4. Aromatic compounds containing one or more
OH groups directly attached with carbon of
benzene ring are called Phenols.
➡ Simplest phenol is Carbolic Acid C6H5OH
Term Phenol is derived from an old name of
benzene
- Phene
➡ Phenyl: C6H5
5. :Nomenclature of Phenols
It is used as a prefix, while benzene
part of the molecule is used as
suffix In IUPAC-OH group is
represented as hydroxyl..
6. STRUCTURE OF PHENOLS
The alcohol functional group consists of O atom bonded
to sp² hybridized aromatic C atom and H atom via a bond
Both C-O and O-H bonds are polar
Conjugation exist between an unshared electron pair of
the O and Benzene Ring
This results in, as compared to alcohols:
A shorter C-O bond
A more basic OH group
A more acidic OH proton
7. Physical properties of phenols:
Colorless, crystalline, poisonous solid
with phenolic odor
Melting point 41°C and Boiling Point
182°C
Sparingly soluble in water forming
pink solution at room temperature
Completely soluble above 68.5°C
Causes blisters on skin
Used as disinfectants and in washrooms
8. ACIDITY OF PHENOLS:
Phenols are more acidic (pKa≈ 10) than alcohol
(pKa 16-20)
Phenols are less acidic than Carboxylic acids
(pKa = 5)
COMPARISON OF ACIDITY OF PHENOLS AND
ALCOHOLS
Phenol exists as resonance hybrid of following
structures
9. Phenols are much more acidic than alcohols
because the negative charge in the phenoxide ion
is not localized on the oxygen atom, as it is in an
alkoxide ion, but is delocalized-it is shared by a
number of carbon atoms in the benzene ring.
Due to resonance O atom acquires a positive
charge and hence attracts electron pair of O-H
bond leading to the release of H+
10. Carbon atom of C-OH group of phenol (sp² hybridized)
is more electrophilic than Carbon atom in Alcohols (sp3
hybridized)
In phenols, a greater inductive effect facilitated release
of proton
Thus phenols are more acidic than alcohols because
resonance is impossible in alcohols
➡ Phenoxide is more resonance stabilized than phenol
but in case of alcohol, alkoxide is not stable because
there is no possibility for the delocalization of negative
11. Preparation of phenols:
1.Hydrolysis of chlorobenzene( Dow Process)
Chlorobenzene is hydrolysed by heating with 10%
NaOH at 360ºC under high pressure to form
sodium phenoxide which on treating with HCl
gives phenol.
12. 2.Preparation of phenol from diazonium salt:
When an aromatic primary amine is treated with
nitrous (NaNO2 + HCl) acid at 273 – 278 K, diazonium
salts are obtained. These diazonium salts are highly
reactive in nature. Upon warming with water, these
diazonium salts finally hydrolyze to phenols. Phenols
can also be obtained from diazonium salts by treating
it with dilute acids.
13. 3. Preparation of Phenols from Benzene Sulphonic Aci
Benzenesulphonic acid can be obtained from benzene
by reacting it with oleum. This benzene sulphonic
acid can be treated with molten sodium hydroxide at
high temperatures to encourage the formation of
sodium phenoxide. Finally, sodium phenoxide on
acidification gives phenol.
14. 4. Preparation of Phenols from Cumene:
Cumene is an organic compound obtained by Friedel-Crafts
alkylation of benzene with propylene. Upon oxidation of cumene
(isopropylbenzene) in the presence of air, cumene hydroperoxide
is obtained. Upon further treatment of cumene hydroperoxide with
dilute acid, phenols are obtained. Acetone is also produced as one
of the by-products of this reaction in large quantities. Hence,
phenols prepared by these methods need purifications.
15. Naturally Occuring Phenols:
1.Methyl phenols(cresols)
Chemical formula:CH3C6H4OH
2.1,2Dihydroxybenzene(Catechol)
Chemical formula: C6H6O2
3.1,3Dihydroxybenzene(Resorcinol)
4.Quinol(Hydroquinone)
16. Reactions of phenol due to OH
group:
1.Salt formation:
Since phenols are more acidic than water,they react with aqueous
solution of sodium or potassium hydroxide to form water soluble
sodium or potassium phenoxides.
17. 2.Ester Formation:
Phenols react with acid chlorides and acid
anhydrides in the presence of a base or an acid
to form phenyl esters
18. 3.Reaction with FeCl3:
Phenols give green, blue or violet colouration
with neutral FeCl3 solution due
to the formation of coloured
complex iron salt.
4.Reaction with zinc
dust
When phenol is distilled with zinc dust,benzene is obtained.
19. Chemical tests for phenols:
1. Phenol reacts with bromine water to form
2,4,6-tribromophenol which is insoluble in water
and gives turbidity in a solution.
2.Almost all phenols in water or alcohol give a
characteristics colour ranging from violet through blue
and green to red with neutral FeCl3 solution.The colour
develops due to the formation of ferric phenoxide
salts.Phenol itself gives a violet colouration .
20. USES OF PHENOL:
Production of plastic
Disinfectant
Chemical intermediate
Wood preservative
Stabilizer in the production of
explosives
In cosmetic industry
