Tuesday, February 5, 2008

Phenols - Preparation

1. Alkali fusion of sodium benzene sulphonate

NaOH is fused with sodium benzene sulphonate at 573 - 623 K. sodium phenoxide is formed. This on acidification gives phenol.

2. From diazonium salts
An acqueous solution of benzene diazonium salt on warming gives phenol

3. 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.

4. From Grignard reagent
When oxygen gas is bubbled through an ethereal solution of phenyl magnesium bromide (Grignard reagent RMgX), it forms an oxy compound which upon hydrolysis with dilute mineral acid gives phenol.

Commercial Preparation of Phenols

1. From chlorobenzene (Dow's Process)
Chlorobenzene is heated with 10% acqueous sodium hydroxide solution at about 623 K under 200 atmospheres and in the presence of copper salt acting as catalyst to form sodium phenoxide.

The sodium salt when treated with dilute HCl, gives phenol.

2. From cumene

Air or oxygen is passed through a suspension of cumene in acqueous sodium carbonate solution in presence of cobalt or manganese naphthenate catalyst. The oxidation product is cumene hydroperoxide.

The hydroperoxide is then decomposed by hot dilute sulphuric acid when phenol is formed with liberation of acetone.

3. From Benzene (Raschig's method)
Vapours of HCl are passed over benzene at 500 K in the presence of copper chloride and excess of air to form chlorobenzene.

Steam is then passed through chlorobenzene at 800 K in the presence of silica as catalyst to give phenol.

4. Phenol prepared using benzene and H2SO4

Benzene is heated with excess of concentrated sulphuric acid at about 388 K to give benzene sulphonic acid.

It is neutralized with sodium hydroxide solution, when sodium benzene sulphonate is obtained.

Dry sodium benzene sulphonate is next fused with excess of caustic soda at about 573 K when it yields sodium phenate (or sodium phenoxide).

Sodium phenate is decomposed by dilute sulphuric acid to give phenol.

***In many methods sodium phenoxide is formed first and from sodium phenoxide, phenol is obtained.

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