Download Alcohols and Phenols

Alcohols and Phenols
Chapter 17 Part 3
17.8 Oxidation of Alcohols
„ Can be accomplished by inorganic reagents,
such as KMnO4, CrO3, and Na2Cr2O7 or by
more selective, expensive reagents
Oxidation of Primary Alcohols
„ To aldehyde: pyridinium chlorochromate
(PCC, C5H6NCrO3Cl) in dichloromethane
„
Other reagents produce carboxylic acids
„ Converts secondary alcohol to ketone
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„ Jones’ Reagent: CrO3 in aqueous sulfuric
CH3O
CH3OH
CH3
PCC
CH2Cl2, 25oC
O
„
Oxidizes primary alcohols to carboxylic acids:
CH3
O
Testosterone
acid.
4-Androstene-3,17-dione
CH3(CH2)8CH2OH
1-Decanol
O
CrO3
CH3(CH2)8COH
H2SO4 Decanoic acid
All of the oxidations occur via an E2 mechanism.
Mechanism of Chromic Acid
Oxidation
„ Alcohol forms a chromate ester followed by
elimination with electron transfer to give ketone
„ The mechanism was determined by observing the
effects of isotopes on rates
„ Na2Cr2O7 in aqueous acetic acid is an
inexpensive oxidizing agent:
CH3
OH
Na2Cr2O7
acetic acid
4-tert-Methylcyclohexanol
What alcohols would give these
products on oxidation?
O
CH3
CH3CHCHO
CH3
O
4-tert-Methylcyclohexanone
What products would you expect from the oxidation
of the following compounds with Jones’ reagent?
With PCC?
„1
-
Hexanol
Jones’:
PCC:
„2
-
Hexanol
Jones’:
PCC:
O
„ Hexanal
Jones’:
PCC:
2
Predict the major organic product:
OH
Cl
17.9 Protection of Alcohols
K2Cr2O7
„ Hydroxyl groups can easily transfer their proton to a
acetic acid
„ This can prevent desired reactions
basic reagent
„ Converting the hydroxyl to a (removable) functional
group without an acidic proton protects the alcohol
OH
PCC
CH2Cl2
Methods to Protect Alcohols
„ Reaction with chlorotrimethylsilane in the
presence of base yields an unreactive
trimethylsilyl (TMS) ether
„ The ether can be cleaved with acid or with
fluoride ion to regenerate the alcohol
Protection
-
Deprotection
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17.10 Preparation and Uses of Phenols
„ Industrial process from readily available
cumene
„ Forms cumene hydroperoxide with oxygen at
high temperature
Laboratory Preparation of Phenols
17.11 Reactions of Phenols
„ From aromatic sulfonic acids by melting with NaOH
„ The hydroxyl group is a strongly activating, making
at high temperature
„ Limited to the preparation of alkyl-substituted phenols
phenols substrates for electrophilic halogenation,
nitration, sulfonation, and Friedel–Crafts reactions
„ Reaction of a phenol with strong oxidizing agents
yields a quinone
„ Fremy's salt [(KSO3)2NO] works under mild
conditions through a radical mechanism
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Quinones in Nature
17.12 Spectroscopy of Alcohols and
Phenols
„ Ubiquinones mediate electron-transfer processes
„ Characteristic O–H stretching absorption at 3300 to
involved in energy production through their redox
reactions
3600 cm−1 in the infrared
„ Sharp absorption near 3600 cm-1 except if H-bonded:
then broad absorption 3300 to 3400 cm−1 range
„ Strong C–O stretching absorption near 1050 cm−1
(See Figure 17.11)
„ Phenol OH absorbs near 3500 cm-1
Butanol (Gas Phase)
Nuclear Magnetic Resonance
Spectroscopy
13C NMR: C bonded to OH absorbs at a lower
field, δ 50 to 80
„ 1H NMR: electron-withdrawing effect of the nearby
oxygen, absorbs at δ 3.5 to 4 (See Figure 17-13)
„
„
„
Usually no spin-spin coupling between O–H proton and
neighboring protons on C due to exchange reactions
with moisture or acids
Spin–spin splitting is observed between protons on the
oxygen-bearing carbon and other neighbors
„ Phenol O–H protons absorb at δ 3 to 8
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