Ester

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https://en.wikipedia.org/wiki/Ester

 

An ester of carboxylic acid. R stands for any group (organic or inorganic, typically hydrogen or organyl) and R′ stands for organyl group.

In chemistry, an ester is a compound derived from an acid (organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group (−OH) of that acid is replaced by an organyl group (−R). Analogues derived from oxygen replaced by other chalcogens belong to the ester category as well. According to some authors, organyl derivatives of acidic hydrogen of other acids are esters as well (e.g. amides), but not according to the IUPAC.

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. 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 like celery and other foods.

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 and trithiocarbonic acid). An example of an ester formation is the substitution reaction between a carboxylic acid (R−C(=O)−OH) and an alcohol (R'−OH), forming an ester (R−C(=O)−O−R'), where R stands for any group (organic or inorganic, typically hydrogen or organyl) and R′ stands for organyl group.

Organyl esters of carboxylic acids 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, and are one of the largest classes of synthetic lubricants on the commercial market. 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.

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 (e.g. according to them, trimethylstannyl acetate (or trimethyltin acetate) CH₃COOSn(CH₃)₃ is a trimethylstannyl ester of acetic acid, and dibutyltin dilaurate (CH₃(CH₂)₁₀COO)₂Sn((CH₂)₃CH₃)₂ is a dibutylstannylene ester of lauric acid).

Nomenclature

Etymology

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

IUPAC nomenclature

Main article: IUPAC nomenclature of organic chemistry § Esters

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

An uncommon class of esters are the orthoesters. One of them are the esters of orthocarboxylic acids. Those esters have the formula RC(OR′)₃, where R stands for any group (organic or inorganic) and R′ stands for 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

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

Esters can also be derived from inorganic acids.

• Phosphoric acid forms phosphate esters, e.g. triphenyl phosphate (O=P(OC₆H₅)₃) and methyl dihydrogen phosphate (O=P(OCH₃)(OH)₂)
• Sulfuric acid forms sulfate esters, e.g., dimethyl sulfate ((CH₃O)₂SO₂) and methyl bisulfate (CH₃OSO₂OH)
• Nitric acid forms nitrate esters, e.g. methyl nitrate (CH₃−O−NO₂) and nitroglycerin (CH(−O−NO₂)(−CH₂−O−NO₂)₂)
• Boric acid forms borate esters , e.g. trimethyl borate (B(OCH₃)₃)
• Carbonic acid forms carbonate esters, e.g. ethylene carbonate ((CH₂O)₂C=O) (if one classifies carbonic acid as an inorganic compound)
• Trithiocarbonic acid forms trithiocarbonate esters, e.g. dimethyl trithiocarbonate ((CH₃S)₂C=S) (if one classifies trithiocarbonic acid as an inorganic compound)
• Thiocyanic acid forms thiocyanate esters, e.g. methyl thiocyanate (CH₃−S−C≡N) (if one classifies thiocyanic acid as an inorganic compound)

Inorganic acids that exist as tautomers form two or more kinds of esters.

• Phosphorous acid forms two kinds of esters: phosphite esters, e.g. triethyl phosphite (P(OCH₂CH₃)₃), and phosphonate esters, e.g. diethyl phosphonate (HP(=O)(−OCH₂CH₃)₂)
• Thiosulfuric acid forms two kinds of thiosulfate esters, e.g. O,O-dimethyl thiosulfate ((CH₃−O−)₂C=S) and O,S-dimethyl thiosulfate (CH₃−O−C(=O)−S−CH₃)

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

• Chromic acid, which has never been detected, forms di-tert-butyl chromate (((CH₃)₃CO)₂CrO₂)
• Sulfurous acid, which is rare, forms dimethyl sulfite ((CH₃O)₂S=O)

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

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. The pK_a of the alpha-hydrogens on esters is around 25.

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. Lactones with small rings are restricted to the s-trans (i.e. E) conformation due to their cyclic structure.

Metrical details for methyl benzoate, distances in picometers.

Physical properties and characterization

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.

Characterization and analysis

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 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 are, in general, triesters derived from glycerol and fatty acids. Esters are responsible for the aroma of many fruits, including apples, durians, pears, bananas, pineapples, and strawberries. Several billion kilograms of polyesters are produced industrially annually, important products being polyethylene terephthalate, acrylate esters, and cellulose acetate.

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

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

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

RCO₂H + R'OH ⇌ RCO₂R' + H₂O

The equilibrium constant for such reactions is about 5 for typical esters, e.g., ethyl acetate. 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.

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

RCO₂H + R'OH + P(C₆H₅)₃ + R₂N₂ → RCO₂R' + OP(C₆H₅)₃ + R₂N₂H₂

Carboxylic acids can be esterified using diazomethane:

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 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

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

RCO₂H + RCHCH₂O → RCO₂CH₂CH(OH)R

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

Alcoholysis of acyl chlorides and acid anhydrides

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

RCOCl + R'OH → RCO₂R' + HCl

(RCO)₂O + R'OH → RCO₂R' + RCO₂H

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

Although rarely employed for esterifications, carboxylate salts (often generated in situ) react with electrophilic alkylating agents, such as alkyl halides, to give esters. Anion availability can inhibit this reaction, which correspondingly benefits from phase transfer catalysts or such highly polar aprotic solvents as DMF. An additional iodide salt may, via the Finkelstein 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

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 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:

(C₆H₄)(CO₂CH₃)₂ + 2 C₂H₄(OH)₂ → 1/n[(C₆H₄)(CO₂)₂(C₂H₄)]_n + 2 CH₃OH

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

(CH₂CO)₂ + ROH → CH₃C(O)CH₂CO₂R

Carbonylation

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

H₂C=CH₂ + ROH + CO → CH₃CH₂CO₂R

A preparation of methyl propionate is one illustrative example.

H₂C=CH₂ + CO + CH₃OH → CH₃CH₂CO₂CH₃

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

CH₃OH + CO → HCO₂CH₃

Addition of carboxylic acids to alkenes and alkynes

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

H₂C=CH₂ + CH₃CO₂H → CH₃CO₂CH=CH₂

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

2 H₂C=CH₂ + 2 CH₃CO₂H + O₂ → 2 CH₃CO₂CH=CH₂ + 2 H₂O

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

H₂C=CH₂ + CH₃CO₂H → CH₃CO₂CH₂CH₃

From aldehydes

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. The method is used in the production of ethyl acetate from acetaldehyde.

Other methods

• Favorskii rearrangement of α-haloketones in presence of base
• Baeyer–Villiger oxidation of ketones with peroxides
• Pinner reaction of nitriles with an alcohol
• Nucleophilic abstraction of a metal–acyl complex
• Hydrolysis of orthoesters in aqueous acid
• Cellulolysis via esterification
• Ozonolysis of alkenes using a work up in the presence of hydrochloric acid and various alcohols.
• Anodic oxidation of methyl ketones leading to methyl esters.
• Interesterification exchanges the fatty acid groups of different esters.

Reactions

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

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.

The alkoxide group may also be displaced by stronger nucleophiles such as ammonia or primary or secondary amines to give amides: (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 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

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.

RCO₂R' + 2 H₂ → RCH₂OH + R'OH

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.

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.

Claisen condensation and related reactions

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

• Methyl esters are often susceptible to decarboxylation in the Krapcho decarboxylation.
• Phenyl esters react to hydroxyarylketones in the Fries rearrangement.
• Specific esters are functionalized with an α-hydroxyl group in the Chan rearrangement.
• Esters with β-hydrogen atoms can be converted to alkenes in ester pyrolysis.
• A direct conversion of esters to nitriles.
• Pairs of esters are coupled to give α-hydroxyketones in the acyloin condensation

Protecting groups

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

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 ionic hydrides.

List of ester odorants

See also: List of esters

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		pineapple
Benzyl acetate		pear, strawberry, jasmine
Bornyl acetate		pine
Butyl acetate		apple, honey
Butyl butyrate		pineapple
Butyl propionate		pear drops, apple
Ethyl acetate		nail polish remover, model paint, model airplane glue, pears
Ethyl benzoate		sweet, wintergreen, fruity, medicinal, cherry, grape
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 butyrate (methyl butanoate)		pineapple, apple, strawberry
Methyl cinnamate		strawberry
Methyl formate		pleasant, ethereal, rum, sweet
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 acetate		pear
Propyl hexanoate		blackberry, pineapple, cheese, wine
Propyl isobutyrate		rum
Terpenyl butyrate		cherry

Ether

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Ether

 

The general structure of an ether. R and R' represent any alkyl or aryl substituent.

In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula R−O−R′, where R and R′ represent the alkyl or aryl groups. Ethers can again be classified into two varieties: if the alkyl or aryl groups are the same on both sides of the oxygen atom, then it is a simple or symmetrical ether, whereas if they are different, the ethers are called mixed or unsymmetrical ethers. A typical example of the first group is the solvent and anaesthetic diethyl ether, commonly referred to simply as "ether" (CH₃−CH₂−O−CH₂−CH₃). Ethers are common in organic chemistry and even more prevalent in biochemistry, as they are common linkages in carbohydrates and lignin.

Structure and bonding

Ethers feature bent C–O–C linkages. In dimethyl ether, the bond angle is 111° and C–O distances are 141 pm. The barrier to rotation about the C–O bonds is low. The bonding of oxygen in ethers, alcohols, and water is similar. In the language of valence bond theory, the hybridization at oxygen is sp³.

Oxygen is more electronegative than carbon, thus the alpha hydrogens of ethers are more acidic than those of simple hydrocarbons. They are far less acidic than alpha hydrogens of carbonyl groups (such as in ketones or aldehydes), however.

Ethers can be symmetrical of the type ROR or unsymmetrical of the type ROR'. Examples of the former are dimethyl ether, diethyl ether, dipropyl ether etc. Illustrative unsymmetrical ethers are anisole (methoxybenzene) and dimethoxyethane.

Vinyl- and acetylenic ethers

Vinyl- and acetylenic ethers are far less common than alkyl or aryl ethers. Vinylethers, often called enol ethers, are important intermediates in organic synthesis. Acetylenic ethers are especially rare. Di-tert-butoxyacetylene is the most common example of this rare class of compounds.

Nomenclature

In the IUPAC Nomenclature system, ethers are named using the general formula "alkoxyalkane", for example CH₃–CH₂–O–CH₃ is methoxyethane. If the ether is part of a more-complex molecule, it is described as an alkoxy substituent, so –OCH₃ would be considered a "methoxy-" group. The simpler alkyl radical is written in front, so CH₃–O–CH₂CH₃ would be given as methoxy(CH₃O)ethane(CH₂CH₃).

Trivial name

IUPAC rules are often not followed for simple ethers. The trivial names for simple ethers (i.e., those with none or few other functional groups) are a composite of the two substituents followed by "ether". For example, ethyl methyl ether (CH₃OC₂H₅), diphenylether (C₆H₅OC₆H₅). As for other organic compounds, very common ethers acquired names before rules for nomenclature were formalized. Diethyl ether is simply called ether, but was once called sweet oil of vitriol. Methyl phenyl ether is anisole, because it was originally found in aniseed. The aromatic ethers include furans. Acetals (α-alkoxy ethers R–CH(–OR)–O–R) are another class of ethers with characteristic properties.

Polyethers

Polyethers are generally polymers containing ether linkages in their main chain. The term polyol generally refers to polyether polyols with one or more functional end-groups such as a hydroxyl group. The term "oxide" or other terms are used for high molar mass polymer when end-groups no longer affect polymer properties.

Crown ethers are cyclic polyethers. Some toxins produced by dinoflagellates such as brevetoxin and ciguatoxin are extremely large and are known as cyclic or ladder polyethers.

Aliphatic polyethers

Name of the polymers with low to medium molar mass	Name of the polymers with high molar mass	Preparation	Repeating unit	Examples of trade names
Paraformaldehyde	Polyoxymethylene (POM) or polyacetal or polyformaldehyde	Step-growth polymerisation of formaldehyde	–CH2O–	Delrin from DuPont
Polyethylene glycol (PEG)	Polyethylene oxide (PEO) or polyoxyethylene (POE)	Ring-opening polymerization of ethylene oxide	–CH2CH2O–	Carbowax from Dow
Polypropylene glycol (PPG)	Polypropylene oxide (PPOX) or polyoxypropylene (POP)	anionic ring-opening polymerization of propylene oxide	–CH2CH(CH3)O–	Arcol from Covestro
Polytetramethylene glycol (PTMG) or Polytetramethylene ether glycol (PTMEG)	Polytetrahydrofuran (PTHF)	Acid-catalyzed ring-opening polymerization of tetrahydrofuran	−CH2CH2CH2CH2O−	Terathane from Invista and PolyTHF from BASF

The phenyl ether polymers are a class of aromatic polyethers containing aromatic cycles in their main chain: polyphenyl ether (PPE) and poly(p-phenylene oxide) (PPO).

Related compounds

Many classes of compounds with C–O–C linkages are not considered ethers: Esters (R–C(=O)–O–R′), hemiacetals (R–CH(–OH)–O–R′), carboxylic acid anhydrides (RC(=O)–O–C(=O)R′).

Physical properties

Ethers have boiling points similar to those of the analogous alkanes. Simple ethers are generally colorless.

Selected data about some alkyl ethers
Ether	Structure	m.p. (°C)	b.p. (°C)	Solubility in 1 liter of H2O	Dipole moment (D)
Dimethyl ether	CH3–O–CH3	−138.5	−23.0	70 g	1.30
Diethyl ether	CH3CH2–O–CH2CH3	−116.3	34.4	69 g	1.14
Tetrahydrofuran	O(CH2)4	−108.4	66.0	Miscible	1.74
Dioxane	O(C2H4)2O	11.8	101.3	Miscible	0.45

Reactions

Structure of the polymeric diethyl ether peroxide

The C-O bonds that comprise simple ethers are strong. They are unreactive toward all but the strongest bases. Although generally of low chemical reactivity, they are more reactive than alkanes.

Specialized ethers such as epoxides, ketals, and acetals are unrepresentative classes of ethers and are discussed in separate articles. Important reactions are listed below.

Cleavage

See also: Ether cleavage

Although ethers resist hydrolysis, they are cleaved by hydrobromic acid and hydroiodic acid. Hydrogen chloride cleaves ethers only slowly. Methyl ethers typically afford methyl halides:

ROCH₃ + HBr → CH₃Br + ROH

These reactions proceed via onium intermediates, i.e. [RO(H)CH₃]⁺Br⁻.

Some ethers undergo rapid cleavage with boron tribromide (even aluminium chloride is used in some cases) to give the alkyl bromide. Depending on the substituents, some ethers can be cleaved with a variety of reagents, e.g. strong base.

Peroxide formation

When stored in the presence of air or oxygen, ethers tend to form explosive peroxides, such as diethyl ether hydroperoxide. The reaction is accelerated by light, metal catalysts, and aldehydes. In addition to avoiding storage conditions likely to form peroxides, it is recommended, when an ether is used as a solvent, not to distill it to dryness, as any peroxides that may have formed, being less volatile than the original ether, will become concentrated in the last few drops of liquid. The presence of peroxide in old samples of ethers may be detected by shaking them with freshly prepared solution of a ferrous sulfate followed by addition of KSCN. Appearance of blood red color indicates presence of peroxides. The dangerous properties of ether peroxides are the reason that diethyl ether and other peroxide forming ethers like tetrahydrofuran (THF) or ethylene glycol dimethyl ether (1,2-dimethoxyethane) are avoided in industrial processes.

Lewis bases

Structure of VCl₃(thf)₃.

Ethers serve as Lewis bases. For instance, diethyl ether forms a complex with boron trifluoride, i.e. diethyl etherate (BF₃·OEt₂). Ethers also coordinate to the Mg center in Grignard reagents. Tetrahydrofuran is more basic than acyclic ethers. It forms complexes with many metal halides.

Alpha-halogenation

This reactivity is similar to the tendency of ethers with alpha hydrogen atoms to form peroxides. Reaction with chlorine produces alpha-chloroethers.

Synthesis

Ethers can be prepared by numerous routes. In general alkyl ethers form more readily than aryl ethers, with the later species often requiring metal catalysts.

The synthesis of diethyl ether by a reaction between ethanol and sulfuric acid has been known since the 13th century.

Dehydration of alcohols

The dehydration of alcohols affords ethers:

2 R–OH → R–O–R + H₂O at high temperature

This direct nucleophilic substitution reaction requires elevated temperatures (about 125 °C). The reaction is catalyzed by acids, usually sulfuric acid. The method is effective for generating symmetrical ethers, but not unsymmetrical ethers, since either OH can be protonated, which would give a mixture of products. Diethyl ether is produced from ethanol by this method. Cyclic ethers are readily generated by this approach. Elimination reactions compete with dehydration of the alcohol:

R–CH₂–CH₂(OH) → R–CH=CH₂ + H₂O

The dehydration route often requires conditions incompatible with delicate molecules. Several milder methods exist to produce ethers.

Williamson ether synthesis

Nucleophilic displacement of alkyl halides by alkoxides

R–ONa + R′–X → R–O–R′ + NaX

This reaction is called the Williamson ether synthesis. It involves treatment of a parent alcohol with a strong base to form the alkoxide, followed by addition of an appropriate aliphatic compound bearing a suitable leaving group (R–X). Suitable leaving groups (X) include iodide, bromide, or sulfonates. This method usually does not work well for aryl halides (e.g. bromobenzene, see Ullmann condensation below). Likewise, this method only gives the best yields for primary halides. Secondary and tertiary halides are prone to undergo E2 elimination on exposure to the basic alkoxide anion used in the reaction due to steric hindrance from the large alkyl groups.

In a related reaction, alkyl halides undergo nucleophilic displacement by phenoxides. The R–X cannot be used to react with the alcohol. However phenols can be used to replace the alcohol while maintaining the alkyl halide. Since phenols are acidic, they readily react with a strong base like sodium hydroxide to form phenoxide ions. The phenoxide ion will then substitute the –X group in the alkyl halide, forming an ether with an aryl group attached to it in a reaction with an S_N2 mechanism.

C₆H₅OH + OH⁻ → C₆H₅–O⁻ + H₂O

C₆H₅–O⁻ + R–X → C₆H₅OR

Ullmann condensation

The Ullmann condensation is similar to the Williamson method except that the substrate is an aryl halide. Such reactions generally require a catalyst, such as copper.

Electrophilic addition of alcohols to alkenes

Alcohols add to electrophilically activated alkenes.

R₂C=CR₂ + R–OH → R₂CH–C(–O–R)–R₂

Acid catalysis is required for this reaction. Often, mercury trifluoroacetate (Hg(OCOCF₃)₂) is used as a catalyst for the reaction generating an ether with Markovnikov regiochemistry. Using similar reactions, tetrahydropyranyl ethers are used as protective groups for alcohols.

Preparation of epoxides

Main article: epoxide

Epoxides are typically prepared by oxidation of alkenes. The most important epoxide in terms of industrial scale is ethylene oxide, which is produced by oxidation of ethylene with oxygen. Other epoxides are produced by one of two routes:

• By the oxidation of alkenes with a peroxyacid such as m-CPBA.
• By the base intramolecular nucleophilic substitution of a halohydrin.

Sabatier reaction

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Sabatier_reaction

 

Paul Sabatier (1854-1941) winner of the Nobel Prize in Chemistry in 1912 and discoverer of the reaction in 1897

The Sabatier reaction or Sabatier process produces methane and water from a reaction of hydrogen with carbon dioxide at elevated temperatures (optimally 300–400 °C) and pressures (perhaps 3 MPa ) in the presence of a nickel catalyst. It was discovered by the French chemists Paul Sabatier and Jean-Baptiste Senderens in 1897. Optionally, ruthenium on alumina (aluminium oxide) makes a more efficient catalyst. It is described by the following exothermic reaction:

${\displaystyle {\text{CO}}_2^{}+4{\text{H}}_2^{}\underset{{\text{pressure}}^{+}\text{catalyst}}{\overset{\genfrac{}{}{0ex}{}{}{400{}^{\circ }\text{C}}}{\to }}{\text{CH}}_4^{}+2{\text{H}}_2^{}\text{O}}$ ∆H = −165.0 kJ/mol

There is disagreement on whether the CO₂ methanation occurs by first associatively adsorbing an adatom hydrogen and forming oxygen intermediates before hydrogenation or dissociating and forming a carbonyl before being hydrogenated.

${\displaystyle \text{CO}+3{\text{H}}_2^{}⟶{\text{CH}}_4^{}+{\text{H}}_2^{}\text{O}}$ ∆H = −206 kJ/mol

CO methanation is believed to occur through a dissociative mechanism where the carbon oxygen bond is broken before hydrogenation with an associative mechanism only being observed at high H₂ concentrations.

Methanation reactions over different metal catalysts including Ni, Ru and Rh have been widely investigated for the production of CH₄ from syngas and other power to gas initiatives. Nickel is the most widely used catalyst owing to its high selectivity and low cost.

Applications

Creation of synthetic natural gas

Main article: Carbon-neutral fuel

Methanation is an important step in the creation of synthetic or substitute natural gas (SNG). Coal or wood undergo gasification which creates a producer gas that must undergo methaneation in order to produce a usable gas that just needs to undergo a final purification step.

The first commercial synthetic gas plant opened in 1984 and is the Great Plains Synfuels plant in Beulah, North Dakota. As of 2016, it is still operational and produces 1500 MW worth of SNG using coal as the carbon source. In the years since its opening, other commercial facilities have been opened using other carbon sources such as wood chips.

In France, AFUL Chantrerie, located in Nantes, in November 2017 opened the demonstrator MINERVE. This unit produces 14 Mm³/day of methane. The plant feeds a compressed natural gas station and injects methane into a natural gas boiler.

The Sabatier reaction has been used in renewable-energy-dominated energy systems to use the excess electricity generated by wind, solar photovoltaic, hydro, marine current, etc. to make methane from hydrogen from water electrolysis. In contrast to a direct usage of hydrogen for transport or energy storage applications, the methane can be injected into the existing gas network. The methane can be used on-demand to generate electricity overcoming low points of renewable energy production. The process is electrolysis of water by electricity to create hydrogen (which can partly be used directly in fuel cells) and the addition of carbon dioxide CO₂ (Sabatier reaction) to create methane. The CO₂ can be extracted from the air or fossil fuel waste gases by the amine process.

A 6 MW power-to-gas plant went into production in Germany in 2013, and powered a fleet of 1,500 Audi A3.

Ammonia synthesis

In ammonia production CO and CO₂ are considered poisons to most commonly used catalysts. Methanation catalysts are added after several hydrogen producing steps to prevent carbon oxide buildup in the ammonia synthesis loop as methane does not have similar adverse effects on ammonia synthesis rates.

International Space Station life support

Oxygen generators on board the International Space Station produce oxygen from water using electrolysis; the hydrogen produced was previously discarded into space. As astronauts consume oxygen, carbon dioxide is produced, which must then be removed from the air and discarded as well. This approach required copious amounts of water to be regularly transported to the space station for oxygen generation in addition to that used for human consumption, hygiene, and other uses—a luxury that will not be available to future long-duration missions beyond low Earth orbit.

NASA is using the Sabatier reaction to recover water from exhaled carbon dioxide and the hydrogen previously discarded from electrolysis on the International Space Station and possibly for future missions. The other resulting chemical, methane, is released into space. As half of the input hydrogen becomes wasted as methane, additional hydrogen is supplied from Earth to make up the difference. However, this creates a nearly-closed cycle between water, oxygen, and carbon dioxide which only requires a relatively modest amount of imported hydrogen to maintain.

${\displaystyle 2{\text{H}}_2^{}\text{O}\stackrel{\text{electrolysis}}{\to }{\text{O}}_2^{}+2{\text{H}}_2^{}\stackrel{\text{respiration}}{\to }{\text{CO}}_2^{}+2{\text{H}}_2^{}+\stackrel{\text{added}}{2{\text{H}}_2^{}}⟶2{\text{H}}_2^{}\text{O}+\stackrel{\text{discarded}}{{\text{CH}}_4^{}}}$

The loop could be further closed if the waste methane was separated into its component parts by pyrolysis, the high efficiency (up to 95% conversion) of which can be achieved at 1200 °C:

${\displaystyle {\text{CH}}_4^{}\stackrel{\text{heat}}{\to }\text{C}+2{\text{H}}_2^{}}$

The released hydrogen would then be recycled back into the Sabatier reactor, leaving an easily removed deposit of pyrolytic graphite. The reactor would be little more than a steel pipe, and could be periodically serviced by an astronaut where the deposit is chiselled out.

Alternatively, the loop could be partially closed (75% of H₂ from CH₄ recovered) by incomplete pyrolysis of the waste methane while keeping the carbon locked up in gaseous form as acetylene:

${\displaystyle 2{\text{CH}}_4^{}\stackrel{\text{heat}}{\to }{\text{C}}_2^{}{\text{H}}_2^{}+3{\text{H}}_2^{}}$

The Bosch reaction is also being investigated by NASA for this purpose and is:

${\displaystyle {\text{CO}}_2^{}+2{\text{H}}_2^{}⟶\text{C}+2{\text{H}}_2^{}\text{O}}$

The Bosch reaction would present a completely closed hydrogen and oxygen cycle which only produces atomic carbon as waste. However, difficulties maintaining its temperature of up to 600 °C and properly handling carbon deposits mean significantly more research will be required before a Bosch reactor could become a reality. One problem is that the production of elemental carbon tends to foul the catalyst's surface (coking), which is detrimental to the reaction's efficiency.

Manufacturing propellant on Mars

The Sabatier reaction has been proposed as a key step in reducing the cost of human mission to Mars (Mars Direct, SpaceX Starship) through in situ resource utilization. Hydrogen is combined with CO₂ from the atmosphere, with methane then stored as fuel and the water side product electrolyzed yielding oxygen to be liquefied and stored as oxidizer and hydrogen to be recycled back into the reactor. The original hydrogen could be transported from Earth or separated from Martian sources of water.

Importing hydrogen

Importing a small amount of hydrogen avoids searching for water and just uses CO₂ from the atmosphere.

"A variation of the basic Sabatier methanation reaction may be used via a mixed catalyst bed and a reverse water gas shift in a single reactor to produce methane from the raw materials available on Mars, utilising carbon dioxide in the Martian atmosphere. A 2011 prototype test operation that harvested CO₂ from a simulated Martian atmosphere and reacted it with H₂, produced methane rocket propellant at a rate of 1 kg/day, operating autonomously for 5 consecutive days, maintaining a nearly 100% conversion rate. An optimised system of this design massing 50 kg "is projected to produce 1 kg/day of O₂:CH₄ propellant ... with a methane purity of 98+% while consuming ~17 kWh per day of electrical power (at a continuous power of 700 W). Overall unit conversion rate expected from the optimised system is one tonne of propellant per 17 MWh energy input."

Stoichiometry issue with importing hydrogen

The stoichiometric ratio of oxidiser and fuel is 2:1, for an oxygen:methane engine:

${\displaystyle {\text{CH}}_4^{}+2{\text{O}}_2^{}⟶{\text{CO}}_2^{}+2{\text{H}}_2^{}\text{O}}$

However, one pass through the Sabatier reactor produces a ratio of only 1:1. More oxygen may be produced by running the water-gas shift reaction (WGSR) in reverse (RWGS), effectively extracting oxygen from the atmosphere by reducing carbon dioxide to carbon monoxide.

Another option is to make more methane than needed and pyrolyze the excess of it into carbon and hydrogen (see above section), where the hydrogen is recycled back into the reactor to produce further methane and water. In an automated system, the carbon deposit may be removed by blasting with hot Martian CO₂, oxidizing the carbon into carbon monoxide (via the Boudouard reaction), which is vented.

A fourth solution to the stoichiometry problem would be to combine the Sabatier reaction with the reverse water-gas shift (RWGS) reaction in a single reactor as follows:

${\displaystyle 3{\text{CO}}_2^{}+6{\text{H}}_2^{}⟶{\text{CH}}_4^{}+2\text{CO}+4{\text{H}}_2^{}\text{O}}$

This reaction is slightly exothermic, and when the water is electrolyzed, an oxygen to methane ratio of 2:1 is obtained.

Regardless of which method of oxygen fixation is utilized, the overall process can be summarized by the following equation:

${\displaystyle 2{\text{H}}_2^{}+3{\text{CO}}_2^{}⟶{\text{CH}}_4^{}+2{\text{O}}_2^{}+2\text{CO}}$

Looking at molecular masses, 16 grams of methane and 64 grams of oxygen have been produced using 4 grams of hydrogen (which would have to be imported from Earth, unless Martian water was electrolysed), for a mass gain of 20:1; and the methane and oxygen are in the right stoichiometric ratio to be burned in a rocket engine. This kind of in situ resource utilization would result in massive weight and cost savings to any proposed crewed Mars or sample-return missions.