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The general structure of an ether

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.

Contents

Structure and bonding

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.

Nomenclature

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

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.

Related compounds, not classified as ethers

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 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

 

Reactions

Structure of the polymeric diethyl ether peroxide

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]

Ether cleavage

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:

ROCH3 + HBr → CH3Br + ROH

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.

Peroxide formation

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.

As 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. Polyethers, including many antibiotics, cryptands, and crown ethers, bind alkali metal cations strongly.

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

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:

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' + X-

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, geneating 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

Chemical structure of ethylene oxide Ethylene oxide The smallest cyclic ether.
Chemical structure of dimethyl ether Dimethyl ether An aerosol spray propellant. A potential renewable alternative fuel for diesel engines with a cetane rating as high as 56-57.
Chemical structure of diethyl ether Diethyl ether A common low boiling solvent (b.p. 34.6°C), and an early anaesthetic.
Chemical structure of dimethoxyethane Dimethoxyethane (DME) A high boiling solvent (b.p. 85°C):
Chemical structure of dioxane Dioxane A cyclic ether and high boiling solvent (b.p. 101.1°C).
Chemical structure of THF Tetrahydrofuran (THF) A cyclic ether, one of the most polar simple ethers that is used as a solvent.
Chemical structure of anisole Anisole (methoxybenzene) An aryl ether and a major constituent of the essential oil of anise seed.
Chemical structure of 18-crown-6 Crown ethers Cyclic polyethers that are used as phase transfer catalysts.
Chemical structure of polyethylene glycol Polyethylene glycol (PEG) A linear polyether, e.g. used in cosmetics and pharmaceuticals.

References

  1. ^ International Union of Pure and Applied Chemistry. "ethers". Compendium of Chemical Terminology Internet edition.
  2. ^ Wilhelm Heitmann, Günther Strehlke, Dieter Mayer "Ethers, Aliphatic" in Ullmann's Encyclopedia of Industrial Chemistry" Wiley-VCH, Weinheim, 2002. doi:10.1002/14356007.a10_023
  3. ^ J. F. W. McOmie and D. E. West (1973), "3,3'-Dihydroxylbiphenyl", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV5P0412 ; Coll. Vol. 5: 412 

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Up to date as of January 22, 2010

From Wikisource

< The Complete Works of Swami Vivekananda | Volume 9/Writings: Prose and Poems(Original and Translated)
The Complete Works of Swami Vivekananda by Swami Vivekananda
Volume 9, Writings: Prose and Poems(Original and Translated)
THE ETHER[6]*

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?


1911 encyclopedia

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Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

See also ether

Contents

English

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Proper noun

Singular
Ether

Plural
-

Ether

  1. The ancient American prophet of Mormon theology who wrote the Book of Ether in the Book of Mormon.

Translations

Anagrams


German

Alternative spellings

Noun

Ether m.

  1. ether

Bible wiki

Up to date as of January 23, 2010

From BibleWiki


One of the cities in the lowland of Judah allotted to Simeon (Josh 15:42, Josh 19:7).

This entry includes text from the Jewish Encyclopedia, 1906.
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