Download Phenols Like alcohols, phenols are starting materials for a wide

Phenols
Like alcohols, phenols are starting materials for a wide range of compounds. The functional group is again
OH but unlike alcohols, it is attached directly to a benzene ring and this affects its reactivity.
Parent compound:
OH
phenol
Nomenclature
Named as derivatives of phenol
OH
OH
OH
CH3
NO2
O 2N
Cl
NO2
2-methylphenol
o-methylphenyl
o-cresol
2,4,6-trinitrophenol
picric acid
4-chlorophenol
p-chlorophenol
Some compounds have 2 or more -OH groups, named as benzenediols:
OH
OH
OH
OH
OH
1,2-benzenediol
catechol
OH
Naphthols
OH
1,4-benzenediol
hydroquinone
1,3-benzenediol
resorcinol
OH
1-naphthol
2-naphthol
Phenols are considerably more acidic than alcohols since the hydroxyl group is directly attached to the
benzene ring. This allows resonance stabilization of the anion formed by loss of the hydroxyl proton.
O
O
O
O
OH
OH-
Notice that the -ve charge can be delocalized into the benzene ring and can be written on the o and p
positions relative to the original hydroxyl group. The carbanion is stabilized further by
electron-withdrawing groups in these positions.
Nitrobenzene is more acidic than phenol
-
OH
O
O
OH-
O
O
O
-
-
NO2
N
+ O
O
N
O + O
N
+
O O
-
-
N
O+O
-
-
-
very important
resonance structure
On the other hand, electron donating groups make the phenol less acidic, e.g.
O
OH
OO
CH3
CH3
CH3
CH3
-
OH-
N
+O
O
-
O
-
CH3
-
unfavored
resonance
structure
o-cresol is a weaker acid than phenol; m-cresol is a stronger acid than o- or p-cresol
Preparation of phenols
Cannot readily replace the halide of aryl halides by -OH, since aryl halides are inert. The reaction can be
performed in industry under high pressure and temperature:
OH
Cl
This reaction is incorrect unless the conditions are
specified
2M NaOH
300oC, 150 atm
Fusion of arylsulfonic acid with KOH
KOH
ArSO3H
ArO-K+
o
300 C
CH3
e.g.
1) KOH, 300oC
H3O+
ArOH
CH3
2) H3O+
SO3 H
OH
SO3 H
1) KOH, 300oC
2) H3O
+
OH
This is the best way of making
2-naphthol and from it naphthalene
compounds substituted in the
2-position
Phenols can be prepared from amines. We will see this reaction when we come to the study of amines.
Industrial preparation of phenol
+ CH2 =CHCH3
H 3C
H 3PO4
CH
H 3C
CH 3
O 2, 100oC
CH 3
C OOH
CH 3
C OOH
H 3C
cumene
H 3O +
CH 3
C OOH
H
+ H 3C C CH 3
~50o C
cumene
hydroperoxide
Mechanism of the oxidation
H 3C
O
H 3O+
250oC
Friedel Crafts type reaction
OH
H 3C
CH 3
H 3C
H 3C
C O
OH 2
C O
-H 2O
CH 3
H O
CH 3
C OH
H 2O
Reactions of phenols
Like alcohols, they readily form a sodium salt, but since the proton on the OH group of phenols is more
acidic, the salt is formed using NaOH, not Na itself. However the salt is not formed with a weak base like
Na2CO3
OH
O -Na +
2
+ H2 O
+ 2NaOH
Phenols are very reactive towards electrophilic aromatic substitution
OH
OH
e.g. OH
1 eq Br2
Br
Br
o
H 2O/0 C
CCl4
+ Br2
but,
fast
Br
Notice that the catalyst FeBr3 is not needed in this reaction
Mechanism
OH
OH
OH
Br+
OH
Br
+
OH
-H+
+
+
+
H Br
Br
OH
H
Br
H Br
very
important
resonance
structure
H
Br
Br
Oxidation of Phenol
OH
O
OH
Na2Cr2 O7 /H2O
SNCl2/H2O
or
(KSO3)2 NO/ H2 O
Fremy's salt
Potassium nitrodisulfonate
Fremy's salt
O
benzoquinone
Redox reaction important in cells
O
CH3
H3C O
H3C O
O
OH
hydroquinone
ubiquinones are ubiquitous in nature and are involved in the electron transfer
process in cells. n is between 1 and 10
(CH2CH=CCH2 )nH
Many phenols are biosynthesized in plants e.g.
OH
CH3
C
O
H
vanillin
The irritants in poison ivy are phenols
HO
(CH2)4CH3
O
tetrahydrocannabinol (marijuana)