Inchemistry, anester is acompound derived from anacid (organic or inorganic) in which thehydrogen atom (H) of at least oneacidichydroxyl group (−OH) of that acid is replaced by anorganyl group (R′).[1] These compounds contain adistinctive functional group. Analogues derived fromoxygen replaced by otherchalcogens belong to the ester category as well.[1] According to some authors, organyl derivatives of acidic hydrogen of other acids are esters as well (e.g.amides), but not according to theIUPAC.[1]
Glycerides arefatty acid esters ofglycerol; they are important in biology, being one of the main classes oflipids and comprising the bulk ofanimal fats andvegetable oils.Lactones are cyclic carboxylic esters; naturally occurring lactones are mainly 5- and 6-membered ring lactones. Lactones contribute to the aroma of fruits, butter, cheese,vegetables likecelery and other foods.
There are compounds in which an acidic hydrogen of acids mentioned in this article are not replaced by an organyl, but by some other group. According to some authors, those compounds are esters as well, especially when the first carbon atom of the organyl group replacing acidic hydrogen, is replaced by another atom from thegroup 14 elements (Si,Ge,Sn,Pb); for example, according to them,trimethylstannyl acetate (or trimethyltin acetate)CH3COOSn(CH3)3 is atrimethylstannyl ester ofacetic acid, anddibutyltin dilaurate(CH3(CH2)10COO)2Sn((CH2)3CH3)2 is adibutylstannylene ester oflauric acid, and thePhillips catalystCrO2(OSi(OCH3)3)2 is a trimethoxysilyl ester ofchromic acid (H2CrO4).[4][5]
The names of esters that are formed from an alcohol and an acid, are derived from the parent alcohol and the parent acid, where the latter may be organic or inorganic. Esters derived from the simplestcarboxylic 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 hexylcaprylate, has the formulaCH3(CH2)6CO2(CH2)5CH3.
Butyl acetate, an ester derived from a residue ofbutanol (CH3CH2CH2CH2OH) (the butanol residue isbutyl group−CH2CH2CH2CH3) (right side of the picture, blue) andacetic acidCH3CO2H (left side of the picture, orange). Theacidichydrogen atom (−H) from acetic acidmolecule is replaced by the butyl group.
The chemical formulas of organic esters formed from carboxylic acids and alcohols usually take the formRCO2R' or RCOOR', where R and R' are theorganyl parts of the carboxylic acid and the alcohol, respectively, and R can be ahydrogen in the case of esters offormic acid. For example,butyl acetate (systematically butyl ethanoate), derived frombutanol andacetic acid (systematically ethanoic acid) would be writtenCH3CO2(CH2)3CH3. Alternative presentations are common including BuOAc andCH3COO(CH2)3CH3.
Cyclic esters are calledlactones, regardless of whether they are derived from an organic or inorganic acid. One example of an organic lactone isγ-valerolactone.
An uncommon class of esters are theorthoesters. One of them are the esters of orthocarboxylic acids. Those esters have the formulaRC(OR′)3, where R stands for any group (organic or inorganic) and R′ stands fororganyl group. For example,triethyl orthoformate (HC(OCH2CH3)3) is derived, in terms of its name (but not its synthesis) fromesterification oforthoformic acid (HC(OH)3) withethanol.
Esters derived fromcarboxylic acids andalcohols contain acarbonyl group C=O, which is adivalent group atC atom, which gives rise to 120° C–C–O and O–C–O angles. Unlikeamides, carboxylic acid 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 correspondingamides.[7] ThepKa of the alpha-hydrogens on esters of carboxylic acids is around 25 (alpha-hydrogen is a hydrogen bound to the carbon adjacent to thecarbonyl group (C=O) of carboxylate esters).[8]
Many carboxylic acid esters have the potential forconformational isomerism, but they tend to adopt anS-cis (orZ) conformation rather than theS-trans (orE) alternative, due to a combination ofhyperconjugation and dipole minimization effects. The preference for theZ conformation is influenced by the nature of the substituents and solvent, if present.[9][10]Lactones with small rings are restricted to thes-trans (i.e.E) conformation due to their cyclic structure.
Esters derived fromcarboxylic acids and alcohols are more polar thanethers but less polar than alcohols. They participate inhydrogen 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.[7]
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 toνC=O. This peak changes depending on the functional groups attached to the carbonyl. For example, a benzene ring or double bond in conjunction with the carbonyl will bring the wavenumber down about 30 cm−1.
Esterification is the general name for achemical reaction in which two reactants (typically an alcohol and an acid) form an ester as thereaction product. Esters are common in organic chemistry and biological materials, and often have a pleasant characteristic, fruity odor. This leads to their extensive use in thefragrance andflavor industry. Ester bonds are also found in manypolymers.
The classic synthesis is theFischer esterification, which involves treating a carboxylic acid with an alcohol in the presence of adehydrating agent:
RCO2H + R'OH ⇌ RCO2R' + H2O
The equilibrium constant for such reactions is about 5 for typical esters, e.g., ethyl acetate.[15] 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 aspolymeric sulfonic acids. Since esterification is highly reversible, the yield of the ester can be improved usingLe 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 asmolecular sieves are also effective.
Reagents are known that drive the dehydration of mixtures of alcohols and carboxylic acids. One example is theSteglich esterification, which is a method of forming esters under mild conditions. The method is popular inpeptide synthesis, where the substrates are sensitive to harsh conditions like high heat. DCC (dicyclohexylcarbodiimide) is used to activate the carboxylic acid to further reaction.4-Dimethylaminopyridine (DMAP) is used as an acyl-transfercatalyst.[16]
Another method for the dehydration of mixtures of alcohols and carboxylic acids is theMitsunobu reaction:
Carboxylic acids can be esterified usingdiazomethane:
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 bygas chromatography. The method is useful in specialized organic synthetic operations but is considered too hazardous and expensive for large-scale applications.
The reactions are irreversible simplifyingwork-up. Since acyl chlorides and acid anhydrides also react with water, anhydrous conditions are preferred. The analogous acylations of amines to giveamides are less sensitive because amines are strongernucleophiles and react more rapidly than does water. This method is employed only for laboratory-scale procedures, as it is expensive.
Although rarely employed for esterifications, carboxylate salts (often generatedin situ) react withelectrophilicalkylating agents, such asalkyl halides, to give esters.[14][18] Anion availability can inhibit this reaction, which correspondingly benefits fromphase transfer catalysts or such highly polaraprotic solvents asDMF. An additional iodide salt may, via theFinkelstein reaction, catalyze the reaction of a recalcitrant alkyl halide. Alternatively, salts of a coordinating metal, such as silver, may improve the reaction rate by easing halide elimination.
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 degradingtriglycerides, e.g. in the production of fatty acid esters and alcohols.Poly(ethylene terephthalate) is produced by the transesterification ofdimethyl terephthalate and ethylene glycol:[14]
n (C6H4)(CO2CH3)2 + 2n C2H4(OH)2 → [(C6H4)(CO2)2(C2H4)]n + 2n CH3OH
A subset of transesterification is the alcoholysis ofdiketene. This reaction affords 2-ketoesters.[14]
Esters are less reactive than acid halides and anhydrides. As with more reactive acyl derivatives, they can react withammonia and primary and secondary amines to give amides, although this type of reaction is not often used, since acid halides give better yields.
Esters can be converted to other esters in a process known astransesterification. Transesterification can be either acid- or base-catalyzed, and involves the reaction of an ester with an alcohol. Unfortunately, because the leaving group is also an alcohol, the forward and reverse reactions will often occur at similar rates. Using a large excess of thereactant alcohol or removing the leaving group alcohol (e.g. viadistillation) will drive the forward reaction towards completion, in accordance withLe Chatelier's principle.[24]
Acid-catalyzed hydrolysis of esters is also an equilibrium process – essentially the reverse of theFischer esterification reaction. Because an alcohol (which acts as the leaving group) and water (which acts as the nucleophile) have similar pKa values, the forward and reverse reactions compete with each other. As in transesterification, using a large excess of reactant (water) or removing one of the products (the alcohol) can promote the forward reaction.
The acid-catalyzed hydrolysis of an ester and Fischer esterification correspond to two directions of an equilibrium process.
Basic hydrolysis of esters, known assaponification, is not an equilibrium process; a full equivalent of base is consumed in the reaction, which produces one equivalent of alcohol and one equivalent of a carboxylate salt. The saponification of esters offatty acids is an industrially important process, used in the production of soap.[24]
Esterification is a reversible reaction. Esters undergohydrolysis under acidic and basic conditions. Under acidic conditions, the reaction is the reverse reaction of theFischer 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 asammonia or primary or secondaryamines to giveamides (ammonolysis reaction):
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 toisocyanates through intermediatehydroxamic acids in theLossen rearrangement.
Sources of carbon nucleophiles, e.g.,Grignard reagents and organolithium compounds, add readily to the carbonyl.
Compared to ketones and aldehydes, esters arerelatively 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 tofatty alcohols.
Especially for fine chemical syntheses,lithium aluminium hydride is used to reduce esters to two primary alcohols. The related reagentsodium borohydride is slow in this reaction.DIBAH reduces esters to aldehydes.[25]
Direct reduction to give the correspondingether is difficult as the intermediatehemiacetal tends to decompose to give an alcohol and an aldehyde (which is rapidly reduced to give a second alcohol). The reaction can be achieved usingtriethylsilane with a variety of Lewis acids.[26][27]
Esters can undergo a variety of reactions with carbon nucleophiles. They react with an excess of a Grignard reagent to give tertiary alcohols. Esters also react readily withenolates. In theClaisen condensation, an enolate of one ester (1) will attack the carbonyl group of another ester (2) to give tetrahedral intermediate3. The intermediate collapses, forcing out an alkoxide (R'O−) and producing β-keto ester4.
The Claisen condensation involves the reaction of an ester enolate and an ester to form a beta-keto ester.
Crossed Claisen condensations, in which the enolate and nucleophile are different esters, are also possible. Anintramolecular Claisen condensation is called aDieckmann condensation or Dieckmann cyclization, since it can be used to form rings. Esters can also undergo condensations with ketone and aldehyde enolates to give β-dicarbonyl compounds.[28] A specific example of this is theBaker–Venkataraman rearrangement, in which an aromaticortho-acyloxy ketone undergoes an intramolecular nucleophilic acyl substitution and subsequent rearrangement to form an aromatic β-diketone.[29] TheChan rearrangement is another example of a rearrangement resulting from an intramolecular nucleophilic acyl substitution reaction.
Esters react with nucleophiles at the carbonyl carbon.[30] The carbonyl is weakly electrophilic but is attacked by strong nucleophiles (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 in esters is weakly basic, less so than the carbonyl oxygen in amides due to resonance donation of an electron pair from nitrogen in amides, but formsadducts.
As foraldehydes, 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 asalkoxides. Deprotonation gives a nucleophilicenolate, which can further react, e.g., theClaisen condensation and its intramolecular equivalent, theDieckmann condensation. This conversion is exploited in themalonic ester synthesis, wherein the diester ofmalonic acid reacts with an electrophile (e.g.,alkyl halide), and is subsequently decarboxylated. Another variation is theFráter–Seebach alkylation.
As a class, esters serve asprotecting groups forcarboxylic acids. Protecting a carboxylic acid is useful in peptide synthesis, to prevent self-reactions of the bifunctionalamino acids. Methyl and ethyl esters are commonly available for many amino acids; thet-butyl ester tends to be more expensive. However,t-butyl esters are particularly useful because, under strongly acidic conditions, thet-butyl esters undergo elimination to give the carboxylic acid andisobutylene, simplifying work-up.
Many esters have distinctive fruit-like odors, and many occur naturally in the essential oils of plants. This has also led to their common use in artificial flavorings and fragrances which aim to mimic those odors.[32]
Depside, a polymeric ester, a type ofpolyphenolic compound composed of two or more monocyclicaromatic units linked by an ester group
Depsipeptide, a type of ester that is apeptide in which one or more of itsamide groups (−C(=O)−NH−) are replaced by the corresponding ester groups (−C(=O)−O−)[33]
^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.
^abMarch, J.Advanced Organic Chemistry 4th Ed. J. Wiley and Sons, 1992: New York.ISBN0-471-60180-2.
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^Isolation of triglyceride from nutmeg: G. D. Beal "Trimyristen" Organic Syntheses, Coll. Vol. 1, p.538 (1941).Link
^McGee, Harold.On Food and Cooking. 2003, Scribner, New York.
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^Raber, Douglas J.; Gariano, Jr, Patrick; Brod, Albert O.; Gariano, Anne L.; Guida, Wayne C. (1977). "Esterification of Carboxylic Acids with Trialkyloxonium Salts: Ethyl and Methyl 4-Acetoxybenzoates".Organic Syntheses.56: 59.doi:10.15227/orgsyn.056.0059.
^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.
^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.
^Yato, Michihisa; Homma, Koichi; Ishida, Akihiko (June 2001). "Reduction of carboxylic esters to ethers with triethyl silane in the combined use of titanium tetrachloride and trimethylsilyl trifluoromethanesulfonate".Tetrahedron.57 (25):5353–5359.doi:10.1016/S0040-4020(01)00420-3.
^Sakai, Norio; Moriya, Toshimitsu; Konakahara, Takeo (July 2007). "An Efficient One-Pot Synthesis of Unsymmetrical Ethers: A Directly Reductive Deoxygenation of Esters Using an InBr3/Et3SiH Catalytic System".The Journal of Organic Chemistry.72 (15):5920–5922.doi:10.1021/jo070814z.PMID17602594.
^Wood, J. L.; Khatri, N. A.; Weinreb, S. M. (1979). "A direct conversion of esters to nitriles".Tetrahedron Letters.20 (51): 4907.doi:10.1016/S0040-4039(01)86746-0.
^Panten, Johannes; Surburg, Horst (2015). "Flavors and Fragrances, 2. Aliphatic Compounds".Ullmann's Encyclopedia of Industrial Chemistry. pp. 1–55.doi:10.1002/14356007.t11_t01.ISBN978-3-527-30673-2.
