Introduction to Ethers

James Richard Fromm


Two alcohol molecules will react under the proper conditions to produce compounds with the general formula R-O-R'.  These compounds are called ETHERS.   The other product of the reaction is water which is taken up by a dehydration agent.

2ROH arrow2.gif (113 bytes) ROR + H2O

  Simple ethers are prepared commercially by dehydration of the corresponding alcohols by sulfuric acid:

A typical reaction is:

2CH3-CH2-OH --H2SO4 --> CH3-CH2-O-CH2-CH3 + H2O

also written:

2C2H5OH --H2SO4--> C2H5OC2H5  + H2O

The ethers are named by naming the radical on either side of the oxygen atom, and adding the word ether.  The two side chains can be the same, of course, in which case the name is mentioned just once, but preceded by the prefix di-.  Thus, in the preceding reaction, 2 moles of ethanol will react to form 1 mole of diethyl ether.

Ethers may also be named as OXY DERIVATIVES of the hydrocarbons.  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 nomenclature of describing the two alkyl groups and appending "ether", e.g. "ethylmethyl ether" is a trivial usage.

The ether radicals for the first four alkanes are:

CH3-O- Methoxy-
CH3-CH2-O- Ethoxy-
CH3-CH2-CH2-O- Propoxy-
CH3-CH2-CH2-CH2-O- Butoxy-

Another name for diethyl ether is ethoxyethaneDimethyl ether is methoxymethane, and methylheptyl ether is 1-methoxyheptane.

CH3-O-CH3 CH3-O-CH2-CH2-CH2-CH2-CH2-CH2-CH3
dimethyl ether methylheptyl ether

The structural formula for 1,4-dimethoxy-2-pentene is

CH3-O-CH2-CH=CH-CH(CH3-O)-CH3

Many ethers are used in industry as solvents. Diethyl ether has found widespread use as an anesthetic since 1846, in which application it is referred to simply as ether.  It and other ethers are valuable solvents for gums, fats, waxes, and resins.

Ethers are highly flammable and have relatively high vapor pressures, so they constitute a significant fire hazard. Moreover, on standing in air many simple ethers react slowly with atmospheric oxygen to form unstable and highly explosive organic peroxides, a significant hazard in distillations which use ethers.

It would be uncommon (but correct) to name diethyl ether as ethoxyethane, but the name paraethoxybenzoic acid would be appropriate for

C2H5OC6H6COOH. 

The two simplest aromatic ethers carry the nonsystematic names of anisole (methoxybenzene) and phenetole (ethoxybenzene).

More complex ether structures are prepared in multistep synthesis.

anisole.gif (2666 bytes)

Physical Properties

Ether molecules cannot form hydrogen bonds among each other, resulting in a relatively low boiling point comparable to that of the analogous alcohols.  However, the differences in the boiling points of the ethers and their isometric alcohols become smaller as the carbon chains become longer, as the hydrophobic nature of the carbon chain becomes more predominant over the presence of hydrogen bonding.

Ethers are slightly polar as the C - O - C bond angle in the functional group is about 110 degrees, and the C - O dipole does 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, 1-butanol) and ethers (ethoxyethane) to be quite dissimilar.

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

tetrahydrofuran.jpg (2234 bytes) 1,4-dioxane.jpg (3239 bytes)
Tetrahydrofuran 1,4-Dioxane

Ethers can act as Lewis bases.  For instance, diethyl ether forms a complex with boron compounds, such as boron trifluoride diethyl etherate (BF3.OEt2).   Ethers also coordinate to magnesium in Grignard reagents (RMgBr).

Similar Structures

structures_not_ethers.jpg (10487 bytes)

Not all compounds of the formula R-O-R are ethers

Ethers are not to be confused with the following classes of compounds with the same general structure R-O-R.

Primary, Secondary & Tertiary Ethers

The terms "primary ether", "secondary ether", and "tertiary ether" are occasionally used and refer to the carbon atom next to the ether oxygen. In a primary ether this carbon is connected to only one other carbon as in diethyl ether CH3-CH2-O-CH2-CH3. An example of a secondary ether is diisopropyl ether (CH3)2CH-O-CH(CH3)2 and that of a tertiary ether is di-tert-butyl ether (CH3)3C-O-C(CH3)3.

 

dimethylether.jpg (1882 bytes)

Dimethyl Ether

diethylether.jpg (2793 bytes)

a primary ether, diethyl ether

diisopropylether.jpg (2938 bytes)

a secondary ether, diisopropyl ether

di-tert-butylether.jpg (2337 bytes)

a tertiary ether, di-tert-butyl ether


Polyethers

Polyethers are compounds with more than one ether group. While the term generally refers to polymers like polyethylene glycol and polypropylene glycol, low molecular compounds such as the crown ethers may sometimes be included.

150-polyethylene_glycol.jpg (2592 bytes) 150-18-crown-6.jpg (4701 bytes)
Polyethylene Glycol Crown Ether

Organic Reactions

Synthesis

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

R-OH + R'-OH rarrow.gif (63 bytes) R-O-R' + H2

This direct reaction requires drastic conditions (heating to 140oCelsius and an acid catalyst, usually concentrated sulphuric acid).  Effective for making symmetrical ethers, but not as useful for synthesising asymmetrical ethers because the reaction will yield a mixture of ethers, making it usually not applicable:

3R-OH + 3R'-OH rarrow.gif (63 bytes) R-O-R + R'-O-R + R'-O-R' + 3H2O

Conditions must also be controlled to avoid overheating to 170 degrees which will cause intramolecular dehydration, a reaction that yields alkenes.  In addition, the alcohol must be in excess.

R-CH2-CH2(OH) rarrow.gif (63 bytes) R-CH=CH2 + H2O

Such conditions can destroy the delicate structures of some functional groups.   There exist several milder methods to produce ethers.

R-O-  + R-X rarrow.gif (63 bytes) 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 anion 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 does not work if R is aromatic like in bromobenzene (Br-C6H5), however, if the leaving group is separated by at least one carbon from the benzene, the reaction should proceed (as in Br-CH2-C6H5). 
As mentioned above, when one of the R groups in the target ether is aromatic, 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.

HO-C6H5 + OH- rarrow.gif (63 bytes) O--C6H5

O--C6H5 + R-X rarrow.gif (63 bytes) R-O-C6H5

R2C=CR2 + R-OH rarrow.gif (63 bytes) R2CH-C(-O-R)-R2

Acid catalysis is required for this reaction.  Tetrahydropyranyl ethers are used as protective groups for alcohols.

Cyclic ethers which are also known as epoxides can be prepared:

Reactions

diethylether_peroxide.jpg (3661 bytes)

Structure of the polymeric diethyl ether peroxide

Ethers in general are of very low chemical reactivity.  Organic reactions are:

Ethers are hydrolyzed only under drastic conditions like heating with boron tribromide or boiling in hydrobromic acid.  Lower mineral acids containing a halogen, such as hydrochloric acid will cleave ethers, but very slowly.  Hydrobromic acid and hydroiodic acid are the only two that do so at an appreciable rate. Certain aryl ethers can be cleaved by aluminum chloride.
Epoxides, or cyclic ethers in three-membered rings, are highly susceptible to nucleophilic attack and are reactive in this fashion.
Primary and secondary ethers with a CH group next to the ether oxygen easily form highly explosive organic peroxides (e.g. diethyl ether peroxide) in the presence of oxygen, light, and metal and aldehyde impurities.  For this reason ethers like diethyl ether and THF are usually avoided as solvents in industrial processes.

Important Ethers

ethyleneoxide.jpg (1747 bytes) Ethylene Oxide The smallest cyclic ether.
dimethylether.jpg (1882 bytes) Dimethyl Ether An aerosol spray propellant.
diethylether.jpg (2793 bytes) Diethyl Ether A common low boiling solvent.
dimethoxyethane.jpg (2012 bytes) Dimethoxyethane (DME) A high boiling solvent:
1,4-dioxane.jpg (3239 bytes) 1,4-Dioxane A cyclic ether and high boiling solvent.
tetrahydrofuran.jpg (2234 bytes) Tetrahydrofuran (THF) A cyclic ether, one of the most polar simple ethers that is used as a solvent.
100-anisole.jpg (3273 bytes) Anisole (methoxybenzene) An aryl ether and a major constituent of the essential oil of anise seed.
150-18-crown-6.jpg (4701 bytes) Crown Ethers Cyclic polyethers that are used as phase transfer catalysts.
150-polyethylene_glycol.jpg (2592 bytes) Polyethylene Glycol (PEG) A linear polyether, e.g. used in cosmetics.

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