Ester

From Wikipedia, the free encyclopedia

A carboxylate ester. R and R' denote any alkyl or aryl group

In chemistry, esters are chemical compounds derived from an acid (organic or inorganic) in which at least one -OH (hydroxyl) group is replaced by an -O-alkyl (alkoxy) group.^[1] Usually, esters are derived from a carboxylic acid and an alcohol. Esters comprise most naturally occurring fats and oils.^{[dubious – discuss]} An important case are glycerides, which are fatty acid esters of glycerol. Esters with low molecular weight are commonly used as fragrances and found in essential oils and pheromones. Phosphoesters form the backbone of DNA molecules. Nitrate esters, such as nitroglycerin, are known for their explosive properties, while polyesters are important plastics, with monomers linked by ester moieties.

Nomenclature

Etymology

The word 'ester' was coined in 1848 by German chemist Leopold Gmelin,^[2] probably as a contraction of the German Essigäther, "acetic ether".

IUPAC nomenclature

Ester names 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.

Ethyl acetate derived from an alcohol (blue) and an acyl group (yellow) derived from a carboxylic acid.

The chemical formulas of organic esters usually take the form RCO₂R', where R and R' are the hydrocarbon parts of the carboxylic acid and the alcohol, respectively. For example butyl acetate (systematically butyl ethanoate), derived from butanol and acetic acid (systematically ethanoic acid) would be written CH₃CO₂C₄H₉. Alternative presentations are common including BuOAc and CH₃COOC₄H₉.

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

Orthoesters

An uncommon class of organic esters are the orthoesters, 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

A phosphoric acid ester

Esters can also be derived from an inorganic acid and an alcohol. Thus, the nomenclature extends to inorganic oxo acids, e.g. phosphoric acid, sulfuric acid, nitric acid and boric acid. For example, triphenyl phosphate is the ester derived from phosphoric acid and phenol. Organic carbonates are derived from carbonic acid; for example, ethylene carbonate is derived from carbonic acid and ethylene glycol.

Structure and bonding

Esters contain a carbonyl center, which gives rise to 120 °C-C-O and O-C-O angles. Unlike amides, 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.^[3] The pKa of the alpha-hydrogens on esters is around 25.^[4]

Physical properties and characterization

Esters 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.^[3]

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.^[5] Esters are responsible for the aroma of many fruits, including apples, durians, pears, bananas, pineapples, and strawberries.^[6] Several billion kilograms of polyesters are produced industrially annually, important products being polyethylene terephthalate, acrylate esters, and cellulose acetate.^[7]

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

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 characteristic pleasant, 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

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.^[8] 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. DMAP (4-dimethylaminopyridine) is used as an acyl-transfer catalyst.^[9]

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 expensive for large scale applications.

Alcoholysis of acyl chlorides and acid anhydrides

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

RCOCl + R'OH → RCO₂R' + HCl

(RCO₂)O + 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 not widely employed for esterifications, salts of carboxylate anions can be alkylating agent with alkyl halides to give esters. 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. 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

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:^[7]

(C₆H₄)(CO₂CH₃)₂ + 2 C₂H₄(OH)₂ → 1/n {(C₆H₄)(CO₂)₂(C₂H₄)}_n + 2 CH₃OH

Carbonylation

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

C₂H₄ + ROH + CO → C₂H₅CO₂R

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 → CH₃O₂CH

Addition of carboxylic acids to alkenes

In the presence of palladium-based catalysts, ethylene, acetic acid, and oxygen react to give vinyl acetate:

C₂H₄ + CH₃CO₂H + 1/2 O₂ → C₂H₃O₂CCH₃ + H₂O

Direct routes to this same ester are not possible because vinyl alcohol is unstable.

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 ^[10]
• Ozonolysis of alkenes using a work up in the presence of hydrochloric acid and various alcohols.^[11]
• Anodic oxidation of methyl ketones leading to methyl esters.^[12]

Reactions

Esters react with nucleophiles at the carbonyl carbon. The carbonyl is weakly electrophilic but 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 is the one that usually initiates condensation reactions. The carbonyl oxygen is weakly basic (less so than in amides) but forms adducts.

Addition of nucleophiles at carbonyl

Esterification is a reversible reaction. Esters undergo hydrolysis under acid 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:

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.^[13]

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.^[14][15]

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

• 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.^[16]

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.

List of ester odorants

Many esters have distinctive fruit-like odors, and many occur naturally in the essential oils of plants. This has also led to their commonplace use in artificial flavorings and fragrances when those odors aim to be mimicked.

Ester Name	Formula	Odor or occurrence
Allyl hexanoate		pineapple
Benzyl acetate		pear, strawberry, jasmine
Bornyl acetate		pine
Butyl acetate		apple, honey
Butyl butyrate		pineapple
Butyl propanoate		pear drops
Ethyl acetate		nail polish remover, model paint, model airplane glue
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 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

The Reality of Quantum Weirdness

FEB. 20, 2015

Original link:  http://www.nytimes.com/2015/02/22/opinion/sunday/the-reality-of-quantum-weirdness.html?_r=0 

Credit Olimpia Zagnoli

By EDWARD FRENKEL

 

In Akira Kurosawa’s film “Rashomon,” a samurai has been murdered, but it’s not clear why or by whom. Various characters involved tell their versions of the events, but their accounts contradict one another. You can’t help wondering: Which story is true?

But the film also makes you consider a deeper question: Is there a true story, or is our belief in a definite, objective, observer-independent reality an illusion?

This very question, brought into sharper, scientific focus, has long been the subject of debate in quantum physics. Is there a fixed reality apart from our various observations of it? Or is reality nothing more than a kaleidoscope of infinite possibilities?

This month, a paper published online in the journal Nature Physics presents experimental research that supports the latter scenario — that there is a “Rashomon effect” not just in our descriptions of nature, but in nature itself.

Over the past hundred years, numerous experiments on elementary particles have upended the classical paradigm of a causal, deterministic universe. Consider, for example, the so-called double-slit experiment. We shoot a bunch of elementary particles — say, electrons — at a screen that can register their impact. But in front of the screen, we place a partial obstruction: a wall with two thin parallel vertical slits. We look at the resulting pattern of electrons on the screen. What do we see?

If the electrons were like little pellets (which is what classical physics would lead us to believe), then each of them would go through one slit or the other, and we would see a pattern of two distinct lumps on the screen, one lump behind each slit. But in fact we observe something entirely different: an interference pattern, as if two waves are colliding, creating ripples.

Astonishingly, this happens even if we shoot the electrons one by one, meaning that each electron somehow acts like a wave interfering with itself, as if it is simultaneously passing through both slits at once.

So an electron is a wave, not a particle? Not so fast. For if we place devices at the slits that “tag” the electrons according to which slit they go through (thus allowing us to know their whereabouts), there is no interference pattern. Instead, we see two lumps on the screen, as if the electrons, suddenly aware of being observed, decided to act like little pellets.

To test their commitment to being particles, we can tag them as they pass through the slits — but then, using another device, erase the tags before they hit the screen. If we do that, the electrons go back to their wavelike behavior, and the interference pattern miraculously reappears.

There is no end to the practical jokes we can pull on the poor electron! But with a weary smile, it always shows that the joke is on us. The electron appears to be a strange hybrid of a wave and a particle that’s neither here and there nor here or there. Like a well-trained actor, it plays the role it’s been called to perform. It’s as though it has resolved to prove the famous Bishop Berkeley maxim “to be is to be perceived.”

Is nature really this weird? Or is this apparent weirdness just a reflection of our imperfect knowledge of nature?

The answer depends on how you interpret the equations of quantum mechanics, the mathematical theory that has been developed to describe the interactions of elementary particles. The success of this theory is unparalleled: Its predictions, no matter how “spooky,” have been observed and verified with stunning precision. It has also been the basis of remarkable technological advances. So it is a powerful tool. But is it also a picture of reality?

Here, one of the biggest issues is the interpretation of the so-called wave function, which describes the state of a quantum system. For an individual particle like an electron, for example, the wave function provides information about the probabilities that the particle can be observed at particular locations, as well as the probabilities of the results of other measurements of the particle that you can make, such as measuring its momentum.

Does the wave function directly correspond to an objective, observer-independent physical reality, or does it simply represent an observer’s partial knowledge of it?

If the wave function is merely knowledge-based, then you can explain away odd quantum phenomena by saying that things appear to us this way only because our knowledge of the real state of affairs is insufficient. But the new paper in Nature Physics gives strong indications (as a result of experiments using beams of specially prepared photons to test certain statistical properties of quantum measurements) that this is not the case. If there is an objective reality at all, the paper demonstrates, then the wave function is in fact reality-based.

What this research implies is that we are not just hearing different “stories” about the electron, one of which may be true. Rather, there is one true story, but it has many facets, seemingly in contradiction, just like in “Rashomon.” There is really no escape from the mysterious — some might say, mystical — nature of the quantum world.

But what, if anything, does all this mean for us in our own lives? We should be careful to recognize that the weirdness of the quantum world does not directly imply the same kind of weirdness in the world of everyday experience. That’s because the nebulous quantum essence of individual elementary particles is known to quickly dissipate in large ensembles of particles (a phenomenon often referred to as “decoherence”). This is why, in fact, we are able to describe the objects around us in the language of classical physics.

Rather, I suggest that we regard the paradoxes of quantum physics as a metaphor for the unknown infinite possibilities of our own existence. This is poignantly and elegantly expressed in the Vedas: “As is the atom, so is the universe; as is the microcosm, so is the macrocosm; as is the human body, so is the cosmic body; as is the human mind, so is the cosmic mind.”

Edward Frenkel, a professor of mathematics at the University of California, Berkeley, is the author of “Love and Math: The Heart of Hidden Reality.”

Ether

From Wikipedia, the free encyclopedia

The general structure of an ether

Ethers /ˈiːθər/ are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'.^[1] A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether" (CH₃-CH₂-O-CH₂-CH₃). Ethers are common in organic chemistry and pervasive in biochemistry, as they are common linkages in carbohydrates and lignin.

Structure and bonding

Ethers feature C-O-C linkage defined by a bond angle of about 104° and C-O distances of about 140 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 hydrogens alpha to ethers are more acidic than in simple hydrocarbons. They are far less acidic than hydrogens alpha to carbonyl groups (such as in ketones or aldehydes), however.

Depending on the groups at R and R', ethers are classified into two types:

1. Simple ethers or symmetrical ethers; e.g., Diethyl ether, dimethyl ether, etc.
2. Mixed ethers or asymmetrical ethers; e.g., Methyl ethyl ether, Methyl phenyl ether, etc.

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 compounds with more than one ether group.

The crown ethers are examples of low-molecular weight polyethers. Some toxins produced by dinoflagellates such as brevetoxin and ciguatoxin are in a class known as cyclic or ladder polyethers.

Polyether generally refers to polymers which contain the ether functional group in their main chain. The term glycol is reserved for low to medium range molar mass polymer when the nature of the end-group, which is usually a hydroxyl group, still matters. The term "oxide" or other terms are used for high molar mass polymer when end-groups no longer affect polymer properties.

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-
Polytetramethylene glycol (PTMG) or Polytetramethylene ether glycol (PTMEG)	Polytetrahydrofuran (PTHF)	Acid-catalyzed ring-opening polymerization of tetrahydrofuran	-CH 2CH 2CH 2CH 2O-	Terathane from Invista and PolyTHF from BASF

Aromatic polyethers

The phenyl ether polymers are a class of 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

Ether molecules cannot form hydrogen bonds with each other, resulting in relatively low boiling points compared to those of the analogous alcohols. The difference, however, in the boiling points of the ethers and their isomeric alcohols becomes lower as the carbon chains become longer, as the van der Waals interactions of the extended carbon chain dominates over the presence of hydrogen bonding.

Ethers are slightly polar. The C-O-C bond angle in the functional group is about 110°, and the C-O dipoles do not cancel out. Ethers are more polar than alkenes but not as polar as alcohols, esters, or amides of comparable structure. However, the presence of two lone pairs of electrons on the oxygen atoms makes hydrogen bonding with water molecules possible.

Cyclic ethers such as tetrahydrofuran and 1,4-dioxane are miscible in water because of the more exposed oxygen atom for hydrogen bonding as compared to linear aliphatic ethers.

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

Other properties are:

• The lower ethers are highly volatile and flammable.
• Lower ethers also act as anaesthetics.
• Ethers act as good organic solvents.

Reactions

Structure of the polymeric diethyl ether peroxide

Ethers are quite stable chemical compounds which do not react with bases, active metals, dilute acids, oxidising agents and reducing agents. Generally, they are of low chemical reactivity, but they are more reactive than alkanes (epoxides, ketals, and acetals are unrepresentative classes of ethers and are discussed in separate articles). Important reactions are listed below.^[2]

Ether cleavage

Although ethers resist hydrolysis, their polar bonds are cloven by mineral acids such as 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.^[3] Depending on the substituents, some ethers can be cloven 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 peroxide. 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.

Lewis bases

Ethers serve as Lewis bases and Bronsted bases. Strong acids protonate the oxygen to give "onium ions." For instance, diethyl ether forms a complex with boron trifluoride, i.e. diethyl etherate (BF₃^.OEt₂). Ethers also coordinate to Mg(II) center in Grignard reagents.

Alpha-halogenation

This reactivity is akin to the tendency of ethers with alpha hydrogen atoms to form peroxides. Chlorine gives alpha-chloroethers.

Synthesis

Ethers can be prepared in the laboratory in several different ways.

Dehydration of alcohols

The Dehydration of alcohols affords ethers:

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

^[4] This direct nucleophillic subsititution 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 SN2 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

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.

Important ethers

	Ethylene oxide	The smallest cyclic ether. Also the simplest epoxide.
	Dimethyl ether	An aerosol spray propellant. A potential renewable alternative fuel for diesel engines with a cetane rating as high as 56-57.
	Diethyl ether	A common low boiling solvent (b.p. 34.6 °C) and an early anaesthetic. Used as starting fluid for diesel engines. Also used as a refrigerant and in the manufacture of smokeless gunpowder, along with use in perfumery.
	Dimethoxyethane (DME)	A high boiling solvent (b.p. 85 °C):
	Dioxane	A cyclic ether and high boiling solvent (b.p. 101.1 °C).
	Tetrahydrofuran (THF)	A cyclic ether, one of the most polar simple ethers that is used as a solvent.
	Anisole (methoxybenzene)	An aryl ether and a major constituent of the essential oil of anise seed.
	Crown ethers	Cyclic polyethers that are used as phase transfer catalysts.
	Polyethylene glycol (PEG)	A linear polyether, e.g. used in cosmetics and pharmaceuticals.