Phenol

These are organic compounds a hydroxyl group attached directly to a benzene ring.

PREPARATION

Industrial Method

(i) From chloro benzene (Dow’s process)

Chlorobenzene is heated with NaOH at 673 K and under pressure of 300 atm to produced sodium phenoxide which on acidification yields phenol.

(ii) Cumene Process

Cumene obtained from propene & benzene cumene on air oxidation followed by acidification with H2SO4 gives phenol & acetone.

(iii) From benzene sulphonic acid

It is fused with NaOH gives sodium salt of phenol.

(iv) From benzene diazonium chloride

This gives Ar SN₁ reaction with H₂O to forms phenol.

PHYSICAL PROPERTIES

Phenol is needle shaped solid, soon liquefies due to high hygroscopic nature. It is less soluble in water, but readily soluble in organic solvents.

Phenol has high boiling point due to presence of hydrogen bonding.

Acidity of phenol

Phenols are weak acid. They react with aqueous NaOH to form sodium phenoxide, but does not react with sodium bicarbonate.

The acidity of phenol is due to the stability of the phenoxide ion, which is resonance stabilized as shown below:

In substituted phenols, the presence of electron withdrawing groups at ortho and para positions such as nitro group, stabilizes the phenoxide ion resulting in an increase in acid strength. It is due to this reason that ortho and para nitro phenols are more acidic than phenol.

On the other hand, electron releasing groups such as alkyl group, do not favour the formation of phenoxide ion – resulting in decrease in acid strength.

For example: (cresol are less acidic then phenol)

CHEMICAL REACTIONS

(A) Reaction due to breaking of O – H bond

Phenol in more reactive than alcohol for this reaction because phenoxide ion is more stable than the alkoxide ion.

Reaction of phenyl due to breaking of ⎯O ⎯ H bond is given below:

Acylation

Fries rearrangement

Phenolic esters are converted in to o ⎯ and p ⎯ hydroxy ketones in the presence of anhydrous AlCl3. Generally low temperature favours the formation of p – isomer and higher temperature favour the o - isomer.

(B) Reactions due to breaking of carbon- Oxygen bond

Nucleophilic substitution reaction

Phenols are less reactive than aliphatic compound because:

(i) ⎯OH group is present on sp2 hybridised carbon. This makes C ⎯ O bond stronger.

(ii) ‘O’ is more electronegative than halogens. This also makes C ⎯ O bond stronger than
C ⎯ X.

(iii) There is some double bond character between carbon and oxygen due to the resonance. This also makes C ⎯ O bond stronger.

However it give SN under drastic condition.

(C) EAS in Phenol: It is strong activating group.