Organic Compounds Oxygen - I Summary

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Alcohols and Phenols are compounds containing -OH as the functional groups. In alcohols, the -OH group is attached to sp³ hybridised carbon. In phenols, it is attached to sp² hybridised carbon of an aromatic ring. In ethers, oxygen atom is attached to two carbons atoms by two separate sigma bonds.

Alcohols may be prepared by:

1) reduction of aldehydes and ketones

2) reduction of carboxylic acids and esters

3) from alkenes by

a) hydration

b) oxy mercuration

c) hydroboration

4) By the reaction of Grignard reagent with aldehydes and ketones.

Phenols may be prepared by

1) Nucleophilic substitution of

a) SO₃H group in aryl sulphonic acids.

b) Halogen atom in halo arenas by -OH group

2) by hydrolysis of diazonium salt.

Alcohols have higher boiling point then other classes of compounds of comparable molecular masses and are soluble in water due intermolecular hydrogen bonding.

Alcohols and Phenols are acidic in nature. The acidity of phenols is due to the resonance stabilisation of phenoxide ions. The presence of substitutents in the aromatic ring effects the acid strength of phenols. Election withdrawing groups in ortho and para positions increase acid strength and electron - releasing groups decrease it.

Alcohols undergo nucleophilic substitution with hydrogen halides to yield alkyl halides. Dehydration of alcohols gives alkenes. On oxidation primary alcohols yield aldehydes with mild oxidising agents and carboxylic acids with strong oxidising agents. Tertiary alcohols are resistant to oxidation.

The presence of -OH group in phenols activates the aromatic ring towards electrophilic substitution (nitration, halogenation etc) and directs the incoming group to ortho and para positions due to mesomeric effect. Reimer Tiemenn reaction of phenol yields salicylaldehyde.

Order of acidity is Phenol > H₂O > ROH. Phenol can be distinguished from alcohols by reaction with

i) FeCl₃ solution

ii) Litmus solution

iii) NaOH solution (Phenol is soluble in NaOH solution).

Ethers are prepared by:

1) dehydration of alcohols

2) Williamson's synthesis.

Aromatic ethers can prepared by reaction of sodium phenoxide with alkyl halides and not by reaction of sodium alkoxide with alkyl halide. C - O - C bond in ether is bent and hence polar. The C-O bond in ethers can be cleaved by hydrogen halides. In electrophilic substitution, the alkoxy group activates the aromatic ring and directs the incoming group to ortho and para positions due to mesomeric affect.

Aromatic ethers on cleavage give phenol and alkyl halide.

Methanol, ethanol, ethene 1-2 diol, propane 1,2,3 triol and phenols are commercially important ethoxy ethene commercially used laboratory solvent. A number of naturally occurring aromatic ethers namely anethale, eugenol and vanillin are used flavouring agents and in perfumes.