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 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:

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

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.

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	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

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.

• Aromatic compounds like furan where the oxygen is part of the aromatic system.
• Compounds where one of the carbon atoms next to the oxygen is connected to oxygen, nitrogen, or sulfur: 
  • Esters R-C(=O)-O-R
  • Acetals R-CH(-O-R)-O-R
  • Aminals R-CH(-NH-R)-O-R
  • Anhydrides R-C(=O)-O-C(=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.

Dimethyl Ether

a primary ether, diethyl ether

a secondary ether, diisopropyl ether

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.

Polyethylene Glycol	Crown Ether

Organic Reactions

Synthesis

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

• Intermolecular Dehydration of alchohols: 

R-OH + R'-OH R-O-R' + H₂O 

This direct reaction requires drastic conditions (heating to 140^oCelsius 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 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) R-CH=CH₂ + H₂O

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

• Nucleophilic Displacement of Alkyl halides by alkoxides

R-O^-  + 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 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). 

• Nucleophilic Displacement of Alkyl halides by phenoxides

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^- O^--C₆H₅

O^--C₆H₅ + R-X R-O-C₆H₅

• Electrophilic addition of alcohols to alkenes.

R₂C=CR₂ + R-OH 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:

• By the oxidation of alkenes with a peroxyacid such as m-CPBA.
• By the base intramolecular nuclephilic substitution of a halohydrin.

Reactions

Structure of the polymeric diethyl ether peroxide

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

• Hydrolysis.

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.

• Nucleophilic Displacement.

Epoxides, or cyclic ethers in three-membered rings, are highly susceptible to nucleophilic attack and are reactive in this fashion.

• Peroxide formation.

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

	Ethylene Oxide	The smallest cyclic ether.
	Dimethyl Ether	An aerosol spray propellant.
	Diethyl Ether	A common low boiling solvent.
	Dimethoxyethane (DME)	A high boiling solvent:
	1,4-Dioxane	A cyclic ether and high boiling solvent.
	Tetrahydrofuran (THF)	A cyclic ether, one of the most polar simple ethers that is used as a solvent.
	Anisole (methoxybenzene)	An aryl ether and a major constituent of the essential oil of anise seed.
	Crown Ethers	Cyclic polyethers that are used as phase transfer catalysts.
	Polyethylene Glycol (PEG)	A linear polyether, e.g. used in cosmetics.

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