Phenols

SYLLABUS
•Phenols* - Acidity of phenols, effect of substituents on
acidity, qualitative tests, Structure and uses of phenol,
cresols, resorcinol, naphthols.
•Aromatic Amines* - Basicity of amines, effect of
substituents on basicity, and synthetic uses of aryl
diazonium salts.
•Aromatic Acids* –Acidity, effect of substituents on
acidity and important reactions of benzoic acid.
Prepared by Mr. N. M. Jangade

Introduction
• Phenols are compounds containing an –OH group
attached directly to an aromatic ring.
• Phenols, sometimes called phenolics, are a class
of chemical compounds consisting of one or
more hydroxyl groups (-OH) bonded directly to
an aromatic hydrocarbon group.
• The simplest form of phenol is, C6H5OH.

• Phenols are usually named by common systems or as
derivatives of the parent phenol.

• Alkyl phenyl ethers are named as alkoxybenzene,
while the C6H5O- are called as Phenoxy

• Compound which contain an –OH group in a
side chain attached to an aromatic ring are not
phenols. They are called aromatic alcohols.

• Phenols containing Carboxy or Carbonyl functional
groups, these take priority over hydroxy group.

Properties
• Most pure phenols are colourless liquids or solids,
although they are often found to contain red tint owing
to the presence of oxidation products.
• Phenols have characteristic ‘Carbolic’, odour, which in
case of phenol is highly toxic.
• The boiling point of the Phenols are higher than that of
the aliphatic alcohols of comparable molecular weight.
This is due to the stronger intermolecular hydrogen
bonding in Phenols relative to alcohols.

• The introduction of a hydroxy group in to already
substituted aromatic ring, especially para position to
methyl, Halogen and Nitro group, produces marked
increase in the boiling point.

• Ortho isomer: Greater volatility and low solubility in to
water is due to intra-molecular hydrogen bonding.

• Para isomer: Greater Boiling point is due to inter-
molecular hydrogen bonding.

• p-nitrophenol is more soluble in water than o-
nitrophenol can associate with itself and also with
water molecules. o-nitrophenol cannot form
intermolecular hydrogen bonds.

• Greater volatility and low solubility of o-nitrophenol
enables it to be easily separated from p-nitrophenol
by steam distillation.

Structure of Phenol

Resonance forms

Synthesis
1. From Chlorobenzene (Dow Process):
This involves the hydrolysis of Chlorobenzene with aq.
NaOH at high temperature and pressure followed by
treatment with dil. HCl.

2. From Sodium Benzenesulfonate:
This involves fusion of sodium benzenesulfonate with
solid NaOH followed by treatment with dil. HCl.

3. From Benzenediazonium salts:
Aniline on diazotization reaction forms Benzenediazonium
chloride. Warm solution of this Benzenediazonium chloride
on water bath forms Phenol.

4. From Coal Tar:
The middle oil fractions of Coal tar contains Phenols,
Cresols and Naphthalene. The oil, when cooled, deposits
solid Naphthalene, which removed by centrifuging the
mixture.
The oil left is agitated with NaOH solution Phenols and
Cresols dissolve as a sodium salts.

• Phenols were recovered when Carbon dioxide is passed
through above solution and isolated by fractional
distillation.

• Chemical reactions:
Reactions of Phenols are involved as reactions of
–OH and reactions of Benzene ring.

Reactions at –OH group
1. Formation of Salts:
Phenol is acidic. It reacts with sodium hydroxide to form
salts.

2. Reaction with FeCl3:
Phenol on reaction with the neutral FeCl3 solution forms
purple colour complex.

3. Formation of Esters:
Phenols on reaction with acid chlorides (or Acid
anhydrides) in aq. Alkali solution to give Phenyl esters.

4. Formation of Ether:
Phenols react with alkyl halide’s in alkali solution to form
Phenyl ethers.

5. Reaction with Zinc dust:
Phenol on distillation with zinc yield’s Benzene.

• Rate of reaction?
• Proffered attack of an Electrophile on Benzene
ring of phenol?
Reactions at Benzene ring
Electrophilic aromatic substitution reaction

Electrophilic aromatic substitution reaction
• Rate of reaction of Phenol is more if you compared it
with Benzene in Electrophilic aromatic substitution
reaction.
• In Phenol Hydroxy group is substituent over Benzene
ring is electron donating (Activating group) of it will
show ortho-para directive effect.
• Some reactions give trisubstitution but by
moderating the reaction conditions, however we can
obtain the monosubstitution product.

Halogenation:
• Phenols react with Bromine water to give 2,4,6-
tribromophenol.
• If the reaction is carried out in CS2 or CCl4 a mixture of o-
and p- bromophenol is obtained

• Nitration:
Phenol reacts with concentrated nitric acid to give Picric
acid.

• Nitration:
Phenol reacts with dilute nitric acid to give a mixture of o-
and p- nitrophenol.

Sulfonation:
• When Phenol is treated with conc. H2SO4 at 200C, o-
Phenolsulfonic acid forms as a main product while at 1000C,
p-Phenolsulfonic acid forms as a main product.

• Reimer-Tiemann reaction:
• This involves the reaction of the Phenol with Chloroform
in aq. NaOH followed by acid hydrolysis, salicylaldehyde
is formed.
• If CCl4 is used in place of Chloroform, salicylic acid is
formed.

• Step I: Removal of proton from Chloroform

• Gatterman reaction:
Phenol on reaction with mixture of Hydrogen
cyanide and Hydrochloric acid in the presence of
AlCl3 catalyst gives Salicylaldehyde.

• Kolbe’s reaction:
This reaction involves the treatment of sodium phenoxide
with Carbon dioxide at 1250C under 6 atm. Pressure
followed by acid-hydrolysis, salicylic acid is formed.

• Fries rearrangement:
The Phenol is first treated with acetic anhydride in the
presence of aq. NaOH to give Phenyl acetate. This ester is
then heated with Aluminium chloride catayst when the acyl
group migrates from the Phenolic oxygen to an ortho and
para position of the ring.

• Reaction with Benzene diazonium salt:
The Phenol on reaction with Benzene diazonium chloride in
an alkaline condition to form P-Hydroxyazobenzene.

• Reaction with Phthalic anhydride:
• Phenol on reaction with phthalic anhydride in the presence
of Sulphuric acid to form Phenolphthalein.

• Reaction with Formaldehyde:
When Phenol is reacted with an alkaline solution of
Formaldehyde, a mixture of o- and p-hydroxybenzyl alcohol
formed.

• Oxidation:
Phenol undergoes oxidation with air or Chromic
acid to form p-Benzoquinone.

• Reduction:
Phenol on catalytic reduction with hydrogen in presence
of Nickel catalyst at 1250C to form Cyclohexanol.

• The pKa value is one method used to indicate
the strength of an acid.
• pKa is the negative log of the acid dissociation
constant or Ka value.
Acidity of Phenols

Acidity of Phenols

Acidity of Phenols
• The acidity of any compound is defined by its proton
releasing ability in the water.
• Phenol when dissolved in water, to form Phenoxide ion.
• The ease with which a Phenol release a proton is explained
by the difference in the resonance stabilization of the
Phenol and its Phenoxide ion.

• As we know Phenol has 5 forms of resonance.
• In Phenol charge forms II, III and IV contribute much
less to the hybrid than Kekule forms I and II.
• This is because Charge separation takes place which
cannot stabilizes hybrid form much.

• Phenoxide anion also have 5 resonance forms.
• In Phenoxide ion there is no charge separation takes
place, the charge is only delocalized, hence form III,
IV and V stabilize the Phenoxide anion very much.
• Since Phenoxide anion is hybrid is more more stable
than the Phenol hybrid.

• Phenoxide ion is more stable than the Phenol hence
equilibrium will shift to the right, releasing H+ ions.
Therefore Phenols behaves as an acids.

Why Alcohols are less acidic than Phenols?
• The negative charge in the Phenoxide anion is
spread through out the Benzene ring, and thereby
effectively dispersed. This charge delocalization is a
stabilizing factor in the Phenoxide ion.
• On the other hand no resonance is possible in the
alkoxide ions (RO) derived from alcohol.

• Alcohols are less acidic
than Phenols as Alkoxide
anion is not favored as it is
not resonance stabilized.

Effect of the substituent on acidity
• Substituent’s have a great effect on acidity and basicity of
organic compounds.
• Substituent’s can increase or decrease acidity of phenols.
Let’s see how substituent’s affect acidity of phenols.
• Let’s get started, Substituent’s are of 2 types i.e.
1. Electron withdrawing
2. Electron donating.

A. An Electron Withdrawing Substituent’s increases
the acidity of the Phenols

• Electron withdrawing substituent's withdraws electron
from the ring system which results in the dispersal of
negative charge of the Phenoxide anion which makes
Phenoxide anion more stable and results in stronger acid.
• As we know that the electron withdrawing groups (such as
–NO2, -CHO,-CN etc.) pull electron towards them, they
ultimately decrease electron density over the oxygen of
hydroxyl group and increases tendency to donate proton
(H+ ions) easily.

A. An Electron Donating (Releasing) Substituent’s
decreases the acidity of the Phenols

• Electron donating substituent's donates electron to
the ring system which results in the increases
negative charge of the Phenoxide anion which makes
Phenoxide anion more unstable and results in weaker
acid.
• As we know that the electron donating groups (such
as –NH2, -OH, -CH3 etc.) will push electron towards
ring system, they ultimately increase electron density
over the oxygen of hydroxyl group and decreases
tendency to donate proton (H+ ions) easily.

Nitrophenol

Acidity order: Para> Ortho > Meta > Phenol

Why p-nitro phenol is more acidic than o-nitrophenol?
o- nitro phenol can form intra molecular hydrogen bond .
Due to formation this intra molecular hydrogen bond , o-
nitro phenol is more stable than phenoxide ion.
• Consequently, o-nitro phenol can not leave proton ( H+ ) in
solution and hence p-nitro phenol is more acidic than o-
nitro phenol.

Multiple substitution by strongly electron withdrawing
groups greatly increases the acidity of Phenols

Methoxy

Cresols

Which is stronger acid phenol or cresol?

Acidity of o-cresol?
• At o-position due to hyperconjugation effect of methyl
group the density of electron on carbon atom as well as
on oxygen atom increases ( reduces acidity).
• O- position is near to –OH group so high electron donating
inductive effect (reduces acidity)

Why p-cresol is more acidic than o-cresol?
• At p-position due to hyperconjugation effect of methyl
group the density of electron on carbon atom as well as
on oxygen atom increases ( reduces acidity).
• p- position is at maximum distance to –OH group so
electron donating inductive effect is less compared to
ortho (acidity more compare to ortho)

• But, in m-position , hyperconjugation (not effective).
• m- position is at away from –OH group so electron
donating inductive effect is less compared to ortho
(acidity less compare to ortho)
• Hence , m-cresol becomes more acidic than o-cresol.
Why m-cresol is more acidic than o & p-cresol?

Qualitative tests
Any of the following test can be carried out to detect the
phenolic functional group.
1. Litmus test
2. Ferric chloride test
3. Libermann’s test
4. Bromine water test
5. Phthalein dye test

(a) Litmus Test:
Phenol turns blue litmus paper red. This shows that
phenol is acidic in nature.

(b) Ferric Chloride Test:
Aqueous solution of phenol reacts with freshly prepared
ferric chloride solution gives coloured complex.
Most phenols give dark coloured solution.

(c) Libermann’s Test:
Phenol reacts with concentrated sulfuric acid and sodium
nitrite forms a yellow colour quinone monoxime complex.
With excess of phenol and sulfuric acid a deep blue
indophenol complex is formed. On dilution a red colour
indophenol is formed
Which on treatment with sodium hydroxide turns to deep
blue colour solution of sodium salt of indophenol

(d) Bromine Water Test:
Phenol undergoes electrophilic substitution reaction with
bromine. When bromine water is added to aqueous solution
of phenol the brown colour of bromine disappears and a
white precipitate of tribromophenol is formed.

(e) Phthalein Dye Test:
Phenol on heating with phthalic anhydride in the
presence of concentrated sulfuric acid forms a colourless
condensation compound called phenolphthalein. On
further reaction with dilute sodium hydroxide solution
gives a pink colour fluorescent compound called
fluorescein. Characteristic colours are produced by
different phenolic compounds which can be viewed
under white background.
Phenol Reddish pink
o-cresol Red
m-cresol blue or violet blue
1-naphthol green
2-naphthol faint green
Resorcinol yellow-green fluorescence
Hydroquinone deep purple

Qualitative tests
(f) Azo Coupling Test:
The Phenol on reaction with Benzene diazonium chloride in
an alkaline condition to form P-Hydroxyazobenzene.

Uses of Phenol
1. For the preparation of Phenol-formaldehyde resins.
2. Starting material in preparation of drugs like
Aspirin.
3. Starting material in the preparation of
Phenolphthalein and several other dyes.
4. In preparation of Explosives.
5. In preparation of wood preservatives, herbicides,
nylon, antiseptics, gasoline additives.

Cresols
• Cresols give creosote its antibacterial and
insecticidal properties, as well as its toxicity and
its ability to irritate bare skin.
• Cresols can be used in preparation of phenol-formaldehyde
resins.
• m-Cresol is also used in making photographic developer
and explosives.

Resorcinol
• It is used as an antiseptic and disinfectant in topical
pharmaceutical products in the treatment of skin
disorders and infections such as acne, seborrheic
dermatitis, eczema, psoriasis, corns, calluses,
and warts. It exerts a keratolytic activity.
• Resorcinol works by helping to remove hard, scaly, or
roughened skin.

Naphthols
• Naphthol has different uses including dyes, insecticides,
pharmaceuticals, perfumes, antiseptics, synthesis of
fungicides, and antioxidants for rubber.

Phenols

More Related Content

What's hot

Similar to Phenols

Recently uploaded

Phenols