Ether
Structure of ether:
The central atom of oxygen is in sp3 hybrid state. Two of the hybrid orbitals of the oxygen are used in forming σ-bonds with the surrounding alkyl groups. The other two sp3 hybrid orbitals of oxygen contain lone pairs of electrons. The electron pairs( lone pairs as well as bond pairs) surrounding oxygen atom, assume tetrahedral arrangement. However, the bond angle around oxygen atom is not exactly 109.5º. The deviation in angle is caused by the repulsive interactions between the alkyl groups. The bond angle between C-O-C in dimethyl ether is found to be 111.7º.
The C-O bond length in ethers is 141 pm which is almost same as that in alcohols. Thus, ethers have bent structure and their dipole moment is greater than zero. Hence, their molecules are polar in nature.
Methods of preparation of ether:
1) Dehydration of alcohols: Symmetrical ethers are prepared by heating excess of alcohol with conc. H2SO4 at 140 ºC.
R – OH + H – OR ——→ R—O—R + H2O
Alcohols ether
e.g. C2H5—OH + H—OC2H5 ———→ C2H5——O——C2H5 + H2O
A l c o h o l s Diethyl ether
2) Catalytic dehydration: Ether may be obtained by passing alcohols vapours over hot activated Al2O3 at 250ºC.
C2H5—OH + H—OC2H5 ——Al2O3 at 250ºc→ C2H5——O——C2H5 + H2O
A l c o h o l s Diethyl ether
3) Williamson ether synthesis: This involves the treatment of a sodium alkoxide with an alkyl halide. Both symmetrical and unsymmetrical ethers can be made by this method.
R——ONa + R——X —————→ R——O——R + NaX
Sodium alkoxide alkyl halide ether
4) Action of Diazomethane on alcohols: Methyl ethers can be prepared by treatment of primary or secondary alcohols with diazomethane in the presence of fluoroboric acid.
R—OH + CH2N2 ————→ R—O—CH3 + N2
Alcohol diazomethane ether
e.g. CH3CH2– OH + CH2N2 —————→ CH3CH2—O—CH3 + N2
Ethyl alcohol diazomethane ethyl methyl ether
Physical properties of ether:
Dimethyl ether and ethyl methyl ether are gases. All others are colourless liquids with pleasant odours. Lower ethers are highly volatile and very flammable. Boiling points of ethers show a gradual increase with the increase in molecular weight. Ethers have lower boiling points than isomeric alcohols. This is because ether molecules cannot form hydrogen bonds with each other as they have no –OH groups. The B.P. of ethers is close to the boiling points of alkanes. Ethers are slightly soluble in water as they do form H-bond with water.
Chemical properties:
Ethers are relatively inert, almost as inert as alkanes. This is inspite of the presence of oxygen atom carrying two pairs of lone pair of electrons in their molecules. Ethers are not easily attacked by alkalis, dilute mineral acids, metallic sodium, etc., under ordinary conditions. They undergo chemical reactions only under specific conditions. Some of the reaction of ethers are as follows:
1) Halogenation (reaction involving alkyl groups): When ether is treated with chlorine or bromine in dark, substitution products are obtained. The extent of substitution depends upon the reaction conditions. Halogenation preferentially takes place at α-carbon atoms.
However, in the presence of light and excess of chlorine all the hydrogen atoms are substituted by chlorine atoms.
2) Reaction with dilute H2SO4 (reactions involving C-O bonds): When ether is heated with dilute sulphuric acid, alcohols are formed.
3) Reaction with PCl5: Ether reacts with phosphorus pentachloride to give alkyl chlorides.
4) Cleavage by halogen acid: On heating with HI or HBr, ethers undergo cleavage in which one of the C – O –C bond breaks giving alcohols and alkyl halides.
e.g. CH3CH2OCH2CH3 + 2HBr ——-> CH3CH2OH + CH3CH2Br
Diethyl ether Ethyl alcohol Ethyl bromide.
However, excess of acid is used, the alcohol first formed reacts further with the halogen acid to form the corresponding alkyl halide.
CH3CH2OCH2CH3 + 2HBr ———>2 CH3CH2Br + H2O
Diethyl ether (excess) Ethyl bromide
5) Reaction with acetyl chloride: Ether reacts with acetyl chloride in the presence of anhydrous zinc chloride to form ethyl chloride and ethyl acetate.
Ring substitution in aromatic ethers:
Alkoxy group is an ortho and para directing and it direct the next incoming groups to ortho and para positions. For example, when methyl phenyl ether is treated with a mixture of conc. HNO3 and H2SO4, a mixture of o-and p- nitro compound is formed.
Similarly, bromination of anisole gives o-and p-bromoderivatives.
Uses of ether:
- Diethyl ether is an industrial solvent for oils, fats, gums, resins etc.
- It is used in surgery as a refrigerant.
- It is used a reaction medium.