Learning Objectives

1. Properties of phenols

2. Reaction of phenols

Preparation of phenol From aryl sulphonic acids

S O H3

+ N aO H

S O N a3 O N a O H

H+

Aryl sulphonic acid gives corresponding phenol on heating with molten sodium hydroxide at 570-620 K.

Preparation of phenol

+ N aO H

O N a O H

H+

C l

623 K

320 atm

Chlorobenzene is hydrolysed by treating it with NaOH at 623 K and 320 atm.

Preparation of phenolFrom hydrolysis of diazonium salt

Diazonium salts are prepared by treating an aromatic primary amine with nitrous acid (NaNO2 + HCl) at low temperature.

O HN H

P heno l

NaNO + HCl

2

2

N C l2

+ –

H O2

B enzened iazon iumchlo ride

Preparation methodsSynthesis from cumene

2O

OH3

C CH3CH3

O OH

OH

CH3 C CH3

O

CHCH3CH3

+

By decarboxylation of salicylic acid with soda lime

OH

COOH

ONa OH

+ 3NaOHCaO, 360 K

-Na2CO3, -2H2O

HCl

-NaCl

Acidity of phenolPhenol is more acidic than aliphatic alcohols because conjugate base is stabilized by resonance.

O

– O

–O

–

O

–

Reactions of phenolElectrophilic aromatic substitution

—OH group is ortho, para- directing group and activates the benzene rings.

Chemical reaction of phenolFries rearrangement

Distillation with Zn dust :

6 5 6 6C H OH C H + ZnO

Nitration

OH

Conc. HNO3

Conc. H2SO4

OH

NO2O2N

NO2

With dilute HNO3, it gives ortho and para-isomers which can be separated easily by distillation.

O H

Dil. HNO3

O HN O2

O H

N O2

+

With concentrated HNO3 phenol is converted to 2,4,6-trinitrophenol.

Bromination of phenol

OHaqueous medium

Br23

CHCl3/CCl4

OH

BrBr

Br

2, 4, 6 tribromophenol

Br2

OH

Br

p-Bromophenol

OH

Br

o-Bromophenol(minor product)

(major product)

+

+ 3HBr

Kolbe’s reaction

ONa

CO2

OH

COONa H+

OH

COOH

+

Salicylic acid

400 K

4 –7 atm

Reimer-Tiemann Reaction Mechanism

OH

CHCl3

NaOH/H2Oheat

C

O

H

OH

salicylaldehyde

CHCl3 + OH- CCl3- + H2O

CCl3- CCl2 + Cl-

a carbene

+ OH-

OOH O

H

O

H CCl2+

O

H

CCl2 CHCl2

O-

CHCl2

O-

H2O

heat

OHC

O

H+ HCl2

Overall:

Reimer Tiemann Reaction

O

CHO

OH OH

CHOCHCl3

O

CHCl2

H+

Salicylaldehyde(main product)

aq. NaOH, 70°C

The mechanism involves dichlorocarbene as an intermediate

3OH CHCl 2 3 2H O : CCl Cl : CCl

On treating phenol with chloroform in presence of sodium hydroxide, a —CHO group is introduced at ortho position of benzene ring.

Fries rearrangementEsters of phenols yield phenolic ketones on treatment with anhydrous aluminium chloride.

O H

3

O HO

+(CH CO ) O3 2

C O C H3

AlC l

O H

O C O C H 3

O C O C H 3

Coupling Reaction

N N Cl OH+

N N OH

–OH

p-hydroxy azo benzene

Phenol Reactions: A Summary

OH

NaOH

or

Na

O- Na+ 1. NaOH

2.RX (primary)

OR

RCOCl

or

(RCO)2O

OC

O

R

AlCl3

OH

C

O

RArN2

+

OH

N

NAr

CO24-7 atmheat

OH

C

O

O-Na+

CHCl3

O-

C

O

H

NaOH

Na+