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Ethers

Ethers are organic compounds in which an oxygen atom is connected to two carbon groups.  Unlike alcohols, ethers are fairly unreactive (except towards combustion).  They are very common organic solvents, and some ethers also make good anesthetics.

Simple ethers are named by naming the alkyl groups in alphabetical order and adding the word "ether" to the end.  In more complicated ethers, the ether group is named as an alkoxy substituent, in which the "yl" ending of alkyl groups is replaced by "oxy."  For example, a methyl group attached to an oxygen, OCH3, is named as a methoxy group.  In cyclic ethers, the prefix oxa- is used to indicate replacement of a carbon by an oxygen.

Since ethers do not have a hydrogen connected to the oxygen atoms, they are not capable of forming hydrogen bonds to each other.  Thus, their boiling points are similar to those of alkanes of comparable molecular mass.  For example, propane, dimethyl ether, and ethanol have very similar molecular masses, but drastically different boiling points:

  CH3—CH2—CH3 CH3—O—CH3 CH3—CH2—OH
  propane dimethyl ether ethanol
Molecular Mass MM 44.10 g/mol 46.07 g/mol 46.07 g/mol
Boiling Point -42.1C -24.9C 78.3C
Water Solubility 0.1 g/mL 3.28 g/mL infinite

Ethers can form hydrogen bonds to water, since the oxygen atom is attracted to the partially-positive hydrogens in water molecules, making them more soluble in water than alkanes.

Dimethyl ether (Methoxymethane) 3D

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Dimethyl ether, or more formally, methoxymethane, is a colorless gas at room temperature, having a boiling point of -42.1C.  It is fairly water soluble, with 328 grams of the gas dissolving in 100 mL of water.  It is used as an aerosol propellant, as a refrigerant, and as a blowing agent for the production of some foams.  It can also be used as a fuel in diesel engines.

 
   
Ethyl methyl ether (Methoxyethane) 3D

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Ethyl methyl ether, or methoxyethane, is a colorless gas at room temperature, having a boiling point of 7.6C.  Like dimethyl ether, it is fairly water soluble.

 
   
Diethyl ether (Ethoxyethane) 3D

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Diethyl ether, or ethoxyethane, or just plain ether, is a colorless liquid at room temperature, having a boiling point of 34.6C.  Unlike dimethyl ether and ethyl methyl ether, it is only slightly soluble in water, with 6.9 grams of diethyl ether dissolving in 100 mL of water.  Diethyl ether has a strong, somewhat sweet, odor, and the vapor can cause drowsiness or unconsciousness.  Since it has such a low boiling point (less than human body temperature), it evaporates easily, and the fumes can quickly become overwhelming.  Diethyl ether is also extremely flammable, especially in the vapor form.

Diethyl ether was first synthesized by the German physician Valerius Cordus in 1540, who obtained it by distilling a mixture of ethanol and sulfuric acid ("oil of vitriol"); he named the substance "oil of sweet vitriol."

Diethyl ether was one of the first commonly used general anesthetics.  Its use as an anesthetic was first demonstrated publicly by Crawford W. Long on March 30, 1842.  (There was a long and bitter priority dispute between William T. G. Morton, Charles T. Jackson, and Horace Wells, who also made public demonstrations of the use of ether in the 1840s, but Long's work is now generally recognized to have been the first.)  Ether was widely used in surgical procedures until the mid 20th century, when it was replaced by nonflammable anesthetics such as halothane, which also reduced post-surgical nausea.

Diethyl ether is commonly used in chemistry labs as a solvent.  It is unreactive towards most oxidizing and reducing agents, doesn't react with acids or bases, and dissolves a wide variety of compounds.  It is particularly useful in the Grignard reaction, in which organomagnesium compounds (called Grignard reagents) react with compounds containing carbon-oxygen double bonds, thus producing new carbon-carbon bonds.  These reactions require extremely dry conditions, because any water which is present will react with the Grignard reagent.  Ether is fairly easy to obtain in a very dry form, either by purchasing it directly from a chemical supply company, or by distilling it from sodium.  In addition, the lone pairs on the oxygen atoms in the ether can complex with the magnesium atoms in the Grignard reagent, stabilizing the reagent somewhat.

There are a number of hazards associated with the use of diethyl ether.  It is extremely flammable, it evaporates very easily, and its vapor is more dense than air.  It is a standing rule in organic chemistry labs (among those who want to remain standing) that when diethyl ether is being used, no open flames are allowed, since a Bunsen burner can light ether vapor which has evaporated from a container some distance away.  For a demonstration of this, see the Demonstrations page on the Ether Trough.  Another hazard associated with diethyl ether is its ability to form peroxides upon standing; ether peroxides are dangerously unstable, and old bottles of ether that have been sitting around for a long time are a potential explosion hazard.  In addition, heating ethers can also cause the formation of peroxides, especially towards the end of a distillation when a large amount of heat is being passed through a decreasing amount of liquid.  For this reason, it is a standing rule in chemistry labs that ethers should never be distilled to dryness.

 

The Ether Bunny

 
   
Ethylene oxide 3D

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Ethylene oxide, or oxacyclopropane (the oxa prefix indicates replacement of a carbon with an oxygen in a cyclic compound), and oxirane, is the simplest of the cyclic ethers.  Compounds which contain two carbon atoms and one oxygen atom in a ring are also known as epoxides.  Ethylene oxide is synthesized industrial by the reaction of ethylene with oxygen; this reaction is carried out at high pressures with a silver catalyst to produce ethylene oxide in high yields.  Ethylene oxide is used in the production of ethylene glycol, and is also used to sterilize medical equipment.  It is also an important reagent in organic synthesis.

 
   
Glyme (1,2-dimethoxyethane; DME) 3D

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Glyme, or 1,2-dimethoxyethane (DME), is a common organic solvent.

 
   
Methyl tert-butyl ether (MTBE) 3D

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Methyl tert-butyl ether (MTBE), or 2-methoxy-2-methylpropane, is a common organic solvent.  It is fairly soluble in water, with 4.8 grams of MTBE dissolving in 100 mL of water.

MTBE has been used as an octane-boosting additive for gasoline, but this use is becoming less common because of concerns about the contamination of drinking water resulting from the leakage of MTBE-laced gasoline from underground storage tanks.  Because MTBE is more soluble in water than the hydrocarbon components of gasoline, it tends to be the component that dissolves first in groundwater.

 
   
Tetrahydrofuran (THF) 3D

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Tetrahydrofuran, or oxacyclopentane, is a common organic solvent (especially in Grignard reactions), often used in place of diethyl ether because of its much lower volatility and flammability.  It is derived from the aromatic molecule furan, by catalytic hydrogenation, in which two hydrogen molecules (or four hydrogen atoms, hence "tetrahydro") are added to the two double bonds in furan, turning them into single bonds.

 
   
Pyran 3D

2H-Pyran

2H-Pyran
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4H-Pyran

4H-Pyran
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The pyrans are heterocyclic rings consisting of five carbon atoms and one oxygen atom, with four of the carbons in carbon-carbon double bonds.  There are two isomers of pyran, which differ in the positions of the double bonds, as shown above.  The pyran structure is found in the cyclic form of many sugar molecules, where it is usually referred to as a pyranose ring.

 
   
1,4-Dioxane 3D

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1,4-Dioxane is a common organic solvent.  Like diethyl ether and THF, it is a good solvent for the Grignard reaction.  Like many small cyclic ethers, it is miscible with water.

 
   
Dibenzo-para-dioxin 3D

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The molecule shown above, dibenzo-para-dioxin, is the parent compound of a group of molecules called the dioxins, which contain two benzene rings links by ether bridges, and containing chlorine atoms on the benzene rings (polychlorinated dibenzodioxins, PCDDs).  Dioxins are produced in the burning of coal, metal smelting, diesel trucks, the burning of treated wood, cigarette smoke, and as side products of the reactions used to synthesize some herbicides.  (See entry below.)  Dioxins are fat-soluble, and can accumulate in the bodies of those who come into contact with them.  Many of the dioxins are toxic and carcinogenic, especially those in which chlorine atoms are substituted at the b-positions (three carbons away from the oxygen atoms) on the benzene rings, rather than the a-positions (two carbons away from the oxygens).

 
   
2,3,7,8-Tetrachlorodibenzo-para-dioxin 3D

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2,3,7,8-Tetrachlorodibenzo-para-dioxin (TCDD) is the most toxic of the dioxins.  It was present as a contaminant in Agent Orange, a herbicide used to clear jungle foliage during the Vietnam War.  Agent Orange itself was a mixture of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T); TCDD was produced during the synthesis of these compounds, and was present in the mixture that was used during the war.

 
   
Anisole 3D

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Anethole, or methoxybenzene, has a smell similar to that of oil of aniseed (see entry for anethole below), and is used in perfumes.

 
   
Anethole 3D

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Anethole, or 1-methoxy-4-(1-propenyl)benzene, is found in anise (Pimpinella anisum), fennel (Foeniculum vulgare), star anise (Illicium verum), and tarragon (Artemisia dracunculus).

 
   
Estragole 3D

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Estragole, or 1-methoxy-4-(2-propenyl)benzene, is the main component of tarragon oil(Artemisia dracunculus), and is also found in pine oil and turpentine.

 
   
18-Crown-6 3D

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18-crown-6, or more formally, 1,4,7,10,13,16-hexaoxacyclooctadecane, is a member of a class of compounds called the crown ethers.  These are large, cyclic poly-ether molecules which are capable of binding metal cations.  They are used in organic reactions as phase-transfer catalysts, which are capable of making ionic salts soluble in nonpolar organic solvents; by surrounding the metal cation with a hydrocarbon ring, the ionic compound can be dragged into an organic solution, allowing the anion portion of the salt to react with the appropriate organic substrate.

The name "crown ether" is derived from the three-dimensional shape of the molecule, in which the oxygen atoms point upwards in a way that suggests the points on a crown.  The Nobel Prize in Chemistry for 1987 was awarded to Charles J. Pedersen, Donald J. Cram, and Jean-Marie Lehn, for their development of crown ethers, and other molecules which are capable of "recognizing" others and binding them in specific and selective ways.

In the "crown" system for naming these molecules, the first number indicates the number of atoms in the ring, and the second indicates the number of oxygen atoms.  Crown ethers of various sizes can be synthesized which are capable of binding particular metal ions:  18-crown-6 selectively binds potassium ions (see structure below), 15-crown-5 binds sodium ions, and 12-crown-4 binds lithium ions.

 

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Anesthetics

General anesthetics, such as diethyl ether, revolutionized medicine by allowing patients to undergo surgery without feeling pain.  The term "anesthesia" was derived by Oliver Wendell Holmes, Sr. in 1846 from the Green words an- ("without") and aisthesis ("perception").

The mechanisms of action of general anesthetics is still uncertain, but it is believed that these compounds disrupt the order arrangement of protein receptors and channel on cell membranes, disrupting signal transmission and thereby causing unconsciousness.

In most cases, rather than using a single anesthetic during the entire procedure, a complex mixture of anesthetics is used to produced the desired level and duration of unconsciousness.

Nitrous oxide 3D

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Nitrous oxide, or dinitrogen monoxide, is better known as laughing gas, the anesthetic most commonly used in dental procedures and minor surgery.  It blocks or minimizes the perception of pain, and causes euphoria, dizziness, and slight hallucinations, and also interferes with the perception of sound (flanging).

Nitrous oxide was the first chemical anesthetic to be discovered.  It was discovered by the English scientist Joseph Priestley in 1775 by heating iron filings that were wet with nitric acid.  (He referred to the gas as "phlogisticated nitrous air" — phlogiston was still all the rage.)  The gas was further investigated by Sir Humphrey Davy in the 1790s, who realized its mild anesthetic effects.

Industrially, nitrous oxide is prepared by heating ammonium nitrate; this must be done cautiously because of the risk of explosion.

In dentistry, a mixture of nitrous oxide and oxygen is inhaled by the patient as a mild sedative to relieve anxiety.  It is also used as a carrier gas (again, combined with oxygen) during general anesthesia.  Nitrous oxide is also used as a propellant in aerosol sprays(such as in whipped cream and cooking sprays), as an oxidant in rocket fuels, and in some race cars to deliver more oxygen (at high temperatures, nitrous oxide breaks down into nitrogen and oxygen) to the car's engine.

 
   
Chloroform 3D

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See entry on Alkyl Halides page.

 
   
Diethyl ether 3D

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See entry on diethyl ether above.

 
   
Divinyl ether 3D

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Divinyl ether is an inhalant gas that is faster acting and less nauseating than diethyl ether.  Like diethyl ether and other hydrocarbon ethers, its use has largely been superseded by halogenated compounds that are faster acting, less nauseating, and less flammable.

 
   
Halothane 3D

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Halothane, or 2-bromo-2-chloro-1,1,1-trifluoroethane, is a nonflammable, non-ether-based anesthetic.  Its use has largely been superseded by sevoflurane, desflurane, and other compounds.

 
    
Methoxyflurane 3D

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Methoxyflurane, or 2,2-dichloro-1,1-difluoro-1-methoxyethane halogenated ether used as an anesthetic in the 1960s; it was taken off the market because it was found to produce harmful fluoride ions when metabolized in the kidneys.

 
   
Enflurane 3D

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Enflurane, or 2-chloro-1-(difluoromethoxy)-1,1,2-trifluoroethane, and its isomer isoflurane are halogenated anesthetics which are much less flammable than diethyl ether.

 
   
Isoflurane 3D

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Isoflurane, or 2-chloro-2-(difluoromethoxy)-1,1,1-trifluoroethane, and its isomer enflurane are an halogenated anesthetics which are much less flammable than diethyl ether.

 
   
Sevoflurane 3D

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Sevoflurane, or 1,1,1,3,3,3-hexafluoro-2-(fluoromethoxy)propane, or fluoromethyl, is a sweet-smelling, non-flammable, halogenated anesthetic.  Sevoflurane and desflurane are largely replacing enflurane and isoflurane.

 
   
Desflurane 3D

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Desflurane, or 2-(difluoromethoxy)-1,1,1,2-tetrafluoroethane, is a non-flammable, halogenated anesthetic.  Desflurane and sevofluraneare largely replacing enflurane and isoflurane.

 
   
Propofol 3D

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Propofol, or 2,6-diisopropylphenol, is a fast acting anesthetic; unlike the other anesthetics listed in this section, it is administered intravenously rather than by inhalation.

 

 

 

References

P. W. Atkins, Molecules, 2nd ed.  Cambridge: Cambridge University Press, 2003, p. 151, 200.

Robert Brunsvold, "Chemical Aspects of General Anesthesia:  Part I.  From Ether to Halothane." (J. Chem. Ed., 2006, 82, p1821).

Robert Brunsvold, "Chemical Aspects of General Anesthesia:  Part II.  Current Practices." (J. Chem. Ed., 2006, 82, p1826).

Richard J. Lewis, Sr., Hawley's Condensed Chemical Dictionary, 13th ed.  New York:  Van Nostrand Reinhold, 1997.

D. W. A. Sharp, The Penguin Dictionary of Chemistry, 2nd ed.  London:  Penguin Books, 1990.

Graham Solomons and Craig Fryhle, Organic Chemistry, 7th ed.  New York:  John Wiley & Sons, Inc., 2000, p. 478-484, 521.

Martha Windholz (ed.), The Merck Index, 10th ed. Rahway: Merck & Co., Inc., 1983.