CHEMCA Free Chemistry Notes for JEE NEET Chemistry MadeEasy: Chemical Properties of Phenol

Phenol behaves both as an acid (due to the -OH group) and as an activated aromatic compound (due to the resonance effect of the -OH group on the benzene ring).

1. Acidic Nature

Why Phenol is Acidic?

Phenol turns blue litmus red and reacts with aqueous NaOH to form Sodium Phenoxide. It is more acidic than alcohols but less acidic than carboxylic acids.

$$ C_6H_5OH + NaOH \rightarrow C_6H_5O^-Na^+ + H_2O $$

Reason: The phenoxide ion formed after losing a proton is stabilized by Resonance, delocalizing the negative charge over the benzene ring.

2. Electrophilic Substitution Reactions

Ortho-Para Directing

The -OH group activates the benzene ring towards electrophilic substitution and directs incoming groups to ortho and para positions.

A. Nitration

With Dilute HNO3: Gives a mixture of ortho and para nitrophenol.

$$ C_6H_5OH + \text{dil. } HNO_3 \rightarrow o\text{-Nitrophenol} + p\text{-Nitrophenol} $$

With Concentrated HNO3: Gives 2,4,6-Trinitrophenol (Picric Acid).

$$ C_6H_5OH + \text{conc. } HNO_3 \xrightarrow{H_2SO_4} \text{Picric Acid} $$

B. Halogenation (Bromination)

In Non-polar solvent (CS2/CHCl3): Mono-substitution occurs.

$$ C_6H_5OH + Br_2 \xrightarrow{CS_2, 273K} p\text{-Bromophenol (Major)} + o\text{-Isomer} $$

In Water (Polar solvent): Poly-substitution occurs.

$$ C_6H_5OH + 3Br_2(aq) \rightarrow \text{2,4,6-Tribromophenol (White ppt)} $$

3. Important Named Reactions

A. Kolbe's Reaction

Sodium phenoxide is heated with $CO_2$ at 400 K and 4-7 atm pressure, followed by acidification to form 2-Hydroxybenzoic acid (Salicylic Acid).

$$ C_6H_5ONa + CO_2 \xrightarrow{400K, 4-7 atm} \xrightarrow{H^+} \underset{\text{Salicylic Acid}}{o\text{-HO-C}_6H_4\text{-COOH}} $$

B. Reimer-Tiemann Reaction

Treatment of phenol with Chloroform ($CHCl_3$) in the presence of aqueous NaOH followed by hydrolysis yields Salicylaldehyde.

$$ C_6H_5OH + CHCl_3 + NaOH \rightarrow \underset{\text{Intermediate}}{C_6H_5(ONa)CHCl_2} \xrightarrow{NaOH, H^+} \underset{\text{Salicylaldehyde}}{o\text{-HO-C}_6H_4\text{-CHO}} $$

The electrophile is Dichlorocarbene ($:CCl_2$).

4. Other Important Reactions

A. Reaction with Zinc Dust

Heating phenol with zinc dust reduces it to Benzene.

$$ C_6H_5OH + Zn \xrightarrow{\Delta} \underset{\text{Benzene}}{C_6H_6} + ZnO $$

B. Oxidation

Oxidation of phenol with chromic acid ($Na_2Cr_2O_7 + H_2SO_4$) produces a conjugated diketone called Benzoquinone.

$$ C_6H_5OH \xrightarrow{Na_2Cr_2O_7/H_2SO_4} \underset{\text{p-Benzoquinone}}{O=C_6H_4=O} $$

Chemical Properties of Phenol | chemca

Chemical Properties of Phenol