Ethers are organic compounds containing an oxygen atom bonded to two carbon containing groups. Their general structural representation is ROR, where the groups attached to oxygen can be alkyl, aryl or other carbon based substituents. Unlike alcohols, ethers do not have an oxygen hydrogen bond and therefore cannot donate hydrogen bonds through the ether oxygen, although their oxygen atom can accept hydrogen bonds. Ethers can be symmetrical or unsymmetrical depending on whether the two carbon groups are identical.
They are widespread in organic chemistry and occur naturally in some biological molecules and synthetic materials. Their relatively stable carbon oxygen carbon linkage makes them useful as solvents, structural components and synthetic intermediates. The simplest examples include dimethyl ether and diethyl ether, while larger cyclic ethers and polyethers display more specialised properties.
Ethers are commonly prepared by reactions that create a carbon oxygen carbon bond. The Williamson ether synthesis is a major laboratory method in which an alkoxide nucleophile reacts with a suitable alkyl halide through nucleophilic substitution. It works particularly well with primary substrates because competing elimination becomes more significant with hindered substrates. Acid catalysed reactions can also form ethers from alcohols under appropriate conditions. Industrial ether production can involve dehydration processes or other catalytic routes. Cyclic ethers can be produced through intramolecular reactions that close a carbon containing chain into a ring. The synthetic method selected depends on the desired substitution pattern, steric environment, functional group compatibility and scale.
Ethers generally have moderate polarity because oxygen possesses lone electron pairs and a significant electronegativity difference exists between oxygen and carbon. They can accept hydrogen bonds from water and other hydrogen bond donors, but they cannot donate hydrogen bonds themselves. Small ethers can therefore dissolve many organic substances while also having some water solubility. Diethyl ether is relatively volatile and has historically been widely used as an organic solvent. Larger ethers generally have higher boiling points and lower volatility. Cyclic ethers such as tetrahydrofuran have important solvent properties because their ring structure and oxygen atom provide a useful combination of polarity and organic compatibility. Polyethers contain multiple ether oxygen atoms and can have substantially different solubility and coordination properties.
Ethers have important biological roles. Some carbohydrates contain cyclic ether like structures in which an oxygen atom forms part of a ring, although their chemistry is more complex than that of simple ethers. Polyether natural products can bind metal ions and transport them across membranes. Ether lipids contain ether linkages connecting hydrocarbon chains to glycerol derived structures and occur particularly prominently in certain organisms and tissues.
The stability of ether bonds under some chemical conditions can make them useful biological structural features. However, biological ether containing molecules can still undergo specialised enzymatic oxidation or cleavage. Their presence demonstrates that the ether functional group is not restricted to laboratory solvents and industrial chemicals.
Ethers are valuable solvents because they can dissolve many nonpolar and moderately polar compounds while remaining relatively chemically unreactive toward many reagents. Diethyl ether has a low boiling point and can be removed readily, but it is highly flammable and can form dangerous peroxides during prolonged storage when exposed to oxygen and light. Tetrahydrofuran is another important ether solvent with higher polarity and strong compatibility with organometallic reagents and polymers.
Glycol ethers contain both ether and alcohol or related functional groups and are used in coatings, cleaning products and industrial formulations. Crown ethers contain repeated ether units arranged in a ring and can selectively coordinate certain metal cations, making them useful in supramolecular chemistry and phase transfer chemistry.
Ether oxygen atoms can coordinate positively charged species through their lone pairs. Crown ethers illustrate this property particularly clearly because their ring cavities can accommodate certain cations according to ionic radius and coordination geometry. This ability can change the apparent solubility and reactivity of ionic substances in organic solvents. Polyethers similarly provide multiple donor atoms capable of coordinating metals. Such interactions are important in analytical chemistry, catalysis, separation science and molecular recognition. The chemical behaviour of an ether therefore depends not only on the basic carbon oxygen carbon connection but also on the number, arrangement and environment of the oxygen atoms present.
Ethers differ structurally and chemically from alcohols, esters and epoxides. An alcohol has an oxygen hydrogen bond, whereas an ether has two carbon groups attached to oxygen. An ester contains a carbonyl group in addition to oxygen, giving it substantially different reactivity. An epoxide is a three membered cyclic ether whose severe ring strain makes it much more reactive toward nucleophilic ring opening than ordinary open chain ethers. Ethers are often relatively resistant to many bases and nucleophiles, but they can react under strongly acidic conditions and can undergo cleavage with powerful reagents such as hydrogen halides under appropriate circumstances. Their apparent inertness is therefore conditional rather than absolute.
Ethers are important in both basic and advanced chemistry because they combine useful polarity with comparatively low reactivity. Their oxygen atoms permit hydrogen bond acceptance and metal coordination, while their carbon containing groups provide compatibility with many organic substances. The same features that make ethers useful solvents can also create safety considerations, particularly flammability and peroxide formation in certain low molecular mass ethers. Structural modifications allow chemists to tune volatility, polarity, water miscibility, coordination ability and thermal behaviour. Ethers consequently form a broad family extending from simple solvents to biological membrane components, cyclic structures, polyethers and specialised molecular recognition systems.
