Transcript Organic Chemistry Fifth Edition

Phenols
Infrared Spectrum of p-Cresol
Proton NMR
H
H
HO
CH3
H
10.0
9.0
8.0
7.0
6.0
H
5.0
4.0
3.0
Chemical shift (, ppm)
2.0
1.0
0
13C
NMR
OH
128.5
OCH3
155.1
121.1
159.7
115.5
114.0
129.8
129.5
120.7
Oxygen of hydroxyl group deshields carbon
to which it is directly attached.
The most shielded carbons of the ring are those that
are ortho and para to the oxygen.
Mass Spectrometry
Prominent peak for molecular ion. Most intense
peak in phenol is for molecular ion.
•+
OH
••
m/z 94
Physical Properties
The OH group of phenols allows hydrogen bonding
to other phenol molecules and to water.
Hydrogen Bonding in Phenols
O H
O
Physical Properties
C6H5CH3
C6H5OH
C6H5F
Molecular weight
92
94
96
Melting point (°C)
–95
43
–41
Boiling
point (°C,1 atm)
111
132
85
Solubility in
H2O (g/100 mL,25°C)
0.05
8.2
0.2
Acidity of Phenols
Their most characteristic property
Compare
••
•• O
•• –
•• O ••
H
pKa = 10
H
pKa = 16
••
CH3CH2O
••
H
+ +
•• –
+ + CH CH O
•
H
3
2 •
••
Delocalized negative charge in phenoxide ion
– ••
•• O ••
••
•• O
H
H
H
H
H
H
H
–
••
H
H
H
Delocalized negative charge in phenoxide ion
••
••
•• O
H
H
••
–
H
•• O
H
H
H
H
–
••
H
H
H
Delocalized negative charge in phenoxide ion
••
••
•• O
H
H
••
–
H
•• O
H
H
H
H
–
H
••
H
H
Phenols are converted to phenoxide ions
in aqueous base
••
•• O
•• –
•• O ••
H
–
+ HO
stronger acid
+ H2O
weaker acid
Substituent Effects
on the
Acidity of Phenols
Question
Which one of the following has the lowest pKa?
A)
B)
C)
D)
Electron-releasing groups have little or no effect
OH
pKa:
10
OH
OH
CH3
OCH3
10.3
10.2
Electron-withdrawing groups increase acidity
OH
pKa:
10
OH
OH
Cl
NO2
9.4
7.2
Effect of electron-withdrawing groups is most
pronounced at ortho and para positions
OH
OH
OH
NO2
NO2
NO2
pKa:
7.2
8.4
7.2
Effect of strong electron-withdrawing groups
is cumulative
OH
OH
OH
NO2
NO2
pKa:
7.2
NO2
4.0
NO2
O2N
NO2
0.4
Picric acid
Resonance
– ••
•• O ••
••
•• O ••
H
H
H
H
H
H
H
H
•• O
••
N
+
••
O ••
•• –
••
•• O
– ••
N
+
••
O ••
•• –
Question
Which of the following compounds is more
acidic?
A) o-Cresol
B) o-Chlorophenol
C) o-Methoxyphenol
D) o-nitrophenol
E) m-nitrophenol
Preparation of Aryl Ethers
Typical Preparation is by Williamson Synthesis
ONa + RX
SN2
OR + NaX
but the other combination
X + RONa
fails because aryl halides are normally unreactive
toward nucleophilic substitution
Example
OH
K2CO3
+ H2C
CHCH2Br
acetone, heat
OCH2CH
CH2
(86%)
Aryl Ethers from Aryl Halides
F
OCH3
+ KOCH3
NO2
CH3OH
+ KF
25°C
NO2
(93%)
nucleophilic aromatic substitution is effective
with nitro-substituted (ortho and/or para) aryl
halides
Claisen Rearrangement
of Allyl Aryl Ethers
Allyl Aryl Ethers Rearrange on Heating
OCH2CH
CH2
200°C
OH
CH2CH
(73%)
CH2
allyl group
migrates to
ortho position
Mechanism
OCH2CH
CH2
O
rewrite as
OH
keto-to-enol
isomerization
O
H
Sigmatropic Rearrangement
Claisen rearrangement is an example of a
sigmatropic rearrangement. A  bond migrates
from one end of a conjugated  electron system
to the other.
this  bond breaks
O
O
“conjugated 
electron system”
is the allyl group
H
this  bond forms
Question
What will be the Claisen rearrangement
product of the carbon-14 labeled ether
shown here?
A)
B)
C)
D)
Oxidation of Phenols:
Quinones
Quinones
The most common examples of phenol oxidations
are the oxidations of 1,2- and 1,4-benzenediols
to give quinones.
OH
O
Na2Cr2O7, H2SO4
H2O
OH
O
(76-81%)
Quinones
The most common examples of phenol oxidations
are the oxidations of 1,2- and 1,4-benzenediols
to give quinones.
OH
O
OH
O
Ag2O
diethyl ether
CH3
CH3
(68%)
Some quinones are dyes
O
OH
OH
O
Alizarin
(red pigment)
UV-VIS
Oxygen substitution on ring shifts max to longer
wavelength; effect is greater in phenoxide ion.
OH
max
204 nm
256 nm
max
210 nm
270 nm
O
max
235 nm
287 nm
–
Some quinones are important biomolecules
O
CH3
CH3
O
CH3
CH3
CH3
Vitamin K
(blood-clotting factor)
CH3
Some quinone precursors are important in foods
Antioxidants can protect against the celldamaging effects of free radicals. Some dietary
phenolics are oxidized to quinones as free
radical-antioxidant traps.
Eg. Reservatrol / Flavonoids