Phenols/OC -II PCI Syllabus/preparation of Phenols/ Sources of Phenols/Qualitative tests of Phenols

 Introduction
 Shape
 Nomenclature
 Sources
 Preparation of phenols
 Physical properties
 Chemical properties
 Structure and uses
 Qualitative tests
Contents

Compounds in which —OH group is directly attached with
benzene ring are called as Phenols.

Phenols
 Phenol is also called as carbolic acid.
 Phenol is discovered by Runge
 Hofmann, another scientist, prepared it first from ‘coal tar’.

sp2 - sp2
(C-C σ bond)
sp3 - s
(O-H σ bond)
sp2 – sp3
(C-O σ bond)
sp2 - s
(C-H σ bond)
Formation of sigma (σ ) bonds in Phenol
All ring carbon atoms also have a p orbital and these are perpendicular to the plane of the σ
bonds of the ring. The lateral overlap of these p orbitals produces a delocalized π molecular
orbital.

Adjacent p orbitals overlap to form the delocalized π molecular orbital.one
of the lone pair electrons on the oxygen atom resides in sp3 orbital and
interacts with the delocalized π molecular orbital to form an extended π
molecular orbital containing 8 electrons.

Sources
Phenols occur widely in nature and have many industrial, pharmaceutical, and biological
applications. phenol is a general disinfectant, commonly called carbolic acid.
Poison ivy, which produces the
phenol urushiol.
Cloves contain the phenol eugenol.

Sources
Peppers, a source
of capsaicin, which
is a phenol.
Polyphenols are
found in olive oil.
Salicylic acid is a phenol
that is used to treat
acne.

Anthocyanins and
flavonoids provide
the bright colors in
certain fruits and
vegetables.

Physical properties
● Have higher boiling points than hydrocarbons of the same molecular weight
● Modest solubility in water
● Form strong intermolecular hydrogen bonds

Physical properties
Figure : Hydrogen bonding in phenols. The weak attraction between a
positively polarized _OH hydrogen and a negatively polarized oxygen holds
molecules together.
 Phenols and alcohols have unusually high boiling points because, like water, they
form hydrogen bonds.
 These forces must be overcome for a molecule to break free from the liquid and
enter the vapor, so the boiling temperature is raised.

a) Dow process
b) From diazonium salt
c) From benzene sulphonate
d) From salicylic acid
e) from oxidation of cumene
f) From benzene
Preparation of phenols

 Nitration
 Halogenation
 Sulfonation
 Kolbe’s Reaction
 Claisen Rearrangement
 Fries Rearrangement
 Reimer - Tiemann Reaction
 Reaction with Neutral FeCl3
 Oxidation
 Reaction with Benzene diazonium chloride
 Reaction with Formaldehyde
 Reaction with Phthalic aanhydride
 Formation of salt
 Formation ofEther
 Formation ofEster

Reactions of Phenols
Formation of salt
Phenols are weak acids and react with strong bases, such as NaOH, to form water-
soluble salts.
Most phenols do not react with weaker bases, such as sodium bicarbonate, and do
not dissolve in aqueous sodium bicarbonate.

Formation of Ether (Williamson’s Ether Synthesis)
Phenols reacts with alkyl halides in alkali solution to form phenyl ethers. the alkali first forms
phenoxide ion which then reacts with alky halide

Formation of Ester
Phenols reacts with acyl halides in alkali solution to form phenyl esters. the alkali
first forms phenoxide ion which then reacts with alky halide

Formation of Ester
Phenols reacts with anhydrides in alkali solution to form phenyl esters. the alkali
first forms phenoxide ion which then reacts with alky halide

Resonance in phenol and phenoxide ion:
 It is evident from the above structures that — OH group of phenol is o- and p-directing, as these
are electron richer places; so electrophiles attack at these positions.
 Phenoxide ion is resonance stabilized. That is why phenol shows acidic character. The hydroxyl
oxygen is less basic, and the hydroxyl proton is more acidic in phenol than in alcohol.
Reactions of Benzene ring of Phenols

Electrophilic aromatic substitution

The hydroxyl group is a powerful activating group—and an ortho–para director in
electrophilic aromatic substitutions.
Monobromination of phenol can be achieved by carrying out the reaction in CS2 at a low
temperature, conditions that reduce the electrophilic reactivity of bromine. The major
product is the para isomer
Halogenation

Nitration
Phenol reacts with dilute nitric acid to yield a mixture of o- and p-nitrophenol.
Although the yield is relatively low (because of oxidation of the ring), the
ortho and para isomers can be separated by steam distillation.

o- nitrophenol and p - nitrophenol can be separated by steam distillation
More Volatile Less Volatile
Due to intramolecular
H - bonding
Due to intermolecular
H - bonding
Low B.P. High B.P.
Nitration

With concentrated HNO3 , phenol is converted to 2,4,6 – trinitrophenol ( Picric acid ).
The yield of reaction product is poor.
Nitration
Nowadays picric acid is prepared by treating phenol first with concentrated
sulphuric acid which converts it to phenol-2,4-disulphonic acid, and then with
concentrated nitric acid to get 2,4,6-trinitrophenol.

Sulfonation
Phenol reacts with concentrated sulfuric acid to yield mainly the ortho-sulfonated product
if the reaction is carried out at 25 oC and mainly the para- sulfonated product at 100 oC.

Kolbe’s Reaction
Phenoxide ion is even more susceptible to SEAr than phenol. The Kolbe–Schmitt reaction
or Kolbe process is a carboxylation chemical reaction that proceeds by heating sodium
phenoxide with carbon dioxide under pressure, then treating the product with sulfuric
acid.

Kolbe’s Reaction
Mechanism
OH
OH- O-
C OO
+
-
O
H
C O
O-
OH
CO2
-
H+OH
CO2H

Claisen Rearrangement
Heating allyl phenyl ether to 2000c effects an intramolecular reaction to
generate o - allylphenol.
Mechanism:

Claisen Rearrangement
When the ortho position is occupied, attack take place on para position.

Fries Rearrangement
Phenolic esters of aliphatic and aromatic carboxylic acids, when treated with a Lewis acid
as catalyst, do undergo a rearrangement reaction to yield ortho- and para-acylphenols
respectively.

Reimer-Tiemann reaction
On treating phenol with chloroform in the presence of sodium hydroxide, a –CHO group is
introduced at ortho position of benzene ring. This reaction is known as Reimer - Tiemann
reaction.The intermediate substituted benzal chloride is hydrolysed in the presence of
alkali to produce salicylaldehyde.

Reaction with Benzene diazonium chloride
Benzene
diazonium chloride
Phenol p-Hydroxyazobenzene
Phenols couples with Benzene diazonium chloride in an alkaline solution to form
p-Hydroxyazobenzene

Reaction with Phthalic anhydride
Phenols reacts with phthalic anhydride in the presence of sulfuric acid to fom
phenolphthalein.
Phthalic anhydride phenolphthalein
Phenol Phenol

Reaction with Formaldehyde
Phenol o and p-Hydroxyazobenzyl alcohol
Phenols is treated with an alkaline solution of formaldehyde, a mixture of o- and p-
Hydroxybenzyl alcohol is formed.

Reaction with Formaldehyde
If the reaction is carried at high temperature and
in excess of formaldehyde, hard thermosetting
plastic Bakelite is formed.

Oxidation
p-Benzoquinone
Phenols undergoes oxidation with air or chromic acid to form p-Benzoquinone

BHT (Butylated Hydroxytoluene)
Antioxidant Disinfectant

58
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