Phenols
Ar-OH
Phenols are compounds with an –OH group
attached to an aromatic carbon. Although they
share the same functional group with alcohols,
where the –OH group is attached to an aliphatic
carbon, the chemistry of phenols is very different
from that of alcohols.
Nomenclature.
Phenols are usually named as substituted phenols. The
methylphenols are given the special name, cresols. Some other
phenols are named as hydroxy compounds.
CH3
OH
OH
OH
OH
COOH
Br
phenol
m-bromophenol
OH
OH
o-cresol
salicylic acid
OH
COOH
OH
OH
OH
OH
catechol
resorcinol
hydroquinone
p-hydroxybenzoic acid
physical properties
phenols are polar and can hydrogen bond
phenols are water insoluble
phenols are stronger acids than water and
will dissolve in 5% NaOH
phenols are weaker acids than carbonic acid and
do not dissolve in 5% NaHCO3
Intramolecular hydrogen bonding is possible in some
ortho-substituted phenols. This intramolecular hydrogen
bonding reduces water solubility and increases volatility.
Thus, o-nitrophenol is steam distillable while the
isomeric p-nitrophenol is not.
OH
O
N
O
H
O
o-nitrophenol
bp 100oC at 100 mm
0.2 g / 100 mL water
volatile with steam
NO2
p-nitrophenol
bp decomposes
1.69 g / 100 mL water
non-volatile with steam
phenols, syntheses:
1. From diazonium salts
OH
N2
H2O,H+
2. Alkali fusion of sulfonates
SO3 Na
NaOH,H2O
300o
ONa
H+
OH
Reactions:
alcohols
phenols
1. HX
NR
2. PX3
NR
3. dehydration
NR
4. as acids
phenols are more acidic
5. ester formation
similar
6. oxidation
NR
Phenols, reactions:
1. as acids
2. ester formation
3. ether formation
4. EAS
a) nitration
f) nitrosation
b) sulfonation
g) coupling with diaz. salts
c) halogenation
h) Kolbe
d) Friedel-Crafts alkylation
i) Reimer-Tiemann
e) Friedel-Crafts acylation
as acids:
with active metals:
OH
ONa
Na
+ H2(g)
sodium phenoxide
with bases:
CH4 < NH3 < HCCH < ROH < H2O < phenols < H2CO3 < RCOOH < HF
ONa
OH
+ NaOH
SA
SB
+ H2O
WB
WA
CH4 < NH3 < HCCH < ROH < H2O < phenols < H2CO3 < RCOOH < HF
ONa
OH
+ NaOH
SB
SA
water insoluble
+ H2O
WB
WA
water soluble
OH
+ NaHCO3
NR
phenol < H2CO3
CH4 < NH3 < HCCH < ROH < H2O < phenols < H2CO3 < RCOOH < HF
water
5% NaOH
5% NaHCO3
phenols
insoluble
soluble
insoluble
carboxylic
acids
insoluble
soluble
soluble
We use the ionization of acids in water to measure acid
strength (Ka):
HBase + H2O
H3O+ + Base-
Ka = [H3O+ ][ Base ] / [ HBase]
ROH Ka ~ 10-16 - 10-18
ArOH Ka ~ 10-10
Why are phenols more acidic than alcohols?
OH
ROH + H2O
H3O+ + RO-
ArOH + H2O
H3O+ + ArO-
OH
O
O
O
O
O
Resonance stabilization of the phenoxide
ion, lowers the PE of the products of the
ionization, decreases the ΔH, shifts the equil
farther to the right, makes phenol more
acidic than an alcohol
effect of substituent groups on acid strength?
O
OH
G
+ H2O
G
+ H3O
Electron withdrawing groups will decrease the negative
charge in the phenoxide, lowering the PE, decreasing the ΔH,
shifting the equil farther to the right, stronger acid.
Electron donating groups will increase the negative charge
in the phenoxide, increasing the PE, increasing the ΔH,
shifting the equilibrium to the left, weaker acid.
Number the following acids in decreasing order of acid
strength (let # 1 = most acidic, etc.)
OH
3
OH
5
OH
OH
OH
NO2
CH3
Br
1
4
2
SO3H
COOH
OH
CH2OH
1
2
3
4
2. ester formation (similar to alcohols)
OH
CH3
O
+ CH3CH2C
OH
H+
O
CH3CH2C
O
H3C
O
H3C C
OH
COOH
O
COOH
+ (CH3CO)2O
salicyclic acid
aspirin
+ H2O
O
H3C C
O
COOH
aspirin
analgesic
anti-inflamatory
antipyrretic
anticoagulant
Reye's syndrome
not to be used by children
with high fevers!
OH
aspirin substitute
Tylenol
H3C C NH
O
acetaminophen
Kidney damage!
3. ether formation (Williamson Synthesis)
Ar-O-Na+ + R-X  Ar-O-R + NaX
note: R-X must be 1o or CH3
Because phenols are more acidic than water, it is possible
to generate the phenoxide in situ using NaOH.
OCH2CH3
OH
+ CH3CH2Br, NaOH
CH3
CH3
4. Electrophilic Aromatic Substitution
The –OH group is a powerful activating group in EAS
and an ortho/para director.
a) nitration
OH
OH
HNO3
O2N
NO2
polynitration!
NO2
OH
OH
OH
dilute HNO3
NO2
+
NO2
b) halogenation
OH
OH
Br2 (aq.)
Br
Br
no catalyst required
use polar solvent
polyhalogenation!
Br
OH
OH
OH
Br2, CCl4
Br
+
non-polar solvent
Br
c) sulfonation
OH
OH
SO3H
H2SO4, 15-20oC
OH
H2SO4, 100oC
SO3H
At low temperature the reaction is non-reversible and the lower Eact orthoproduct is formed (rate control).
At high temperature the reaction is reversible and the more stable paraproduct is formed (kinetic control).
d) Friedel-Crafts alkylation.
OH
OH
+
CH3
H3C C CH3
Cl
AlCl3
H3C C CH3
CH3
e) Friedel-Crafts acylation
OH
OH
O
+
CH3CH2CH2C
AlCl3
Cl
O
Do not confuse FC acylation with esterification:
OH
O
O
+
CH3CH2CH2C
Cl
O
Fries rearrangement of phenolic esters.
OH
O
O
+
CH3CH2CH2C
O
Cl
AlCl3
OH
O
f) nitrosation
OH
OH
HONO
p-nitrosophenol
NO
EAS with very weak electrophile NO+
OH
OH
CH3
CH3
NaNO2, HCl
NO
g) coupling with diazonium salts
(EAS with the weak electrophile diazonium)
N2 Cl
OH
OH
CH3
CH3
+
benzenediazonium
chloride
an azo dye
N
N
h) Kolbe reaction (carbonation)
OH
ONa
+ CO2
COONa
125oC, 4-7 atm.
sodium salicylate
H+
EAS by the weakly
electrophilic CO2

O C O
OH
COOH
salicylic acid
i) Reimer-Tiemann reaction
OH
OH
CHCl3, aq. NaOH
H+
CHO
70oC
salicylaldehyde
The salicylaldehyde can be easily oxidized to salicylic acid
Spectroscopy of phenols:
Infrared:
O—H stretching, strong, broad 3200-3600 cm-1
C—O stretch, strong, broad ~1230 cm-1
(alcohols ~ 1050 – 1200)
nmr: O—H 4-7 ppm (6-12 ppm if intramolecular
hydrogen bonding)
o-cresol
C--O
O--H
o-cresol
OH
b
CH3
a
c
c
b
a
ethyl salicylate (intramolecular hydrogen bonding)
d
OH O
C
O CH2CH3
b
a
c
d
c
b
a