Ester
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Esters are chemical compounds derived formally from a petrol (one containing an oxo group, X=O); and a hydrocarbon compound, e.g. an alcohol or phenol.[1] Esters consist of an inorganic or organic acid in which at least fifty -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; low molecular weight esters 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, which are the most common.
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[edit] 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, e.g., formate, acetate, propionate, 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.
[edit] 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].
[edit] 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:
[edit] 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 e.g. 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.
[edit] Preparation
[edit] Fischer esterification
The most classic method is the Fischer esterification: refluxing a carboxylic acid in an alcohol, which acts as both solvent and reactant:
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.
[edit] 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.
[edit] 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
[edit] Reactions
Esters may react primarily at two locations: at the carboxyl, and at the carbon adjacent the carbonxyl group.
[edit] 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.
[edit] 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]
[edit] 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.
[edit] 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.
[edit] 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
[edit] References
- ^ International Union of Pure and Applied Chemistry. "esters". Compendium of Chemical Terminology Internet edition.
- ^ IUPAC parent groups using traditional names
- ^ IUPAC naming of esters
- ^ http://www.acdlabs.com/iupac/nomenclature/93/r93_511.htm
- ^ Damsté JS, Rijpstra WI, Hopmans EC, Schouten S, Balk M, Stams AJ (Dec 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: . PMID 17643227.
- ^ Najafi A, Fawcett HD, Hutchison N (Aug 1986). "Sarcoplasmic reticulum interacts with the Ca(2+) indicator precursor fura-2-am". Biochem Biophys Res Commun. 138 (3): 1153–62. doi: . PMID 3755905.
- ^ 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.
- ^ Chan WH, Lee AW, Kwong DW, Tam WL, Wang KM (Apr 1996). "Potassium ion-selective optodes based on the calix[6]arene hexaester and application in human serum assay". Analyst. 121 (4): 531–4. doi: . PMID 8633794.
- ^ Evanics F, Hohmann J, Rédei D, Vasas A, Günther G, Dombi G (Oct 2001). "[New diterpene polyesters isolated from Hungarian Euphorbia species] [Article in Hungarian]". Acta Pharm Hung. 71 (3): 289–92. PMID 11961895.
- ^ Dueno EE, Bisht KS (Apr 2004). "Intramolecular inclusion in novel octaester cavitands". Chem Commun (Camb). (8): 954–5. doi: . PMID 15069491.
- ^ Xian M, Fuerst MM, Shabalin Y, Reusch RN (Nov 2007). "Sorting signal of Escherichia coli OmpA is modified by oligo-(R)-3-hydroxybutyrate". Biochim Biophys Acta. 1768 (11): 2660–6. doi: . PMID 17659252.
- ^ W. Reusch. "Carboxyl Derivative Reactivity". Virtual Textbook of Organic Chemistry. http://www.cem.msu.edu/~reusch/VirtualText/crbacid2.htm#react2.
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