Tuesday, February 5, 2008

IIT JEE Revision - Ch.29 Phenols - Core Points

JEE syllabus

Phenols:
Preparation, Physical and Chemical properties

specially highlighted topics

Acidity,
electrophilic substitution reactions (halogenation, nitration and sulphonation);
Reimer-Tieman reaction,
Kolbe reaction.
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Phenols are aromatic hydroxy compounds. In phenols, one or more hydroxyl group is directly attached to the aromatic (benzene) nucleus.

If OH group is not directly attached to be carbon atom in the benzene ring, but present in the molecule as a part of the alkyl side chain group, then the compound is not termed as phenol.It is called aromatic alcohol because it resembles aliphatic alcohols in its characteristics.

By decarboxylation of sodium salt of salicyclic acid

Fusion of sodium salicylate with soda lime (NaOH and CaO mixture).
sodium phenoxide is formed. This on acidification gives phenol.

State and smell: Phenols are colourless crystalline solids or liquids. They have characteristic phenolic odours.

Boiling points: Higher than the boiling points of the aromatic hydrocarbons of comparable molecular masses.


Bromination

Action of Bromine water on phenol: When phenol is treated with bromine water, it gets decolourised giving a white precipitate of 2,4,6, tribromophenol.

Action of Bromine in CS-2 on phenol:o-Bromophenol + p-Bromophenol mixture is obtained. p-Bromophenol is the major product.

Nitration

Action of dilute nitirc acid on phenol: a mixture of o-nitrophenol and p-nitrophenol is formed.

Action of conc. nitric acid in the presence of conc. sulphuric acid on phenol: 2,4,6-trinitrophenol is formed. This is picric acid.

Sulphonation

Action of conc. sulphuric acid at different temperatures on phenol:
Pheno reacts with conc. sulphuric acid to form a mixture of o-, and p-phenol sulphonic acid.
At low temperature about 288 to 293 K, o-phenol sulphonic acid is the main product formed.

At high temperature about 373 K, p-phenol sulphonic acid is the main product formed.



Acidity of Phenols

Phenols are weakly acidic in nature (Ka = 10^-10).
They turn blue litmus read and react with alkali metals and alkalies to form their salts.
The acidic character of phenol is due to polar OH bond.

Kolbe's reaction

When sodium phenoxide is heated with carbon dioxide at about 400K and under 4 ot 7 atmospheric pressure, sodium salicylate is formed as a major product. This on acidification gives salicylic acid. A small amount of para isomer is also obtained and if the temperature is allowed to rise above 410 K, the para isomer dominates.


Reimer-Tiemann reaction



When phenol is treated with choloroform and aqueous sodium hydroxide at 340 K follwoed by hydrolysis, an aldehydic group, -CHO group is introduced in the ring at a position ortho to the phenol group (OH group).

Ortho hydroxy benzaldehyde or salicylaldehyde is formed as the product of the reaction.

In addition, small amount of p-salicylaldehyde is also formed
In place of chloroform, carbon tetrachloride can be used. In this case o-salicylic acid is formed as the major product.

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