ether (thing)

"Ether" is the familiar name for the compound discussed above, but it is also the name for a class of organic compounds of which diethyl ether, or ethoxyethane, is the best known.

The ether functional group is R-O-R', where R and R' are two (not necessarily identical) alkyl groups, e.g. methyl, CH₃. In other words, ethers are compounds in which an oxygen atoms bonds singly to two alkyl groups. Compare esters, where the bridging oxygen atom connects an alkyl group to an acyl group:

 
                            O
  |       |                 ||      |
- C - O - C -             - C - O - C -
  |       |                         |

 an ether                    an ester

In official nomenclature, simple chain ethers are named by splitting the structure R-O-R' into two parts: one which contains the oxygen atom (R-O), and the other which does not (R'). The first part is an alkoxy group named after the corresponding alkyl group: CH₃O is methoxy (after methyl), CH₃CH₂O is ethoxy (after ethyl) and so on. The second part is simply given the name of the corresponding alkane. Thus the simplest ether, CH₃-O-CH₃, is technically called methoxymethane, while the compound CH₃CH₂-O-CH₂CH₂CH₃ is ethoxypropane.

In everyday language chemists rarely use these names, however, but an older system of the form "alkyl alkyl ether", where the two "alkyls" are the relevant alkyl groups. Therefore ethoxypropane is "ethyl propyl ether", methoxymethane is "dimethyl ether", and ethoxyethane, the "ether" of common language, is "diethyl ether".

Symmetrical ethers can be formed by acidifying the alcohol whose alkyl group corresponds to those in the ether: for example, diethyl ether can be formed by the reaction of ethyl alcohol (ethanol) with sulphuric acid at high temperature. However a more general synthesis for straight-chain ethers is the Williamson ether synthesis, developed by Alexander Williamson in the 1850s. First an alcohol, R-O-H, is reacted with sodium (sometimes in the form of sodium hyride or hydroxide) to produce the sodium alkoxide R-O^-Na⁺, which effectively exists as the alkoxide ion, R-O^-. This is then reacted with a haloalkane (alkyl halide) R'-CH₂-X, where X is a halogen atom. In a bimolecular substitution reaction, the oxygen on the alkoxide attacks the carbon (emboldened above) in the alkyl halide, and replaces the halogen atom. Thus the alkyl group from the alcohol, and that from the alkyl halide, are linked by an oxygen atom to give R-O-CH₂-R'.

For example, reaction of methanol, CH₃-OH, with sodium followed by propyl chloride, CH₃CH₂CH₂Cl, would give methoxypropane, CH₃-O-CH₂CH₂CH₃.

Because of the strength of the carbon-oxygen bond, ethers are unreactive compounds. This makes them useful as solvents for carrying out other organic reactions: two very commonly used ether solvents are diethyl ether and THF, tetrahydrofuran. This second one is an example of a cyclic ether: it is a symmetrical, five-membered ring in which one of the substituents is an oxygen atom, so the carbon groups bonded to the oxygen are also bonded to each other.

Perhaps the easiest way to get ethers to react is to treat them with a hydrogen halide, HX. The halide ion, X^-, attacks one of the carbon atoms attached to the oxygen in the ether, cleaving it into the corresponding alcohol, R-O-H, and alkyl halide, R'-X. If the ether is unsymmetrical, the halide will preferentially attack the less substituted carbon.