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.
Dr. Talat R.
Al-Ramadhany
Phenol Benzylalcohol
−OH is directly linked −OH is not directly linked
to the aromatic ring
carbon to the aromatic ring
carbon
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Al-Ramadhany
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.
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.
Dow process,
in which Chlorobenzene is allowed to react the aqueous sodium hydroxide at a
temperature of about 360ºC.
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Al-Ramadhany
Oxidation of
Cumene: Cumene is converted by air oxidation into cumene hydroperoxide, which
is converted by aqueous acid into Phenol and acetone.
Preparation
of Phenols in the laboratory
Hydrolysis
of diazonium salts
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Al-Ramadhany
Oxidation of
arylthallium compounds
This method
has two advantages over the diazonium route:
The speed
and high yield.
Orientation
control in the thallation step.
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Al-Ramadhany
Alkaline
hydrolysis of aryl halides
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Al-Ramadhany
Dr. Talat R.
Al-Ramadhany
Reaction of
Phenols
There are
two type of reaction:
Reaction of
(O–H) bond.
Reaction of
aromatic ring
(Electrophilic Aromatic
Substitution).
Dr. Talat R.
Al-Ramadhany
Reaction of
(O–H) bond.
Acidity,
salt formation.
Ether
formation (Williamson Synthesis)
Phenols are
converted into ethers by reaction in alkaline solution with alkyl halides.
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Al-Ramadhany
Aryl halide
must be containing strong electron-withdrawing group to form corresponding
ether.
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Al-Ramadhany
Ester
formation
Phenols are usually converted into their
esters by the reaction with carboxylic acids, acid chlorides or
anhydrides.
Phenol Benzoyl chloride Phenyl
benzoate
(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.
B. Reaction
of aromatic ring
(Electrophilic
Aromatic Substitution).
Halogenation
(Bromination)
Treatment of Phenols with aqueous
solution of bromine results in replacement of every hydrogen ortho or para to
the –OH group
If
halogenation is carried out in a solvent of low polarity:
Some group
can replace by bromine
Sulfonation:
Nitration:
Friedel-Crafts
alkylation
Alkyl phenols can be prepared by
Friedel-Crafts alkylation of Phenols, but the yields are often poor.
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.
Nitrosation:
Nitrous acid
converts Phenols into nitrosophenols
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Al-Ramadhany
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.
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.
Dr. Talat R.
Al-Ramadhany
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