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Ether is 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.
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Ethers feature C-O-C linkage defined by a bond angle of about 120° and C-O distances of about 1.5 Å. 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 ketones, however.
The names for simple ethers (i.e. those with none or few other functional groups) are a composite of the two substituents followed by "ether." Methyl ethyl ether (CH3OC2H5), diphenylether (C6H5OC6H5). IUPAC rules are often not followed for simple ethers. 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.
In the IUPAC nomenclature system, which is rarely encountered, 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). The nomenclature of describing the two alkyl groups and appending "ether", e.g. "ethyl methyl ether" in the example above, is a trivial usage.
Polyethers are compounds with more than one ether group. The term generally refers to polymers like polyethylene glycol and polypropylene glycol. The crown ether are examples of low-molecular polyethers.
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).
Ether molecules cannot form hydrogen bonds amongst each other, resulting in a relatively low boiling point compared to that of the analogous alcohols. The difference, however, in the boiling points of the ethers and their isometric alcohols become smaller as the carbon chains become longer, as the van der waals interactions of the extended carbon chain dominate over the presence of hydrogen bonding.
Ethers are slightly polar, as the COC bond angle in the functional group is about 110 degrees, 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, causing the solubility of alcohols (for instance, butan-1-ol) and ethers (ethoxyethane) to be quite dissimilar.
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 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 |
Ethers in general 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]
Although ethers resist hydrolysis, they are cleaved by mineral acids such as hydrobromic acid and hydroiodic acid. Hydrogen chloride cleaves ethers only slowly. Methyl ethers typically afford methyl halides:
These reactions proceed via onium intermediates, i.e. [RO(H)CH3]+Br-.
Some ethers rapidly cleave 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 cleaved with a variety of reagents, e.g. strong base.
Primary and secondary ethers with a CH group next to the ether oxygen form peroxides, e.g. diethyl ether peroxide. The reaction requires oxygen (or air) and is accelerated by light, metal catalysts, and aldehydes. The resulting peroxides can be explosive. For this reason, diisopropyl ether and THF are often avoided as solvents.
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. Polyethers, including many antibiotics, cryptands, and crown ethers, bind alkali metal cations strongly.
This reactivity is akin to the tendency of ethers with alpha hydrogen atoms to form peroxides. Chlorine gives alpha-chloroethers.
Ethers can be prepared in the laboratory in several different ways.
The Dehydration of alcohols affords ethers:
This direct 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. Diethyl ether is produced from ethanol by this method. Cyclic ethers are readily generated by this approach. Such reactions must compete with dehydration of the alcohol:
The dehydration route often requires conditions incompatible with delicate molecules. Several milder methods exist to produce ethers.
Nucleophilic displacement of alkyl halides by alkoxides
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.
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.
Alcohols add to electrophilically activated alkenes.
Acid catalysis is required for this reaction. Often, mercury trifluoroacetate (Hg(OCOCF3)2) is used as a catalyst for the reaction, geneating an ether with Markovnikov regiochemistry. Using similar reactions, tetrahydropyranyl ethers are used as protective groups for alcohols.
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:
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Ethylene oxide | The smallest cyclic ether. |
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Dimethyl ether | An aerosol spray propellant. A potential renewable alternative fuel for diesel engines with a cetane rating as high as 56-57. |
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Diethyl ether | A common low boiling solvent (b.p. 34.6°C), and an early anaesthetic. |
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Dimethoxyethane (DME) | A high boiling solvent (b.p. 85°C): |
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Dioxane | A cyclic ether and high boiling solvent (b.p. 101.1°C). |
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Tetrahydrofuran (THF) | A cyclic ether, one of the most polar simple ethers that is used as a solvent. |
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Anisole (methoxybenzene) | An aryl ether and a major constituent of the essential oil of anise seed. |
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Crown ethers | Cyclic polyethers that are used as phase transfer catalysts. |
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Polyethylene glycol (PEG) | A linear polyether, e.g. used in cosmetics and pharmaceuticals. |
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| The Complete Works
of Swami Vivekananda by Volume 9, Writings: Prose and Poems(Original and Translated) |
Notes→ |
This article first appeared anonymously in the February 1895
issue of the New York Medical Times, a prestigious monthly medical
journal founded and edited by Dr. Egbert Guernsey.
Classification or grouping of phenomena by their similarities is
the first step in scientific knowledge — perhaps it is all. An
organized grouping, revealing to us a similarity running through
the whole group, and a conviction that under similar circumstances
the group will arrange itself in the same form — stretched over all
time, past, present and future — is what we call law.
This finding of unity in variety is really what we call
knowledge. These different groups of similars are stowed away in
the pigeon-holes of the mind, and when a new fact comes before us
we begin to search for a similar group already existing in one of
the pigeon-holes of the mind. If we succeed in finding one
ready-made, we take the newcomer in immediately. If not, we either
reject the new fact, or wait till we find more of his kind, and
form a new place for the group.
Facts which are extraordinary thus disturb us; but and when we
find many like them, they cease to disturb, even when our knowledge
about their cause remains the same as before.
The ordinary experiences of our lives are no less wonderful than
any miracles recorded in any sacred book of the world; nor are we
any more enlightened as to the cause of these ordinary experiences
than of the so-called miracles. But the miraculous is
"extraordinary", and the everyday experience is "ordinary". The
"extraordinary" startles the mind, the "ordinary" satisfies.
The field of knowledge is so varied, and the more the difference
is from the centre, the more widely the radii diverge.
At the start the different sciences were thought to have no
connection whatever with each other; but as more and more knowledge
comes in — that is, the more and more we come nearer the centre —
the radii are converging more and more, and it seems that they are
on the eve of finding a common centre. Will they ever find
it?
The study of the mind was, above all, the science to which the sages of India and Greece had directed their attention. All religions are the outcome of the study of the inner man. Here we find the attempt at finding the unity, and in the science of religion, as taking its stand upon general and massive propositions, we find the boldest and the most vigorous manifestation of this tendency at finding the unity.
Some religions could not solve the problem beyond the finding of a duality of causes, one good, the other evil. Others went as far as finding an intelligent personal cause, a few went still further beyond intellect, beyond personality, and found an infinite being.
In those, and only those systems which dared to transcend beyond
the personality of a limited human consciousness, we find also an
attempt to resolve all physical phenomena into unity.
The result was the "Akâsha" of the Hindus and the "Ether" of the Greeks.
This "Akasha" was, after the mind, the first material manifestation, said the Hindu sages, and out of this "Akasha" all this has been evolved.
History repeats itself; and again during the latter part of the nineteenth century, the same theory is coming with more vigour and fuller light.
It is being proved more clearly than ever that as there is a
co-relation of physical forces there is also a co-relation of
different [branches of] knowledge, and that behind all these
general groups there is a unity of knowledge.
It was shown by Newton (Isaac Newton, 1642 – 1727.) that if
light consisted of material particles projected from luminous
bodies, they must move faster in solids and liquids than in air, in
order that the laws of refraction might be satisfied.
Huyghens, (Christian Huyghens, 1629 – 1695.) on the other hand,
showed that to account for the same laws on the supposition that
light consisted in the undulating motion of an elastic medium, it
must move more slowly in solids and fluids than in gases. Fizeau
(Armand Hippolyte Louis Fizeau, 1819 – 1896.) and Foucault (Jean
Bernard Léon Foucault, 1819 – 1868.) found Huyghens's predictions
correct.
Light, then, consists in the vibrating motion of a medium, which must, of course, fill all space. This is called the ether.
In the fact that the theory of a cosmic ether explains fully all the phenomena of radiation, refraction, diffraction and polarization of light is the strongest argument in favour of the theory.
Of late, gravitation, molecular action, magnetic, electric, and electro-dynamic attractions and repulsions have thus been explained.
Sensible and latent heat, electricity and magnetism themselves
have been of late almost satisfactorily explained by the theory of
the all-pervading ether.
Zöllner, (Johann K. F. Zöllner, 1834 – 1882.) however, basing his calculations upon the data supplied by the researches of Wilhelm Weber (Wilhelm Eduard Weber, 1804 – 1891.), thinks that the transmission of life force between the heavenly bodies is effected both ways, by the undulation of a medium and by the actual evidence of particles.
Weber found that the molecules, the smallest particles of bodies, were composed of yet smaller particles, which he called the electric particles, and which in the molecules are in a constant circular motion. These electric particles are partly positive, partly negative.
Those of the same electricity repulse those of different
electricity; attracting each other, each molecule contains the same
amount of electric particles, with a small surplus of either
positive or negative quickly changing the balance.
Upon this Zöllner builds these propositions:
(1) The molecules are composed of a very great number of
particles — the so-called electric particles, which are in constant
circular motion around each other within the molecule.
(2) If the inner motion of a molecule increases over a certain
limit, then electric particles are emitted. They then travel from
one heavenly body through space until they reach another heavenly
body, where they are either reflected or absorbed by other
molecules.
(3) The electric particles thus traversing space are the ether
of the physicist.
(4) These ether particles have a twofold motion: first, their
proper motion; second, an undulatory motion, for which they receive
the impulse from the ether particles rotating in the
molecules.
(5) The motion of the smallest particles corresponds to that of
the heavenly bodies.
The corollary is:
The law of attraction which holds good for the heavenly bodies
also holds good for the smallest particles.
Under these suppositions, that which we call space is really
filled with electric particles, or ether.
Zöllner also found the following interesting calculation for the
electric atoms:
Velocity: 50,143 geographical miles per second.
Amount of ether particles in a water molecule: 42,000 million.
Distance from each other: 0.0032 millimeter.
So far as it goes, then, the theory of a universal cosmic ether is the best at hand to explain the various phenomena of nature.
As far as it goes, the theory that this ether consists of
particles, electric or otherwise, is also very valuable. But on all
suppositions, there must be space between two particles of ether,
however small; and what fills this inter-ethereal space? If
particles still finer, we require still more fine ethereal
particles to fill up the vacuum between every two of them, and so
on.
Thus the theory of ether, or material particles in space, though accounting for the phenomena in space, cannot account for space itself.
And thus we are forced to find that the ether which comprehends the molecules explains the molecular phenomena, but itself cannot explain space because we cannot but think of ether as in space. And, therefore, if there is anything which will explain this space, it must be something that comprehends in its infinite being the infinite space itself. And what is there that can comprehend even the infinite space but the Infinite Mind?
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Singular |
Plural |
Ether
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Ether m.
One of the cities in the lowland of Judah allotted to Simeon (Josh 15:42, Josh 19:7).
   | An Account of the Extraordinary Medicinal Fluid, called Aether - An Account of the Extraordinary Medicinal Fluid, called Aether. by Turner - Project Gutenberg |
| | The Ether Monument - Ether Monument (The Good Samaritan) in the Boston Public Gardens in Massachusetts by John Quincy Adams Ward |
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