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Friday, 24 February 2017

Alcohols, Phenols and Ethers - IV

Differentiate between organic compounds:

a) Alcohols and phenols
Phenol on reaction with neutral FeCl3 gives purple colour whereas alcohols do not give purple colour.
b) Primary, secondary and tertiary alcohols
Lucas reagent test:

If it is a primary alcohol, no turbidity appears at room temperature. Turbidity appears only on heating. If it is a secondary alcohol, turbidity appears in 5 minutes. If it is a tertiary alcohol, turbidity appears immediately.

c) Methanol and ethanol
Iodoform test: Ethanol when reacted with (I2 and NaOH) or NaOI gives yellow ppt of iodoform since it has the presence of CH3-CH (OH)- group.

 Structure of ethers:
Preparation of ethers:
a) From alcohols

b) From alkyl halide and sodium alkoxide

Here, the alkyl halide should be primary and alkoxide should be tertiary. In case of aromatic ether, the aromatic part should be with phenoxide ion.

Physical properties of ethers:
a) Miscibility:
Miscibility of ethers with water resembles those of alcohols of the same molecular mass. This is due to the fact that just like alcohols, oxygen of ether can also form hydrogen bonds with water molecule.

b) Boiling points:
Ethers have much lower boiling points than alcohols. This is due to the presence of hydrogen bonding in alcohols. Hydrogen bonding is absent in ethers.
Chemical properties of ethers:
a) Cleavage of C–O bond in ethers:

The order of reactivity of hydrogen halides is as follows: HI > HBr > HCl
Alkyl halide formed is always the lower alkyl group. But if a tertiary alkyl group is present, the alkyl halide is always tertiary. In case of phenolic ethers, the cleavage occurs with the formation of phenol and alkyl halide.

b) Electrophilic substitution reaction in aromatic ethers:

The electrophilic substitution reaction of aromatic ether involves the following reaction:

Other conversion reactions:
a) Phenol to salicyldehyde

b) Phenol to benzene diazonium chloride

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