Esters are chemical compounds A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together in a defined spatial arrangement by chemical bonds derived by reacting an oxoacid Under Lavoisier's original theory, all acids contained oxygen, which was named from the Greek οξυς (acid, sharp) and γεινομαι (geinomai) (engender). It was later discovered that some acids, notably hydrochloric acid, did not contain oxygen and so acids were divided into oxoacids and these new hydracids (one containing an oxo group, X=O) with a hydroxyl In chemistry, hydroxyl is a compound containing an oxygen atom bound covalently with a hydrogen atom. The neutral form of this group is a hydroxyl radical. The hydroxyl anion is called hydroxide; it is a diatomic ion with a single negative electronic charge. In discussions of synthetic methods in organic chemistry the hydroxyl group (–OH) is compound such as an alcohol In chemistry, an alcohol is any organic compound in which a hydroxyl functional group (-O or phenol In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of a hydroxyl group (-O .^[1] Esters are usually derived from an inorganic acid A mineral acid is an acid derived from one or more inorganic compounds. A mineral acid is not organic and all mineral acids release hydrogen ions when dissolved in water or organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids whose acidity is associated with their carboxyl group -COOH. Sulfonic acids, containing the group -SO2OH, are relatively stronger acids. The relative stability of the conjugate base of the acid determines its acidity. Other groups in which at least one -OH (hydroxyl) group is replaced by an -O-alkyl In chemistry, an alkyl group is an alkane missing one hydrogen. It has the general formula CnH2n+1. Typically an alkyl is a part of a larger molecule. In structural formulae, the symbol R is used to designate a generic alkyl group. The smallest alkyl group is methyl, with the formula CH3— (alkoxy In chemistry, the alkoxyl group is an alkyl group linked to oxygen thus: R-O. The range of alkoxy groups is great, the simplest being methoxy (-OCH3). An ethoxy group (-OCH2CH3) is found in the organic compound phenetol, C6H5OCH2CH3 which is also known as ethoxy benzene. Related to alkoxy groups are aryloxy groups) group, and most commonly from carboxylic acids Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. A carboxyl group is a functional group consisting of a carbonyl and a hydroxyl, which has the formula -C(=O)OH, usually written -COOH or -CO2H. Carboxylic acids are Brønsted-Lowry acids — they are proton donors. Salts and anions of carboxylic acids and alcohols In chemistry, an alcohol is any organic compound in which a hydroxyl functional group (-O . Basically, esters are formed by condensing A condensation reaction is a chemical reaction in which two molecules or moieties combine to form one single molecule, together with the loss of a small molecule. When this small molecule is water, it is known as a dehydration reaction; other possible small molecules lost are hydrogen chloride, methanol, or acetic acid. The word "condensation& an acid with an alcohol.

Esters are ubiquitous. Many naturally occurring fats and oils are the fatty acid In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. The most occurring natural fatty acids have an even number of carbon atoms because their biosynthesis involves acetyl-CoA, a coenzyme carrying a two-carbon-atom group (see fatty acid synthesis) esters of glycerol Glycerol is an organic compound, also called glycerin or glycerine. It is a colourless, odourless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydrophilic hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol substructure is a central component of many. Esters with low molecular weight are commonly used as fragrances and found in essential oils An essential oil is a concentrated, hydrophobic liquid containing volatile aroma compounds from plants. Essential oils are also known as volatile, ethereal oils or aetherolea, or simply as the "oil of" the plant from which they were extracted, such as oil of clove. An oil is "essential" in the sense that it carries a and pheromones A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual. There are alarm pheromones, food trail pheromones, sex pheromones, and many others that. Phosphoesters A phosphodiester bond is a group of strong covalent bonds between a phosphate group and two 5-carbon ring carbohydrates over two ester bonds. Phosphodiester bonds are central to all life on Earth, as they make up the backbone of the strands of DNA. In DNA and RNA, the phosphodiester bond is the linkage between the 3' carbon atom of one sugar form the backbone of DNA Deoxyribonucleic acid ( /diːˌɒksɨˌraɪbɵ.nuːˈkleɪ.ɪk ˈæsɪd/ (help·info)) (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information. DNA is often compared to a set of molecules. Nitrate esters, such as nitroglycerin Nitroglycerin , (United States spelling) also known as nitroglycerine, (UK spelling), trinitroglycerin, trinitroglycerine, 1,2,3-trinitroxypropane and glyceryl trinitrate, is a heavy, colorless, oily, explosive liquid obtained by nitrating glycerol. Since the 1860s, it has been used as an active ingredient in the manufacture of explosives,, are known for their explosive properties, while polyesters Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate . Polyesters include naturally-occurring chemicals, such as in the cutin of plant cuticles, as well as are important plastics A plastic material is any of a wide range of synthetic or semi-synthetic organic amorphous solids[citation needed] used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce costs. Monomers of plastic are either natural or synthetic, with monomers linked by ester moieties.

Nomenclature

The names for esters are derived from the names of the parent alcohol and the carboxylic acid. For esters derived from the simplest carboxylic acids, the traditional names are used, such as formate, acetate, propionate, and butyrate. For esters from more complex carboxylic acids, the systematic name for the acid is used, followed by the suffix -oate. For example, hexyl octanoate, also called hexyl caprylate Caprylic acid is the common name for the eight-carbon saturated fatty acid known by the systematic name octanoic acid. It is found naturally in the milk of various mammals, and it is a minor constituent of coconut oil and palm kernel oil. It is an oily liquid that is minimally soluble in water with a slightly unpleasant rancid-like smell, has the formula CH₃(CH₂)₆CO₂(CH₂)₅CH₃.

The chemical formulas of esters are typically written in the format of RCO₂R', where R and R' are the organic parts of the carboxylic acid and alcohol respectively. For example butyl acetate n-Butyl acetate, also known as butyl ethanoate, is an organic compound commonly used as a solvent in the production of lacquers and other products. It is also used as a synthetic fruit flavoring in foods such as candy, ice cream, cheeses, and baked goods. Butyl acetate is found in many types of fruit, where along with other chemicals it imparts, derived from butanol n-Butanol or n-butyl alcohol is a primary alcohol with a 4-carbon structure and the molecular formula C4H10O. It is one of the group of "fusel alcohols" (from the German for "bad liquor"), which have more than two carbon atoms and have significant solubility in water and acetic acid Acetic acid, CH3COOH, also known as ethanoic acid, is an organic acid that gives vinegar its sour taste and pungent smell. It is a weak acid, in that it is only a partially dissociated acid in an aqueous solution. Pure, water-free acetic acid is a colourless liquid that absorbs water from the environment (hygroscopy), and freezes at 16.5 °C (62 ° would be written CH₃CO₂C₄H₉. Alternative presentations are common including BuOAc and CH₃COOC₄H₉.

Cyclic esters are called lactones. One example of many is valerolactone.

Orthoesters

An uncommon class of organic esters are the orthoesters In organic chemistry, an orthoester is a functional group containing three alkoxy groups attached to one carbon atom. The name can also refer to any organic compound that contains this functional group. An example of an orthoester is ethyl orthoacetate, CH3C3, more correctly known as 1,1,1-triethoxyethane. Orthoesters are used in organic synthesis, 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"

Ester is a general term for the product derived from the condensation of an oxo acid and an alcohol. Thus, the nomenclature extends to inorganic acids, especially phosphoric acid Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral (inorganic) acid having the chemical formula H3P , sulfuric acid Sulfuric acid is a strong mineral acid with the molecular formula H2SO4(aq). It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in 2001 was 165 million tonnes, with an approximate value of US$8 billion. Principal uses include lead-acid, and nitric acid Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming nitric acid. Fuming nitric acid is characterized as white fuming nitric acid and red fuming nitric acid, depending on the amount of nitrogen dioxide present. Cyclic esters are called lactones In chemistry, a lactone is a cyclic ester which can be seen as the condensation product of an alcohol group -OH and a carboxylic acid group -COOH in the same molecule. It is characterized by a closed ring consisting of two or more carbon atoms and a single oxygen atom, with a ketone group =O in one of the carbons adjacent to the other oxygen. The preparation of an ester is known generally as an esterification Esterification is the general name for a chemical reaction in which two reactants 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 reaction. For example, triphenyl phosphate Triphenyl phosphate is the chemical compound with the formula OP3. This colourless solid is the ester (triester) of phosphoric acid and phenol. It is used as a plasticizer and a fire retardant is the ester derived from phosphoric acid Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral (inorganic) acid having the chemical formula H3P and phenol Phenol, also known as carbolic acid, is an organic compound with the chemical formula C6H5OH. It is a white, crystalline solid. This functional group consists of a phenyl, bonded to a hydroxyl . It is produced on a large scale (about 7 billion kg/year) as a precursor to many materials and useful compounds. It is a mildly acidic compound that. Organic carbonates, such as ethylene carbonate Ethylene carbonate is an ester of ethylene glycol and carbonic acid. At room temperature ethylene carbonate is a transparent crystal-like solid. In the liquid state (m.p. 34-37 °C) it is a colorless odorless liquid are derived from carbonic acid Carbonic acid is the inorganic compound with the formula H2CO3 (equivalently OC2). It is also a name sometimes given to solutions of carbon dioxide in water, which contain small amounts of H2CO3. The salts of carbonic acids are called bicarbonates (or hydrogen carbonates) and carbonates. It is a weak acid. Carbonic acid is sometimes confused with and ethylene glycol Ethylene glycol is an organic compound widely used as an automotive antifreeze and a precursor to polymers. In its pure form, it is an odorless, colorless, syrupy, sweet-tasting liquid. Ethylene glycol is toxic, and ingestion can result in death. The term boronic ester A boronic acid is an alkyl or aryl substituted boric acid containing a carbon-boron bond belonging to the larger class of organoboranes. Boronic acids act as Lewis acids. Their unique feature are that they are capable of forming reversible covalent complexes with sugars, amino acids, hydroxamic acids, etc. (molecules with vicinal, or occasionally ( is widely used but these compounds are not formally esters because they lack a B=O bond.

Structure and bonding

Esters contain a carbonyl In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O center, which gives rise to 120° C-C-O and O-C-O angles. Unlike amides In chemistry, an amide is usually an organic compound that contains the functional group consisting of an acyl group linked to a nitrogen atom (N). The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia (NH3) or an amine, often represented as anions R2N-, 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.

Physical properties and characterization

Esters are more polar than ethers but less polar than alcohols. They participate in hydrogen bonds A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, like nitrogen, oxygen or fluorine . The hydrogen must be covalently bonded to another electronegative atom to create the bond. These bonds can occur between molecules (intermolecularly), or within different parts of a single molecule (intramolecularly) 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 Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. A carboxyl group is a functional group consisting of a carbonyl and a hydroxyl, which has the formula -C(=O)OH, usually written -COOH or -CO2H. Carboxylic acids are Brønsted-Lowry acids — they are proton donors. Salts and anions of carboxylic acids of similar molecular weight.

Characterization and analysis

Esters are usually identified by gas chromatography, taking advantage of their volatility. IR spectra Infrared spectroscopy is the subset of spectroscopy that deals with the infrared region of the electromagnetic spectrum. It covers a range of techniques, the most common being a form of absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify compounds and investigate sample composition. A common laboratory for esters feature an intense sharp band in the range 1730-1750 cm^-1 assigned to ν_C=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

Esters are widespread in nature and are widely used in industry. In nature, fats Fats consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water. Chemically, fats are generally triesters of glycerol and fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure and composition. Although the words "oils", "fats", are generally triesters derived from glycerol Glycerol is an organic compound, also called glycerin or glycerine. It is a colourless, odourless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydrophilic hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol substructure is a central component of many and fatty acids In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. The most occurring natural fatty acids have an even number of carbon atoms because their biosynthesis involves acetyl-CoA, a coenzyme carrying a two-carbon-atom group (see fatty acid synthesis). Esters are responsible for the aroma of many fruits, including apples, pears, bananas, pineapples, and strawberries.^[2] Several billion kilograms of polyesters are produced industrially annually, important products being polyethylene terephthalate Polyethylene terephthalate (sometimes written poly), commonly abbreviated PET, PETE, or the obsolete PETP or PET-P), is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination with glass fiber, acrylate esters, and cellulose acetate Cellulose acetate , first prepared in 1865, is the acetate ester of cellulose. Cellulose acetate is used as a film base in photography, as a component in some adhesives, and as a frame material for eyeglasses; it is also used as a synthetic fiber and in the manufacture of cigarette filters.^[3]

Preparation

Fischer esterification

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

R¹COOH + R²OH R¹COOR² + H₂O

Strong acids, typically sulfuric acid, catalyze this reaction. Many other acids are also used. Esterification is highly reversible. The yield of the product may 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 like molecular sieves can also be used.
• removal of water by physical means such as distillation as a low-boiling azeotropes with toluene, in conjunction with a Dean-Stark apparatus.

Alcoholysis of acyl chlorides and acid anhydrides

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

RCOCl + R'OH → RCOOR' + HCl

(RCO)₂O + R'OH → RCO₂R' + RCO₂H

These reactions are irreversible, simplifying work-up. Since, acyl chlorides and acid anhydrides react also 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.

Steglich esterification

The Steglich esterification is method of forming esters under mild conditions. The method 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 methods

• Transesterifications between other esters. This method is widely used industrially.
• 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.

Reactions

Esters react primarily at one of two locations, the carbonyl at the carbon adjacent the carbonyl group. The carbonyl is weakly electrophilic and 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 that usually initiates condensation reactions.

Nucleophilic substitutions

Esters undergo hydrolysis under acid and basic conditions. Under acid 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.

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, whereas sodium borohydride does so more slowly. DIBAH reduces esters to aldehydes, while forcing conditions are required for hydrogenation.^[4] In the Bouveault-Blanc reduction, sodium in the presence of proton sources is the reducing agent. Such reactions can also be conducted by hydrogenation.

RCO₂R' + 2 H₂ → RCH₂OH + R'OH

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 condensation. 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

• 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.

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