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Chapter 24 Phenols

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Title: Chapter 24 Phenols


1
Chapter 24Phenols
2
24.1Nomenclature
3
Nomenclature
5-Chloro-2-methylphenol
  • named on basis of phenol as parent
  • substituents listed in alphabetical order
  • lowest numerical sequence first point
    ofdifference rule

4
Nomenclature
1,2-Benzenediol
1,3-Benzenediol
1,4-Benzenediol
(common namepyrocatechol)
(common nameresorcinol)
(common namehydroquinone)
5
Nomenclature
p-Hydroxybenzoic acid
  • name on basis of benzoic acid as parent
  • higher oxidation states of carbon
    outrankhydroxyl group

6
24.2Structure and Bonding
7
Structure of Phenol
  • phenol is planar
  • CO bond distance is 136 pm, which isslightly
    shorter than that of CH3OH (142 pm)

8
24.3Physical Properties
The OH group of phenols allows hydrogen
bondingto other phenol molecules and to water.
9
Hydrogen Bonding in Phenols
10
Physical Properties (Table 24.1)
Compared to compounds of similar size
andmolecular weight, hydrogen bonding in phenol
raises its melting point, boiling point,
andsolubility in water.
11
Physical Properties (Table 24.1)
C6H5CH3
C6H5OH
C6H5F
Molecular weight
92
94
96
95
43
41
Melting point (C)
Boilingpoint (C,1 atm)
111
132
85
Solubility inH2O (g/100 mL,25C)
0.05
8.2
0.2
12
24.4Acidity of Phenols
  • most characteristic property of phenols is their
    acidity

13
Compare


14
Delocalized negative charge in phenoxide ion


O


H
H
H
H
H
15
Delocalized negative charge in phenoxide ion


O


H
H
H
H
H
16
Delocalized negative charge in phenoxide ion
17
Delocalized negative charge in phenoxide ion
18
Delocalized negative charge in phenoxide ion
19
Delocalized negative charge in phenoxide ion

O

H
H


H
H
H
20
Phenols are converted to phenoxide ionsin
aqueous base


H2O
stronger acid
weaker acid
21
24.5Substituent Effectson theAcidity of Phenols
22
Electron-releasing groups have little or no effect
Ka
1 x 10-10
5 x 10-11
6 x 10-11
23
Electron-withdrawing groups increase acidity
Ka
1 x 10-10
4 x 10-9
7 x 10-8
24
Effect of electron-withdrawing groups is
mostpronounced at ortho and para positions
Ka
6 x 10-8
4 x 10-9
7 x 10-8
25
Effect of strong electron-withdrawing groupsis
cumulative
Ka
7 x 10-8
1 x 10-4
4 x 10-1
26
Resonance Depiction
27
24.6Sources of Phenols
  • Phenol is an important industrial chemical.
  • Major use is in phenolic resins for adhesives
    and plastics.
  • Annual U.S. production is about 4 billion pounds
    per year.

28
IndustrialPreparationsof Phenol
1. NaOH heat
2. H
1. O2
1. NaOH heat
2. H2O H2SO4
2. H
29
Laboratory Synthesis of Phenols
  • from arylamines via diazonium ions

(81-86)
30
24.7Naturally Occurring Phenols
  • Many phenols occur naturally

31
Example Thymol
OH
CH3
CH(CH3)2
Thymol(major constituent of oil of thyme)
32
Example 2,5-Dichlorophenol
OH
Cl
Cl
2,5-Dichlorophenol(from defensive secretion ofa
species of grasshopper)
33
24.8Reactions of PhenolsElectrophilic Aromatic
Substitution
Hydroxyl group strongly activates the ringtoward
electrophilic aromatic substitution
34
Electrophilic Aromatic Substitution in Phenols
  • Halogenation
  • Nitration
  • Nitrosation
  • Sulfonation
  • Friedel-Crafts Alkylation
  • Friedel-Crafts Acylation

35
Halogenation

Br2
(93)
  • monohalogenation in nonpolar solvent(1,2-dichloro
    ethane)

36
Halogenation
H2O

3Br2
25C
(95)
  • multiple halogenation in polar solvent(water)

37
Electrophilic Aromatic Substitution in Phenols
  • Halogenation
  • Nitration
  • Nitrosation
  • Sulfonation
  • Friedel-Crafts Alkylation
  • Friedel-Crafts Acylation

38
Nitration
(73-77)
  • OH group controls regiochemistry

39
Electrophilic Aromatic Substitution in Phenols
  • Halogenation
  • Nitration
  • Nitrosation
  • Sulfonation
  • Friedel-Crafts Alkylation
  • Friedel-Crafts Acylation

40
Nitrosation
NO
OH
(99)
  • only strongly activated rings undergo
    nitrosation when treated with nitrous acid

41
Electrophilic Aromatic Substitution in Phenols
  • Halogenation
  • Nitration
  • Nitrosation
  • Sulfonation
  • Friedel-Crafts Alkylation
  • Friedel-Crafts Acylation

42
Sulfonation
H2SO4
100C
SO3H
  • OH group controls regiochemistry

(69)
43
Electrophilic Aromatic Substitution in Phenols
  • Halogenation
  • Nitration
  • Nitrosation
  • Sulfonation
  • Friedel-Crafts Alkylation
  • Friedel-Crafts Acylation

44
Friedel-Crafts Alkylation
  • (CH3)3COH reacts with H3PO4 to give (CH3)3C

(63)
45
Electrophilic Aromatic Substitution in Phenols
  • Halogenation
  • Nitration
  • Nitrosation
  • Sulfonation
  • Friedel-Crafts Alkylation
  • Friedel-Crafts Acylation

46
24.9Acylation of Phenols
Acylation can take place either on the ringby
electrophilic aromatic substitution or onoxygen
by nucleophilic acyl substitution
47
Friedel-Crafts Acylation

ortho isomer
AlCl3
  • under Friedel-Crafts conditions, acylation of the
    ring occurs(C-acylation)

(74)
(16)
48
O-Acylation
(95)
  • in the absence of AlCl3, acylation of the
    hydroxyl group occurs (O-acylation)

49
O- versus C-Acylation
AlCl3
formed faster
more stable
  • O-Acylation is kinetically controlled process
    C-acylation is thermodynamically controlled
  • AlCl3 catalyzes the conversion of the aryl ester
    to the aryl alkyl ketones this is called the
    Fries rearrangement
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