2. PHENOLS
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
3. NOMENCLATURE OF PHENOLS
In IUPAC –OH group is represented as hydroxyl. It is
used as a prefix, while benzene part of the molecule is
used as suffix.
1,2- dihydorxybenzene
O-hydroxyl phenol
(Catechol)
Phenol 1,3- dihydorxybenzene
m-hydroxyl phenol
(Resorcinol)
1,4- dihydorxybenzene
p-hydroxyl phenol
(Hydroquinone)
5. STRUCTURE OF PHENOLS
The alcohol functional group consists of O atom bonded to
sp2 hybridized aromatic C atom and H atom via σ 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
6. PHYSICAL PROPERTIES OF PHENOLS
Colorless, crystalline, poisonous solid with phenolic
odor
Melting point 41oC and Boiling Point 182oC
Sparingly soluble in water forming pink solution at
room temperature
Completely soluble above 68.5oC
Causes blisters on skin
Used as disinfectants .
7. 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
8. Due to resonance O atom acquires a positive charge and hence attracts electron pair of
O-H bond leading to the release of H+
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
charge.
9. COMPARISON OF ACIDITY OF PHENOLS AND CARBOXYLIC
ACIDS
Resonating structures of carboxylic acids:
Carboxylic acids ionize as: R-COOH R-COO- +H+
The carboxylate anion exhibits following resonating structures:
The resonating structures of RCOOH (I and II) are not equivalent and hence less stable
The resonating structures of RCOO- ion (III and IV) are equivalent and hence more
stable
Thus RCOOH have tendency to undergo ionization and form more stable carboxylate ion
and proton.
10. The resonating structures of phenoxide ion are not equivalent as shown below:
The resonating structures of RCOO- ion are equivalent. Hence RCOO- ion is
relatively more resonance stabilized that Phenoxide ion.
Thus a carboxylic acid is more acidic than a phenol.
RELATIVE ACIDITY ORDER OF SOME COMMON
COMPOUNDS: RCOOH > H2CO3 > C6H5OH > H2O >ROH
11. EFFECT OF SUBSTITUENTS ON THE ACIDITY OF
PHENOLS
Electron attracting substituents tend to disperse negative charge of
the phenoxide ion thus stabilize the ion and increase the acidity of
phenols.
Electron releasing substituents tend to intensify the charge,
destabilize the ion, diminish the resonance and decrease the acidity.
12. PREPARATION OF PHENOL
1. REACTION OF SODIUM SALT OF BENZENE SULFONIC
ACID WITH NaOH:
Sodium benzene sulfonate on fusion with strong alkali like
NaOH at 300oC give sodium phenoxide which on treatment
with HCl gives phenol
13. PREPARATION OF PHENOL
2. BASE HYDROLYSIS OF CHLOROBENZEN
(DOW’S METHOD)
Chlorobenzene is hydrolysed by heating with 10% NaOH at
360oC under high pressure to form sodium phenoxide which
on treating with HCl gives phenol
14. PREPARATION OF PHENOL
3. ACIDIC OXIDATION OF CUMENE
It is recently developed commercial method for
preparation of phenol.
Cumene is oxidized by atmospheric oxygen is presence of
metal catalyst
into Cumene Hydroperoxide.
The hydroperoxide is converted into phenol through
acid catalyzed arrangement
15. PREPARATION OF PHENOL
4. Preparation of phenol from Aryl Diazonium salts
Aryl diazonium salts are prepared by reaction of aryl
amines with nitrous acid
Aryl diazonium salts can be converted into phenols using
H2O/H2SO4/ heat
16. PREPARATION OF PHENOL
5. From phenolic acids:
OH
COOH
Sodium salicylate
+ 2NaOH
OH
CaO
Heat
Phenol
+ Na2CO3 + H2O
17. REACTIVITY AND REACTIONS OF PHENOLS
Phenols are very reactive towards electrophilic aromatic
substitution.
OH group is strongly activating ortho-/para- directing group.
ELECTROPHILIC AROMATIC SUBSTITUTION
18. Strong activation means milder reaction conditions than those used for
benzene.
Phenols are so activated that poly-substitution can be a problem.
OXIDATION OF PHENOLS
Phenols are very reactive towards oxidizing agent.
The oxidation takes place through several steps eventually destroying
the ring.
Reaction Benzene Phenol
Nitration Conc. HNO3 / H2SO4 Dil. HNO3 in H2O or CH3COOH
Sulfonation H2SO4 or SO3/H2SO4 Conc. H2SO4
Halogenation X2/Fe or FeX3 X2
Alkylation RCl/AlCl3 ROH/H+ or RCl/AlCl3
Acylation RCOCl/AlCl3 RCOCl/AlCl3
Nitrosation - Aq. NaNO2/H+
19. REACTION WITH SODIUM METAL / CARBOXYLATION OF PHENOLS
(KOLBE- SCHMITT REACTION)
“The reaction of sodium salt of phenol with CO2 is called Kolbe Reaction. It
is carbonation of phenol.”
At low temperature Sodium salicylate (sodium-o-hydroxyl benzoate) is formed,
at higher temperature o-product is isomerizes to p-isomer
CO2 act as electrophilic center in this reaction. Acidification of the salt gives
corresponding hydroxyl acid.
20. PHENOL - REACTIONS OF THE OH GROUP
Water phenol is a weak acid
it dissolves very slightly in water to form a weak acidic solution
it is a stronger acid than aliphatic alcohols
the ring helps weaken the O-H bond and stabilises the resulting anion
C6H5OH(aq) C6H5O¯(aq) + H+(aq)
NaOH phenol reacts with sodium hydroxide to form a salt - sodium phenoxide
it is ionic and water soluble
C6H5OH(aq) + NaOH(aq) ——> C6H5O¯ Na+(aq) + H2O(l)
Sodium phenol reacts with sodium to form an ionic salt - sodium phenoxide
hydrogen is also produced
this reaction is similar to that with aliphatic alcohols such as ethanol
2C6H5OH(s) + 2Na(s) ——> 2C6H5O¯ Na+(s) + H2(g)
22. 3) Acylation (Formation of esters)
OH
+ Cl
C
O
Acetyl chloride
Phenol
Pyridine
O
C
O
CH3
+ HCl
H2SO4
CH3COONa
O
C
O
Phenyl acetate
H3C
Phenyl acetate
+ CH3COOH
Acetic acid
Acetic anhydride
CH3
C
OO
C
O
CH3
23. :Fries Rearrangement
OCOCH3
Phenyl acetate
acetic anhydride
heat
OH
COCH3
OH
COCH3
O-Hydroxy acetophenone
P-Hydroxy acetophenone
+
4) Benzoylation Reaction:
OH
+
C
O Cl
Benzoyl chloridePhenol
NaOH
O
C
O
C6H5
+ HCl
Phenyl benzoate
25. ELECTROPHILIC SUBSTITUTION / Halogenation reaction:
Bromine the OH group is electron releasing
it increases the electron density of the delocalised system
it makes substitution much easier compared to benzene
the electron density is greatest at the 2,4 and 6 positions
substitution takes place at the 2,4 and 6 positions
phenol reacts readily with bromine water WITHOUT A CATALYST
it is so easy that multiple substitution takes place
other electrophiles such as NO2
+ react in a similar way
PHENOL - REACTIONS OF THE AROMATIC RING
34. STRUCTURE AND USES OF COMPOUNDS:
1. Phenol: (C6H5OH)(Carbolic Acid)
• Aromatic compound
• Slightly acidic nature
• Soluble in water have high
• May be toxic and caustic
Uses:
• Powerful antiseptic in soap, lotion.
• Synthesis of Bakelite plastic , resin etc.
• Partial Hydrogenation of phenol gives cyclohexanone which
is a precursor of nylon.
• Extraction of nucleic acid.
• Manufacturing of sunscreens, skin lightening cream, hair
coloring lotions.
• Manufacturing of Drugs like salol, aspirin, salicylic acid.
• Manufacturing of picric acid, preservative for ink.
35. 2. Cresols: (C7H8O) Hydroxy Toluene:
• It is yellowish to brown red tint in colour.
• Liquid or solid depends on temperature.
•Exposure to cresol in high concentration can cause irritation
and burning of skin, eye, mouth and abdominal pain etc.
Uses:
• Chlorination and etherification of o- cresol gives 2-methyl-
4-chlorophenoxy acetic acid (MCPA) which is used as
herbicides.
• Synthesis of vitamin E.
• Antiseptic
•Solvent for polymers.
•Preservative in some insulin.
• P-cresols is used in the production of butylated
hydroxytoulene(BHT) which is an antioxidant.
36. 3. Resorcinol (C6H6O2):
• It is colorless, readily soluble in water, alcohol and ethers but
insoluble in chloroform.
•It becomes pink with exposure to light.
• It has sweet taste.
Uses:
• Antiseptic, disinfectant, Analgesics, bactericidal, preservative.
• Its ointment up to 05 to 10 % used for treatment of psoriases,
eczema.
• Manufacturing of antidandruff shampoo and sunscreens
cosmetics.
• Production of diazodyes, plasticizers and UV absorber in resins.
• Manufacturing of glue, adhesive.
37. 4. Naphthols ( C10 H7OH)
• It is highly polar in nature.
• α-Naphthol is white solid, fluorescent compound and
soluble in alcohols, ethers, and chloroform.
• ß-Naphthol is colorless or yellow crystalline solid
fluorescent compound and soluble in alcohols, ethers, and
chloroform.
Uses:
• Used for detection of carbohydrates(content of
molisch’s reagent)
• Precursor of Insecticides like carbanyl.
• Detection of presence of arginine in proteins by
Sakaguchi test.
• Preparlation of sudan dyes.
• Synthesis of BINOL which is used as ligand for
transition of metal catalysed synthesis.
• Antiseptic, fungicide, antioxidant , in perfumes.
