Post on 20-Jan-2016
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+