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Phenols (Ar-OH)
Phenols (Ar-OH)
Phenols are compounds of the general formula Ar-OH, where Ar- is phenyl, substituted phenyl, or one of the other aryl groups.
I. Structure and nomenclature:
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.
OH
G
OH
G
Dr. Talat R. Al-Ramadhany
OH CH2OH
Phenol Benzylalcohol −OH is directly linked −OH is not directly linked to the aromatic ring carbon to the aromatic ring carbon
Dr. Talat R. Al-Ramadhany
OH
Cl
o-Chlorophenol
OH
CH3
o-cresol
OH
CH3
m-Cresol
COOH
OH
o-Hydroxybenzoic acid
COOH
OHp-Hydroxybenzoic acid
Dr. Talat R. Al-Ramadhany
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 ArO-OH-
H+
A Phenol A Phenoxide ion(acid) (salt)
Insoluble Solublein water in water
Dr. Talat R. Al-Ramadhany
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.
Dr. Talat R. Al-Ramadhany
Cl
NaOH, 360o
4500 lb/in.2
O-Na+
HCl
OH
Sodium phenoxideChlorobenzene Phenol
ii. Dow process, in which Chlorobenzene is allowed to react the aqueous sodium hydroxide at a temperature of about 360ºC.
iii. Oxidation of Cumene: Cumene is converted by air oxidation into cumene hydroperoxide, which is converted by aqueous acid into Phenol and acetone.
C HH3C
CH3
C OOHH3C
CH3
OH
C OCH3
H3C+
Cumene Cumenehydroperoxide
PhenolAcetone
O2 H2O, H+
Dr. Talat R. Al-Ramadhany
Preparation of Phenols in the laboratory
i. Hydrolysis of diazonium salts
ArN2+ + H2O ArOH + H+ + N2
N2+HSO4
-
Cl
OH
Cl
H2O, H++ N2heat
Dr. Talat R. Al-Ramadhany
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.
Tl(OOCCF3)3
Tl(O
Pb(OAc)4
Ph3P
OC
O
CF3)2 C CF3
O
Aryltrifluoroacetate
H2O, OH-
heatH+
O- OH
Arylthalliumtrifluoroacetate
Dr. Talat R. Al-Ramadhany
iii. Alkaline hydrolysis of aryl halides
Cl OH
NaOH (10%)
350o
10%
Cl OH
NaOH (15%)
160o
15%NO2 NO2
Dr. Talat R. Al-Ramadhany
Cl O-Na+
NO2 NO2
NO2NaOH
NO2
H+
OH
NO2
NO2
OH
NO2
NO2HNO3H2SO4
O2N
Picric acid
Cl
NO2
NO2O2N
H2O
OH
NO2
NO2O2N
90%
Dr. Talat R. Al-Ramadhany
Reaction of Phenols
There are two type of reaction:
A. Reaction of (O–H) bond.
B. Reaction of aromatic ring (Electrophilic Aromatic Substitution).
O H
A. Reaction of (O–H) bond.
1) Acidity, salt formation.
OH
+ NaOH
O Na
+ H2O
OH
+ NaHCO3
O Na
+ H2CO3
2) Ether formation (Williamson Synthesis)
aqueous NaOH
OH O-
R X
O R
+ Xheat
Phenols are converted into ethers by reaction in alkaline solution with alkyl halides.
OH
+ C2H5I aqueous NaOHheat
OC2H5
Dr. Talat R. Al-Ramadhany
heat
OH
aq.NaOH
O-
phCH2Br
OCH2ph
Cl
+ phCH2O-
Dr. Talat R. Al-Ramadhany
Aryl halide must be containing strong electron-withdrawing group to form corresponding ether.
Cl
+ CH3O- Na+
NO2
NO2
OCH3
NO2
NO2
3) Ester formation
Phenols are usually converted into their esters by the reaction with carboxylic acids, acid chlorides or anhydrides.
OH + COCl O C
O
NaOH
Phenol Benzoyl chloride Phenyl benzoate
OH + (CH3CO)2O O C
O
CH3COONaO2N CH3O2N
p-Nitrophenyl acetatep-Nitrophenol Acetic anhydride
(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.
OH O
C2H5COCl
C C2H5
O
AlCl3CS2
OH
C C2H5
O
+
OH
CO
C2H5Phenol Phenyl propionate o-Hydroxyphenyl
ethyl ketone
p-Hydroxyphenylethyl 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
+ 3Br2H2O
OHBr
Br
Br
2,4,6-tribromophenol
+ 3HBr
If halogenation is carried out in a solvent of low polarity:
OH
+ 2Br2CHCl3
OHBr
+ 2HBr
OH
Br
+nonpolarsolvent
OH
+ 2Br2CS2 + HBr
OH
Br
0oC
OHCH3
+ 2Br2 (aq)
OHCH3Br
Br2,4-Dibromo-6-methylphenol
+ 2HBr
o-Cresol
Some group can replace by bromine
+ 3Br2 (aq)
OHBrBr
Br
+ 3HBr + H2SO4
OH
SO3Hp-Phenolsulfonic acid 2,4,6-Tribromophenol
2) Sulfonation:
OH
H2SO4
15-20oC
100oC
OHSO3H
OH
SO3H
o-Phenolsulfonic acid
p-Phenolsulfonic acid
H2SO4, 100oC
Conc.
Conc.
3) Nitration:OH OH
NO2dilute HNO3
20oC +
OH
NO2p-Nitrophenolo-Nitrophenol
40% yield 13% yield
OH OHNO2
NO2
O2NConc.HNO3
Picric acid
20oC
4) Friedel-Crafts alkylation
OH + H3C C
CH3
CH3
Cl
HO C
CH3
CH3
CH3
p-tert-Butylphenoltert-Butylchloride
HF
Alkyl phenols can be prepared by Friedel-Crafts alkylation of Phenols, but the yields are often poor.
OH
CH3
m-Cresol
O
CH3
CO
CH3
m-Cresyl acetate
AlCl3
25oC
160oC
(CH3CO)2O
OH
CH3
C CH3O2-Methyl-4-hydroxyacetophenone
OH
CH3
CH3CO
4-Methyl-2-hydroxyacetophenone
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.
Dr. Talat R. Al-Ramadhany
5) Nitrosation:
Nitrous acid converts Phenols into nitrosophenols
OH
NaNO2 , H2SO4
7 - 8oC
OH
NOp-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
+OCO
125oC4-7 atm.
OHCOONa
Sodium salicylate(Chief product)
H+
OHCOOH
Salicylic acid
Dr. Talat R. Al-Ramadhany
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.
OH O-
CHCl2CHCl3, aq.NaOH
70oC
O-
CHOOH
CHOHCl
Salicyladehyde(Chief product)
