Download Phenol

Phenols (Ar-OH)
I. Structure and nomenclature:

Phenols are compounds of the general formula
Ar-OH, where Ar- is phenyl, substituted phenyl, or
one of the other aryl groups.
O
H
G

O
H
G
Phenols differ from alcohols in having the -OH
group attached directly to an aromatic ring.
Hydroxybenzene, the simplest member of the
phenols, is generally referred to as phenol.
Dr. Talat R. Al-Ramadhany
OH
Phenol
−OH is directly linked
to the aromatic ring carbon
Dr. Talat R. Al-Ramadhany
CH2OH
Benzylalcohol
−OH is not directly linked
to the aromatic ring carbon
OH
OH
OH
CH3
Cl
CH3
o-Chlorophenol
o-cresol
m-Cresol
COOH
COOH
OH
OH
o-Hydroxybenzoic acid
Dr. Talat R. Al-Ramadhany
p-Hydroxybenzoic acid
II. Physical properties:
 The simplest Phenols are liquid or low-melting solids.
 Phenols have high boiling points.
 Phenol itself is somewhat soluble in water
 most other Phenols are essentially insoluble in water.
 Phenols are colorless, but they easily oxidized by atmospheric
air and become colored compounds.
Dr. Talat R. Al-Ramadhany
m- and p- isomers have higher boiling point because of
the intermolecular hydrogen bonding and their solubility in
water is due to the hydrogen bonding with water.
Dr. Talat R. Al-Ramadhany
For o-nitrophenol, the –NO2 and –OH groups are closed to
each other and they form intramolecular hydrogen bonding
(within a single molecule). Therefore o-nitrophenol does not have
the low volatility of an associated liquid, cannot form hydrogen
bonding with water, therefore it have lower solubility in water.
Dr. Talat R. Al-Ramadhany
III. Acidity of Phenols:
 Phenols are fairly acidic compounds, and in this respect
differ markedly from alcohols, which are even more weakly
acidic than water.
Carboxylic acid > Phenol > Water > Alcohol
 Aqueous hydroxides convert Phenols into their salts; aqueous
mineral acids convert salts back into the free Phenols.
Ar-OH
A Phenol
(acid)
Insoluble
in water
Dr. Talat R. Al-Ramadhany
OHH+
ArOA Phenoxide ion
(salt)
Soluble
in water
The acidity of phenols is mainly due to an electrical charge
distribution in phenols that causes the -OH oxygen to be more
positive. As a result, the proton is held less strongly, and phenols
can easily give this loosely held proton away to form a phenoxide
ion as outlined below.
Dr. Talat R. Al-Ramadhany
Industrial sours of Phenols:
i.
Conc.
ii. Dow process, in which Chlorobenzene is allowed to react the
aqueous sodium hydroxide at a temperature of about 360ºC.
O-Na+
Cl
NaOH, 360o
4500 lb/in.2
Chlorobenzene
Dr. Talat R. Al-Ramadhany
OH
HCl
Sodium phenoxide
Phenol
iii. Oxidation of Cumene: Cumene is converted by air oxidation
into cumene hydroperoxide, which is converted by aqueous
acid into Phenol and acetone.
H2O, H+
O2
+ H3C C O
CH3
H3C C H
CH3
Cumene
H3C C OOH
CH3
Cumene
hydroperoxide
OH
Phenol
Acetone
Preparation of Phenols in the laboratory
i.
Hydrolysis of diazonium salts
ArN2+ + H2O
ArOH + H+ + N2
N2+HSO4-
OH
H2O, H+
+ N2
heat
Cl
Dr. Talat R. Al-Ramadhany
Cl
ii. Oxidation of arylthallium compounds
This method has two advantages over the diazonium route:
A.The speed and high yield.
B.Orientation control in the thallation step.
O
O
Tl(O C CF3)2
Tl(OOCCF3)3
Pb(OAc)4
H2O, OH-
Ph3P
heat
Arylthallium
trifluoroacetate
Dr. Talat R. Al-Ramadhany
O C CF3
Aryltrifluoroacetate
O-
OH
H+
iii. Alkaline hydrolysis of aryl halides
Cl
OH
NaOH (10%)
350o
10%
Cl
OH
NaOH (15%)
160o
NO2
NO2
15%
Dr. Talat R. Al-Ramadhany
O-Na+
Cl
NO2
OH
NO2
NO2
NO2 O2N
HNO3
H2SO4
H+
NaOH
OH
NO2
NO2
NO2
NO2
Picric acid
Cl
O2 N
OH
NO2
O2N
NO2
H 2O
NO2
NO2
90%
Dr. Talat R. Al-Ramadhany
Reaction of Phenols
There are two type of reaction:
O H
A. Reaction of (O–H) bond.
B. Reaction of aromatic ring
(Electrophilic Aromatic Substitution).
Dr. Talat R. Al-Ramadhany
A. Reaction of (O–H) bond.
1) Acidity, salt formation.
O
H
O
N
a
+N
a
O
H
O
H
+H
O
2
O
N
a
+N
a
H
C
O
3
+H
C
O
2
3
2)
Ether formation (Williamson Synthesis)
Phenols are converted into ethers by reaction in alkaline
solution with alkyl halides.
O-
OH
O R
aqueous NaOH
heat
R X
OH
+ X
OC2H5
+ C2H5I
aqueous NaOH
heat
O-
OH
aq.NaOH
heat
OCH2ph
phCH2Br
Cl
+ phCH2O-
Dr. Talat R. Al-Ramadhany
X
Aryl halide must be containing strong electron-withdrawing
group to form corresponding ether.
OCH3
Cl
NO2
NO2
+ CH3O- Na+
NO2
Dr. Talat R. Al-Ramadhany
NO2
3) Ester formation
Phenols are usually converted into their esters by the
reaction with carboxylic acids, acid chlorides or anhydrides.
OH +
NaOH
COCl
O C
O
Phenol
O2N
Benzoyl chloride
OH + (CH3CO)2O
Phenyl benzoate
CH3COONa
O2N
O C CH3
O
p-Nitrophenol
Acetic anhydride
p-Nitrophenyl acetate
(Fries rearrangement)
When esters of Phenols are heated with aluminum
chloride, the acyl group migrates from the Phenolic oxygen to an
ortho or para position of the ring and yield a ketone. This reaction
is called the Fries rearrangement, is often used to prepare
phenolic ketones.
O
OH
O C C2H5
C2H5COCl
Phenol
OH
AlCl3
CS2
Phenyl propionate
OH
O
C C2H5
o-Hydroxyphenyl
ethyl ketone
+
C C2H5
O
p-Hydroxyphenyl
ethyl ketone
B. Reaction of aromatic ring
(Electrophilic Aromatic Substitution).
1. Halogenation (Bromination)
Treatment of Phenols with aqueous solution of bromine
results in replacement of every hydrogen ortho or para to the
–OH group
OH
OH
+ 3Br2
H2O
Br
Br
+ 3HBr
Br
2,4,6-tribromophenol
 If halogenation is carried out in a solvent of low polarity:
OH
OH
OH
Br
+ 2Br2
CHCl3
nonpolar
solvent
+
+ 2HBr
Br
O
H
O
H
+2B
r2
C
S2
+H
B
r
o
0C
B
r
OH
OH
Br
CH3
CH3
+ 2Br2 (aq)
+ 2HBr
o-Cresol
Br
2,4-Dibromo-6-methylphenol
 Some group can replace by bromine
OH
OH
Br
Br
+ 3Br2 (aq)
SO3H
p-Phenolsulfonic acid
+ 3HBr + H2SO4
Br
2,4,6-Tribromophenol
2) Sulfonation:
OH
SO3H
15-20oC
o-Phenolsulfonic acid
OH
Conc. H2SO4, 100oC
H2SO4
Conc.
OH
100oC
SO3H
p-Phenolsulfonic acid
3) Nitration:
OH
OH
OH
NO2
dilute HNO3
+
20oC
NO2
p-Nitrophenol
13% yield
o-Nitrophenol
40% yield
OH
OH
Conc.HNO3
O2N
NO2
20oC
NO2
Picric acid
4) Friedel-Crafts alkylation
Alkyl phenols can be prepared by Friedel-Crafts
alkylation of Phenols, but the yields are often poor.
CH3
OH + H3C C CH3
Cl
tert-Butyl
chloride
CH3
HF
HO
C CH3
CH3
p-tert-Butylphenol
Phenolic ketones can be made by direct Friedel-Crafts
acylation of Phenols, they are more often prepared in two steps
by means of the Fries rearrangement.
OH
25oC
CH3
C CH3
O
O C CH3
OH
(CH3CO)2O
CH3
m-Cresol
AlCl3
O
2-Methyl-4-hydroxyacetophenone
CH3
m-Cresyl acetate
160oC
O
H3C C
OH
CH3
4-Methyl-2-hydroxyacetophenone
5) Nitrosation:
Nitrous acid converts Phenols into nitrosophenols
OH
OH
NaNO2 , H2SO4
7 - 8 oC
NO
p-Nitrosophenol
80% yield
Dr. Talat R. Al-Ramadhany
6) Synthesis of Phenolic acids (Kolbe reaction):
Treatment of the salts of a Phenol with carbon dioxide
brings about substitution of the carboxyl group, -COOH, for
hydrogen of the ring. This reaction is known as the Kolbe
reaction; its most important application is in the conversion of
Phenol into o-Hydroxybenzoic acid, known as salicylic acid.
ONa
OH
O
+ C
O
o
125 C
4-7 atm.
OH
COONa
H
Sodium salicylate
(Chief product)
Dr. Talat R. Al-Ramadhany
+
COOH
Salicylic acid
7) Synthesis of Phenolic aldehydes (Reimer-Tiemann reaction):
Treatment of Phenol with chloroform and aqueous
hydroxide introduces an aldehyde group, –CHO, into the
aromatic ring, generally ortho to the –OH. This reaction is
known as the Reimer-Tiemann reaction.
O-
OH
CHCl3, aq.NaOH
OCHCl2
OH
CHO
HCl
CHO
70oC
Salicyladehyde
(Chief product)
Dr. Talat R. Al-Ramadhany