualText/crbacid2.htm#react2. 

 
 

 

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