Chapter
Phenols 5
COURSE NAME: Chemistry of Aromatic Compounds
By the end of this chapter, you should understand:
1. The definition and nomenclature of phenols.
2. How the hydroxyl group can be introduced into the aromatic ring.
3. The acidity of phenols.
4. The influence of the hydroxyl group on the aromatic reactions.
Nomenclature
OH
OH
OH
OH HO
HO
Phloroglucinol Pyrogallol
Polyhydric phenols
Methods of preparation
heat H2O
OH N2Cl
Benzenediazonium chloride
Worked problem:
Suggest the
mechanism for the displacement of sulfonic group by hydroxide ion
1-From benzenesulfonic acids
2-From benzenediazonium salts
CaO/NaOH heat Salicyclic acid
OH
COOH
OH
3- from aryl halides
4- from hydroxy acids (decarboxylation)
5.1. Dakin reaction:
The rearrangement of 2-hydroxybenzaldehydes brought about by reaction with alkaline hydrogen peroxide and leading to dihydroxybenzenes
5- Miscellaneous Methods
5.2. Bamberger rearrangement:
The acid-catalyzed rearrangement of phenylhydroxylamines, known as the , is useful for the synthesis of 4-aminophenols
Physical Properties
The OH group of phenols allows hydrogen bonding to other phenol molecules and to water.
Compared to compounds of similar size and molecular weight, hydrogen bonding in phenol raises its melting point, boiling point, and solubility in water.
+ +
− −
Properties of phenols:
Most phenols are solid at room temp., they have high boiling points (why ?) p-nitrophenol is more soluble in water than o-nitrophenol (why ?)
p-nitrophenol has high boiling point than o-nitrophrenol.
C6H5CH3 C6H5OH C6H5F
Molecular weight 92 94 96
–95 43 –41
Melting point (°C)
Boiling
point (°C,1 atm) 111 132 85
Solubility in
H2O (g/100 mL,25°C)
0.05 8.2 0.2
Physical Properties
( a ) ( b ) (c ) ( d ) (e)
OH
OH OH OH OH
(hybride str.)
Carboxylic acids > phenols > alcohols
The acidity of phenols arises from the greater resonance stabilization of the phenoxide anion compared with phenol itself
Reactions of phenols
** Acidic properties
Substituent Effects on the Acidity of Phenols
Electron-releasing groups have little or no effect
Electron-withdrawing groups increase acidity
O
O N O
O
O N O
O
O N O
O
O N O
O
O N O
Presence of electron with-drawing group will increase the acidity
* Esterification
OH CH3
+ (CH3CO)2O
OCOCH3 CH3
+ CH3COOH H
o-Cresol o-Tolyl acetate
Synthesis of Aspirin
Synthesis of Paracetamol
* Effect of FeCl
3Phenols form intensely colored complexes with ferric ion in neutral solution. The formed blue, purple or green colors are
* Effect of Zn-dust
OH
+ Zn-dust
heat
+ ZnO
ONa
+ CH3Br
OCH3
Anisole
* Etherification (Williamson reaction)
* Oxidation
20
Ring substitution reactions
2,4,6-Trinitrophenol
OH OH
O2N
NO2
NO2 Conc. HNO3
Conc H2SO4
( picric acid )
* Nitration
a) Strong conditions
b) Mild conditionds
4-Nitrophenol
OH OH
NO2 dil HNO3
20 oC
2-Nitrophenol OH
NO2 +
OH is o- & p-directing group
* Nitrosation
* Sulfonation
Sulfonic group is a bulky substituent so at higher temperature the strike hindrance between OH and SO3H become too high, as a result the sulfonic group prefer para-position
OH
Conc. H2SO4 20 oC
OH
SO3H
2-Hydroxybenzenesulfonic acid (o-Hydroxybenzenesulfonic acid) Conc. H2SO4
100 oC OH
SO3H
4-Hydroxybenzenesulfonic acid (p-Hydroxybenzenesulfonic acid)
* Halogenation
* Friedel-Crafts acylation
OH OH
p-Cresol Anhyd. AlCl3
OCOCH3
Phenylacetate CH3
CH3COCl
Anhyd. AlCl3 CH3Cl
Friedel-Crafts acylation Friedel-Crafts alkylation
AlCl3
(Fries rearrangment)
OH
OH
COCH3 +
* Reimer-Tiemann reaction
* Kolbe-Schmidt reaction
Aromatic alcohols
Nomenclature
Benzyl alcohol 2-Phenylethanol 1-Phenylethanol
CH2OH CH2CH2OH H3C CH OH
Reactions
1. Write the structural formula for each of the following compounds:
a) 2,4-dinitrophenol b) m-cresol
c) hydroquinone d) resorcinol
e) catechol f) picric acid
2. Give the reagents and any critical conditions necessary to prepare phenol from:
a) aniline b) benzene
c) chlorobenzene
d) cumene (isopropylbenzene)
