Ether

* R ⎯ O ⎯ R′ Alkoxy alkane (Di alkyl ether)

* R = R′ Symmetrical ether.

R ≠ R′ Unsymmetrical or mixed ether.

‘O’ is to be counted with least number of C atom.

Example:

CH₃ ⎯ O ⎯ C₂H₅ Methoxy ethane

CH₃ ⎯ O ⎯ C₆H₅ Methoxy benzene

There are various types of cyclic ethers also.

PREPARATION OF ETHERS

(i) From 1° alcohol

(a) With H₂SO₄

Order of dehydration 1° > 2° > 3° alcohol

(b) With diazomethane

(c) Alcohol having at least one hydrogen at fourth carbon gives five membered cyclic ether with Pb(OAC)₄. The reaction is free radical reaction which is initiated by heat or light.

Williamson’s synthesis

SN₂ reaction of a sodium alkoxide with alkyl halide, alkyl sulphonate or alkyl sulphate is known as Williamson synthesis of ethers.

• In this reaction alkoxide may be alkoxide of primary, secondary as well as tertiary alcohol.
• Alkyl halide must be primary.
• In case of tertiary alkyl halide, elimination occurs giving alkenes
• With a secondary alkyl halide, both elimination and substitution products are obtained.

(3) From Alkane

(4) From Grignard reagent

Higher ethers can be prepared by treating α - halo ethers with suitable reagents.

(5) From Alkyl halide

PROPERTIES OF ETHERS

Dipole nature of ether

Ethers have a tetrahedral geometry i.e. oxygen is sp³ hybridized. The C ⎯ O ⎯ C bond angle in ether is 110°. Because of the greater electronegativity of oxygen than carbon, the C ⎯ O bonds are slightly polar and are inclined to each other at an angle of 110°C, resulting in a net dipole moment.

The bond angle is slightly greater than the tetrahedral angle group to repulsive interaction between the two bulky groups.

CHEMICAL REACTION

Dialkyl ethers reacts with very few reagents other than acids. The only active site for other reagents are the C ⎯ H bonds of the alkyls. Ethers has ability to solvate cations (electrophile) by donating an electron pair from their oxygen atom. These properties make ether as solvents for many reactions.

On standing in contact with air, most aliphatic ethers are converted slowly into unstable peroxides.

Ether gives following reactions:

1. Nucleophilic substitution reactions

Note:

Type of ethers also make a difference in the mechanism followed during the cleavage of C—O by HI/HBr.

Methyl cation is stabler than phenyl cations

(B) Dehydration with H₂SO_4/Δ and Anhy Al₂O₃/Δ

(i) When both alkyl groups has β - hydrogen.

(ii) When only alkyl group has β - hydrogen.

• Hot conc. H₂SO₄ react with secondary and tertiary ethers to give a mixture of alcohols and alkenes.

(CH₃)₃CO—CH₃ (CH₃)₂—C=CH₂ + CH₃OH

(C) Miscellaneous reactions

(1) Halogenation:

Monohalogenation takes place at α carbon (with small amount)

(a)
(b)

(2) Reaction with CO: give ester