Phenols

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Phenols
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Infrared Spectrum of p-Cresol
3
Proton NMR
Chemical shift (?, ppm)
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13C NMR
OCH3
OH
128.5
159.7
155.1
114.0
115.5
129.5
129.8
120.7
121.1
Oxygen of hydroxyl group deshields carbonto
which it is directly attached.
The most shielded carbons of the ring are those
thatare ortho and para to the oxygen.
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Mass Spectrometry
Prominent peak for molecular ion. Most intense
peak in phenol is for molecular ion.
m/z 94
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Physical 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
C6H5CH3
C6H5OH
C6H5F
Molecular weight
92
94
96
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Acidity of Phenols

• Their most characteristic property

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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
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Delocalized negative charge in phenoxide ion
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Phenols are converted to phenoxide ionsin
aqueous base


H2O
stronger acid
weaker acid
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Substituent Effectson theAcidity of Phenols
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Question

• Which one of the following has the lowest pKa?
• A) B)
• C) D)

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Electron-releasing groups have little or no effect
pKa
10
10.3
10.2
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Electron-withdrawing groups increase acidity
pKa
10
9.4
7.2
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Effect of electron-withdrawing groups is
mostpronounced at ortho and para positions
pKa
7.2
8.4
7.2
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Effect of strong electron-withdrawing groupsis
cumulative
pKa
7.2
4.0
0.4
Picric acid
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Resonance
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Question

• Which of the following compounds is more acidic?
• A) o-Cresol
• B) o-Chlorophenol
• C) o-Methoxyphenol
• D) o-nitrophenol
• E) m-nitrophenol

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Preparation of Aryl Ethers
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Typical Preparation is by Williamson Synthesis
SN2


RX
NaX
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Example

K2CO3
acetone, heat
(86)
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Aryl Ethers from Aryl Halides
CH3OH


KOCH3
KF
25C
(93)

• nucleophilic aromatic substitution is effective
  with nitro-substituted (ortho and/or para) aryl
  halides

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Claisen Rearrangementof Allyl Aryl Ethers
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Allyl Aryl Ethers Rearrange on Heating

• allyl group migrates to ortho position

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Mechanism
rewrite as
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Sigmatropic Rearrangement
Claisen rearrangement is an example of a
sigmatropic rearrangement. A ? bond
migratesfrom one end of a conjugated ? electron
systemto the other.
conjugated ? electron system is the allyl group
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Question

• What will be the Claisen rearrangement product of
  the carbon-14 labeled ether shown here?
• A) B)
• C) D)

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Oxidation of PhenolsQuinones
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Quinones

• The most common examples of phenol oxidationsare
  the oxidations of 1,2- and 1,4-benzenediolsto
  give quinones.

(76-81)
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Quinones

• The most common examples of phenol oxidationsare
  the oxidations of 1,2- and 1,4-benzenediolsto
  give quinones.

O
O
Ag2O
diethyl ether
(68)
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Some quinones are dyes
Alizarin(red pigment)
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UV-VIS
Oxygen substitution on ring shifts ?max to
longerwavelength effect is greater in
phenoxide ion.

O
?max204 nm256 nm
?max210 nm270 nm
?max235 nm287 nm
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Some quinones are important biomolecules
Vitamin K (blood-clotting factor)
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Some quinone precursors are important in foods
Antioxidants can protect against the
cell-damaging effects of free radicals. Some
dietary phenolics are oxidized to quinones as
free radical-antioxidant traps.
Eg. Reservatrol / Flavonoids