Chapter 24 Phenols

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Chapter 24Phenols
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24.1Nomenclature
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Nomenclature
5-Chloro-2-methylphenol

• named on basis of phenol as parent
• substituents listed in alphabetical order
• lowest numerical sequence first point
  ofdifference rule

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Nomenclature
1,2-Benzenediol
1,3-Benzenediol
1,4-Benzenediol
(common namepyrocatechol)
(common nameresorcinol)
(common namehydroquinone)
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Nomenclature
p-Hydroxybenzoic acid

• name on basis of benzoic acid as parent
• higher oxidation states of carbon
  outrankhydroxyl group

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24.2Structure and Bonding
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Structure of Phenol

• phenol is planar
• CO bond distance is 136 pm, which isslightly
  shorter than that of CH3OH (142 pm)

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24.3Physical Properties
The OH group of phenols allows hydrogen
bondingto other phenol molecules and to water.
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Hydrogen Bonding in Phenols
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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.
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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
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24.4Acidity of Phenols

• most characteristic property of phenols is their
  acidity

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Compare


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Delocalized negative charge in phenoxide ion


O


H
H
H
H
H
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Delocalized negative charge in phenoxide ion


O


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

O

H
H


H
H
H
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Phenols are converted to phenoxide ionsin
aqueous base


H2O
stronger acid
weaker acid
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24.5Substituent Effectson theAcidity of Phenols
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Electron-releasing groups have little or no effect
Ka
1 x 10-10
5 x 10-11
6 x 10-11
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Electron-withdrawing groups increase acidity
Ka
1 x 10-10
4 x 10-9
7 x 10-8
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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
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Effect of strong electron-withdrawing groupsis
cumulative
Ka
7 x 10-8
1 x 10-4
4 x 10-1
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Resonance Depiction
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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.

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IndustrialPreparationsof Phenol
1. NaOH heat
2. H
1. O2
1. NaOH heat
2. H2O H2SO4
2. H
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Laboratory Synthesis of Phenols

• from arylamines via diazonium ions

(81-86)
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24.7Naturally Occurring Phenols

• Many phenols occur naturally

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Example Thymol
OH
CH3
CH(CH3)2
Thymol(major constituent of oil of thyme)
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Example 2,5-Dichlorophenol
OH
Cl
Cl
2,5-Dichlorophenol(from defensive secretion ofa
species of grasshopper)
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24.8Reactions of PhenolsElectrophilic Aromatic
Substitution
Hydroxyl group strongly activates the ringtoward
electrophilic aromatic substitution
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Electrophilic Aromatic Substitution in Phenols

• Halogenation
• Nitration
• Nitrosation
• Sulfonation
• Friedel-Crafts Alkylation
• Friedel-Crafts Acylation

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Halogenation

Br2
(93)

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

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Halogenation
H2O

3Br2
25C
(95)

• multiple halogenation in polar solvent(water)

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Electrophilic Aromatic Substitution in Phenols

• Halogenation
• Nitration
• Nitrosation
• Sulfonation
• Friedel-Crafts Alkylation
• Friedel-Crafts Acylation

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Nitration
(73-77)

• OH group controls regiochemistry

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Electrophilic Aromatic Substitution in Phenols

• Halogenation
• Nitration
• Nitrosation
• Sulfonation
• Friedel-Crafts Alkylation
• Friedel-Crafts Acylation

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Nitrosation
NO
OH
(99)

• only strongly activated rings undergo
  nitrosation when treated with nitrous acid

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Electrophilic Aromatic Substitution in Phenols

• Halogenation
• Nitration
• Nitrosation
• Sulfonation
• Friedel-Crafts Alkylation
• Friedel-Crafts Acylation

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Sulfonation
H2SO4
100C
SO3H

• OH group controls regiochemistry

(69)
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Electrophilic Aromatic Substitution in Phenols

• Halogenation
• Nitration
• Nitrosation
• Sulfonation
• Friedel-Crafts Alkylation
• Friedel-Crafts Acylation

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Friedel-Crafts Alkylation

• (CH3)3COH reacts with H3PO4 to give (CH3)3C

(63)
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Electrophilic Aromatic Substitution in Phenols

• Halogenation
• Nitration
• Nitrosation
• Sulfonation
• Friedel-Crafts Alkylation
• Friedel-Crafts Acylation

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

ortho isomer
AlCl3

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

(74)
(16)
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O-Acylation
(95)

• in the absence of AlCl3, acylation of the
  hydroxyl group occurs (O-acylation)

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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