Physical and Chemical Properties of ethers

Physical Properties:

Ethers are polar in nature. The dipolemoment of ether is the vector sum of two polar C-O bonds with significant contribution from two lone pairs of electrons. For example, the dipole moment of diethyl ether is 1.18D. Boiling point of ethers are slightly higher than that of alkanes and lower than that of alcohols of comparable masses.

Oxygen of ether can also form Hydrogen bond with water and hence they are miscible with water. Ethers dissolve wide range of polar and non-polar substances.

Chemical Properties of ethers:

1. Nucleophilic substitution reactions of ethers.

Ethers can undergo nucleophilic substitution reactions with HBr or HI . HI is more reactive than HBr .

Mechanism:

Ethers having primary alkyl group undergo SN² reaction while tertiary alkyl ether undergo SN¹ reaction.Protonation of ether is followed by the attack of halide ion.The halide ion preferentially attacks the less sterically hindered of the two alkyl groups which are attached to etherial oxygen.

When excess HBr or HI is used, the alcohol formed will further react with HBr or HI to form alkyl halides.

Evaluate Yourself:

1mole of HI is allowed to react with t-butyl methylether. Identify the product and write down the mechanism of the reaction.

Autooxidation of ethers:

When ethers are stored in the presence of atmospheric oxygen, they slowly oxidise to form hydroperoxides and dialkylperoxides. These are explosive in nature. Such a spontaneous oxidation by atmospheric oxygen is called autooxidation.

Some of the reaction of diethyl ether.

Aromatic electrophilic substitution reactions:

The alkoxy group ( -OR ) is an ortho, para directing group as well as activating group.It activates the aromatic ring towards electrophilic substitution.

i) Halogenation:

Anisole undergoes bromination with bromine in acetic acid even in the absence of a catalyst, para isomer is obtained as the major product.

ii) Nitration:

Anisole reacts with a mixture of conc. H₂SO₄/Conc.HNO₃ to yield a mixture of ortho nitro anisole and para nitro anisole.

iii) Friedel Craft’s reaction:

Anisole undergoes Fridel Craft’s reaction in presence of anhydrous AlCl₃ as a catalyst.