Ethers

Ethers

The general formula of the ethers is C_nH_2n+2O and since their general structure is R-O-R, they may be regarded as alkyl oxides or the anhydrides of the alcohols.

When two alkyl groups in ether are the same, the ether is said to be symmetrical or simple, e.g., diethyl ether, C₂H₅-O-C₂H₅. When the two alkyl groups are different, the ether is said to be unsymmetrical or mixed, e.g., ethyl methyl ether, CH₃-O-C₂H₅.

Nomenclature

In this system of nomenclature all the members are known as ethers, and the individuals are named according to the alkyl groups attached to the oxygen atom, e.g., CH₃-O-CH₃, dimethyl ether;  C₂H₅-O-CH(CH₃)₂, ethyl isopropyl ether.

According to the I.U.P.A.C system of nomenclature, the ethers are regarded as hydrocarbons in which a hydrogen atom is replaced by an alkoxyl group, -OR, the larger group being chosen as the alkane. For symmetrical ethers, method 1 is to be used, e.g., C₂H₅OC₂H₅, diethyl ether; CH₃OC₂H₅, methoxyethane.

General methods of preparation

1.  Ethers may be prepared from alcohols in acid media, and the generally accepted mechanisms are that straight-chain primary alcohols react by the bimolecular mechanism (S_N2), t-alcohols by the unimolecular mechanism (S_N1), and s-alcohols by either of these mechanisms (the alcohol is the nucleophilic reagent):

On the other hand, it still appears to be uncertain whether the preparation of ethers from primary alcohols and concentrated sulphuric acid proceeds by the S_N2 mechanism or via an intermediate alkyl hydrogen sulphate.

Dehydration of alcohols to ethers may also be carried out by passing the alcohol vapour over a heated catalyst such as alumina, aluminium phosphate, etc.

2.  Ethers may be prepared by the addition of alcohols to alkenes in the presence of acid, e.g., ethanol and isobutene give t-butyl ethyl ether:

This reaction gives very good yields with alkenes which can produce t-carbonium ions, and is very useful for preparing mixed ethers.

3. From halogeno-ethers and Grignard reagents.

4. Methyl ethers are readily prepared by the action of diazomethane on alcohols.

General properties of the ethers

The lower members are gases or volatile liquids, and their vapors are highly inflammable. Their boiling points are much lower than those of the alcohols containing the same number of carbon atoms, and this is due to the fact that ethers cannot associate through hydrogen bonding. All the ethers are less dense than water in which they are not very soluble, but their solubility is very much increased in the presence of small amounts of alcohol.

General reactions of Ethers

1.  Ethers dissolve in concentrated solutions of strong inorganic acids to form oxonium salts, i.e., ethers behave as Bronsted-Lowry bases, e.g.,

R₂O   +   H₂SO₄    ⟶   (R₂OH)⁺HSO₄^-

It is the presence of the Ione-pair electrons on the oxygen atom which predominantly characterizes the reactions of the ethers. Thus, ethers also readily form co-ordination complexes (etherates) with Lewis acids, and treatment of an etherate with an alkyl fluoride produces a tertiary oxonium salt.

2.  when ethers heated with dilute sulphuric acid under pressure, ethers form the corresponding alcohols:

R₂O   +   H₂O    (H₂SO₄) ⟶    2ROH

3.  Ethers react with carbon monoxide at 125-180oC and at a pressure of 500 atmospheres, in the presence of boron trifluoride plus a little water:

R₂O   +   CO    ⟶    RCO₂R