Ester

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A carboxylate ester. R and R′ denote organyl groups, or H in the case of R.

In chemistry, an ester is a compound derived from an acid (organic or inorganic) in which at least one acidic hydrogen atom (−H) of that acid is replaced by an organyl group (−R).[1][2] An example of an ester formation is the substitution reaction of a carboxylic acid (RC(=O)−O−H) and an alcohol (R'OH), forming an ester (RC(=O)−O−R'), where R and R′ denote organyl groups, or H in the case of R. Glycerides are fatty acid esters of glycerol; they are important in biology, being one of the main classes of lipids and comprising the bulk of animal fats and vegetable oils. Esters can be formed from oxoacids (e.g. esters of acetic acid, carbonic acid, sulfuric acid, phosphoric acid, nitric acid, xanthic acid), but also from acids that do not contain oxygen (e.g. esters of thiocyanic acid, trithiocarbonic acid, tricyanomethane).

Esters of organic oxoacids typically have a pleasant smell; those of low molecular weight are commonly used as fragrances and are found in essential oils and pheromones. They perform as high-grade solvents for a broad array of plastics, plasticizers, resins, and lacquers,[3] and are one of the largest classes of synthetic lubricants on the commercial market.[4] Polyesters are important plastics, with monomers linked by ester moieties. Esters of phosphoric acid form the backbone of DNA molecules. Esters of nitric acid, such as nitroglycerin, are known for their explosive properties.

Nomenclature[edit]

Etymology[edit]

The word ester was coined in 1848 by a German chemist Leopold Gmelin,[5] probably as a contraction of the German Essigäther, "acetic ether".

IUPAC nomenclature[edit]

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 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.

Butyl acetate, an ester derived from butanol (right side of the picture, blue) and acetic acid (left side of the picture, orange). The acidic hydrogen atom from acetic acid is replaced by a butyl group.

The chemical formulas of organic esters formed from carboxylic acids and alcohols usually take the form RCO2R' or RCOOR', where R and R' are the organyl parts of the carboxylic acid and the alcohol, respectively, and R can be a hydrogen in the case of esters of formic acid. For example, butyl acetate (systematically butyl ethanoate), derived from butanol and acetic acid (systematically ethanoic acid) would be written CH3CO2(CH2)3CH3. Alternative presentations are common including BuOAc and CH3COO(CH2)3CH3.

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

Orthoesters[edit]

An uncommon class of organic esters are the orthoesters. One of them are orthocarboxylic acids, which have the formula RC(OR′)3, where R and R' stand for hydrogen or any organyl group. For example, triethyl orthoformate (HC(OCH2CH3)3) is derived, in terms of its name (but not its synthesis) from esterification of orthoformic acid (HC(OH)3) with ethanol.

Esters of inorganic acids[edit]

A phosphoric acid ester, where R stands for an organyl group.

Esters can also be derived from inorganic acids.

Inorganic acids that exist as tautomers form diverse esters.

Some inorganic acids that are unstable or elusive form stable esters.

In principle, all metal and metalloid alkoxides, of which many hundreds are known, could be classified as esters of the hypothetical acids, e.g. aluminium triethoxide (Al(OCH2CH3)3) could be classified as an ester of aluminic acid which is aluminium hydroxide, tetraethyl orthosilicate (Si(OCH2CH3)4) could be classified as an ester of orthosilicic acid, and titanium ethoxide (Ti(OCH2CH3)4) could be classified as an ester of orthotitanic acid.

Structure and bonding[edit]

Esters derived from carboxylic acids and alcohols contain a carbonyl group C=O, which is a divalent group at C atom, which gives rise to 120° C–C–O and O–C–O angles. Unlike amides, 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 corresponding amides.[6] The pKa of the alpha-hydrogens on esters is around 25.[7]

Many carboxylic acid esters have the potential for conformational isomerism, but they tend to adopt an S-cis (or Z) conformation rather than the S-trans (or E) alternative, due to a combination of hyperconjugation and dipole minimization effects. The preference for the Z conformation is influenced by the nature of the substituents and solvent, if present.[8][9] Lactones with small rings are restricted to the s-trans (i.e. E) conformation due to their cyclic structure.

Ester conformers.png
Metrical details for methyl benzoate, distances in picometers.[10]

Physical properties and characterization[edit]

Esters derived from carboxylic acids and alcohols 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.[6]

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.[11] Esters are responsible for the aroma of many fruits, including apples, durians, pears, bananas, pineapples, and strawberries.[12] Several billion kilograms of polyesters are produced industrially annually, important products being polyethylene terephthalate, acrylate esters, and cellulose acetate.[13]

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

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 pleasant characteristic, 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 with alcohols[edit]

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

The equilibrium constant for such reactions is about 5 for typical esters, e.g., ethyl acetate.[14] 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. 4-Dimethylaminopyridine (DMAP) is used as an acyl-transfer catalyst.[15]

Steglich-1.svg

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

Carboxylic acids can be esterified using diazomethane:

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 hazardous and expensive for large-scale applications.

Esterification of carboxylic acids with epoxides[edit]

Carboxylic acids are esterified by treatment with epoxides, giving β-hydroxyesters:

This reaction is employed in the production of vinyl ester resin from acrylic acid.

Alcoholysis of acyl chlorides and acid anhydrides[edit]

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

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.[13] 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.[16] 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:

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:[13]

A subset of transesterification is the alcoholysis of diketene. This reaction affords 2-ketoesters.[13]

Carbonylation[edit]

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

A preparation of methyl propionate is one illustrative example.

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

Addition of carboxylic acids to alkenes and alkynes[edit]

In hydroesterification, alkenes and alkynes insert into the H-O bond of carboxylic acids. Vinyl acetate is produced industrially by the addition of acetic acid to acetylene in the presence of zinc acetate catalysts:[17] Presently, zinc acetate is used as the catalyst:

Vinyl acetate can also be produced by palladium-catalyzed reaction of ethylene, acetic acid, and oxygen:

Silicotungstic acid is used to manufacture ethyl acetate by the alkylation of acetic acid by ethylene:

From aldehydes[edit]

The Tishchenko reaction involve disproportionation of an aldehyde in the presence of an anhydrous base to give an ester. Catalysts are aluminium alkoxides or sodium alkoxides. Benzaldehyde reacts with sodium benzyloxide (generated from sodium and benzyl alcohol) to generate benzyl benzoate.[18] The method is used in the production of ethyl acetate from acetaldehyde.[13]

Other methods[edit]

Reactions[edit]

Esters react with nucleophiles at the carbonyl carbon. 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 forms adducts.

Hydrolysis and saponification[edit]

Esterification is a reversible reaction. Esters undergo hydrolysis under acidic 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: (ammonolysis reaction)

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.

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.[22]

Direct reduction to give the corresponding ether is difficult as the intermediate hemiacetal tends to decompose to give an alcohol and an aldehyde (which is rapidly reduced to give a second alcohol). The reaction can be achieved using triethylsilane with a variety of Lewis acids.[23][24]

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.

Hazards[edit]

Esters react with strong oxidizing acids, which may cause a violent reaction that is sufficiently exothermic to ignite the esters and the reaction products. Heat is also generated by the interaction of esters with alkali solutions. Very flammable hydrogen gas is generated by mixing esters with alkali metals and hydrides.[26]

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 common use in artificial flavorings and fragrances which aim to mimic those odors.

Ester name Structure 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 propionate Butyl propionate.png pear drops, apple
Ethyl acetate Ethyl acetate2.svg nail polish remover, model paint, model airplane glue, pears
Ethyl benzoate Ethyl benzoate.svg sweet, wintergreen, fruity, medicinal, cherry, grape
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 Structural Formulae.svg lemon, rum, strawberry
Ethyl heptanoate Ethyl-heptanoate.svg apricot, cherry, grape, raspberry
Ethyl isovalerate Ethyl isovalerate structure.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 formate Methyl formate.png pleasant, ethereal, rum, sweet
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". doi:10.1351/goldbook.E02219
  2. ^ https://www.toppr.com/guides/chemistry/alcohols-phenols-and-ethers/ester/
  3. ^ Cameron Wright (1986). A worker's guide to solvent hazards. The Group. p. 48. ISBN 9780969054542.
  4. ^ E. Richard Booser (21 December 1993). CRC Handbook of Lubrication and Tribology, Volume III: Monitoring, Materials, Synthetic Lubricants, and Applications. CRC. p. 237. ISBN 978-1-4200-5045-5.
  5. ^ 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.

  6. ^ a b March, J. Advanced Organic Chemistry 4th Ed. J. Wiley and Sons, 1992: New York. ISBN 0-471-60180-2.
  7. ^ "Chemistry of Enols and Enolates – Acidity of alpha-hydrogens".
  8. ^ Diwakar M. Pawar; Abdelnaser A. Khalil; Denise R. Hooks; Kenneth Collins; Tijuana Elliott; Jefforey Stafford; Lucille Smith; Eric A. Noe (1998). "E and Z Conformations of Esters, Thiol Esters, and Amides". J. Am. Chem. Soc. 120 (9): 2108–2112. doi:10.1021/ja9723848.
  9. ^ Christophe Dugave; Luc Demange (2003). "Cis−Trans Isomerization of Organic Molecules and Biomolecules: Implications and Applications". Chem. Rev. 103 (7): 2475–2932. doi:10.1021/cr0104375. PMID 12848578.
  10. ^ A. A. Yakovenko, J. H. Gallegos, M. Yu. Antipin, A. Masunov, T. V. Timofeeva (2011). "Crystal Morphology as an Evidence of Supramolecular Organization in Adducts of 1,2-Bis(chloromercurio)tetrafluorobenzene with Organic Esters". Cryst. Growth Des. 11 (9): 3964–3978. doi:10.1021/cg200547k.{{cite journal}}: CS1 maint: uses authors parameter (link)
  11. ^ Isolation of triglyceride from nutmeg: G. D. Beal "Trimyristen" Organic Syntheses, Coll. Vol. 1, p.538 (1941). Link
  12. ^ McGee, Harold. On Food and Cooking. 2003, Scribner, New York.
  13. ^ a b c d e Riemenschneider, Wilhelm; Bolt, Hermann M. "Esters, Organic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a09_565.pub2.
  14. ^ Williams, Roger J.; Gabriel, Alton; Andrews, Roy C. (1928). "The Relation Between the Hydrolysis Equilibrium Constant of Esters and the Strengths of the Corresponding Acids". J. Am. Chem. Soc. 50 (5): 1267–1271. doi:10.1021/ja01392a005.
  15. ^ B. Neises & W. Steglich. "Esterification of Carboxylic Acids with Dicyclohexylcarbodiimide/4-Dimethylaminopyridine: tert-Butyl ethyl fumarate". Organic Syntheses.; Collective Volume, vol. 7, p. 93
  16. ^ Matsumoto, Kouichi; Shimazaki, Hayato; Miyamoto, Yu; Shimada, Kazuaki; Haga, Fumi; Yamada, Yuki; Miyazawa, Hirotsugu; Nishiwaki, Keiji; Kashimura, Shigenori (2014). "Simple and Convenient Synthesis of Esters from Carboxylic Acids and Alkyl Halides Using Tetrabutylammonium Fluoride". Journal of Oleo Science. 63 (5): 539–544. doi:10.5650/jos.ess13199. ISSN 1345-8957. PMID 24770480.
  17. ^ Bienewald, Frank; Leibold, Edgar; Tužina, Pavel; Roscher, Günter (2019). "Vinyl Esters". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. pp. 1–16. doi:10.1002/14356007.a27_419.pub2.
  18. ^ Kamm, O.; Kamm, W. F. (1922). "Benzyl benzoate". Organic Syntheses. 2: 5. doi:10.15227/orgsyn.002.0005.; Collective Volume, vol. 1, p. 104
  19. ^ 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. S2CID 101737591.
  20. ^ 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.
  21. ^ 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.
  22. ^ W. Reusch. "Carboxyl Derivative Reactivity". Virtual Textbook of Organic Chemistry. Archived from the original on 2016-05-16.
  23. ^ 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.
  24. ^ 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. PMID 17602594.
  25. ^ 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.
  26. ^ https://pubchem.ncbi.nlm.nih.gov/

External links[edit]