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15.12
Thiols
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Nomenclature of Thiols
1) Analogous to alcohols, but suffix is -thiol
rather than –ol.
2) Final -e of alkane name is retained, not
dropped as with alcohols.
CH3CHCH2CH2SH
CH3
3-Methyl-1-butanethiol
Properties of Thiols
1. Hydrogen bonding is much weaker in thiols than
in alcohols (S—H bond is less polar than O—H).
2. Low molecular weight thiols have foul odors.
3. Thiols are stronger acids than alcohols.
4. Thiols are more easily oxidized than alcohols;
oxidation takes place at sulfur.
Thiols are Less Polar than Alcohols
Methanol
Methanethiol
bp: 65°C
bp: 6°C
Thiols are Stronger Acids than Alcohols
Have pKas of about 10; can be deprotonated in
aqueous base.
••
RS
••
– ••
H + •• OH
stronger acid
(pKa = 10)
••
•• –
RS ••
••
+
H
••
OH
••
weaker acid
(pKa = 15.7)
RS– and HS – are Weakly Basic and
Good Nucleophiles
C6H5S H
H Cl
C6H5SNa
(75%)
SN2
KSH
Br
SN2
SH
(67%)
Oxidation of Thiols Take Place at Sulfur
••
RS
••
thiol
H
••
RS
••
••
SR
••
disulfide
Thiol-disulfide redox pair is important in
biochemistry.
Other oxidative processes place 1,2, or 3
oxygen atoms on sulfur.
Oxidation of Thiols Take Place at Sulfur
••
RS
••
H
thiol
••
RS
••
••
SR
••
disulfide
•• –
•• O ••
••
RS
••
OH
sulfenic acid
+
RS OH
••
sulfinic acid
•• –
•• O ••
2+
RS
OH
•• O ••
••
–
sulfonic acid
Example: Sulfide-disulfide Redox Pair
SH
O
HSCH2CH2CH(CH2)4COH
O2, FeCl3
S
S
O
(CH2)4COH
-Lipoic acid (78%)
15.13
Spectroscopic Analysis of
Alcohols and Thiols
Infrared Spectroscopy
O—H stretching: 3200-3650 cm–1 (broad)
C—O stretching: 1025-1200 cm–1 (broad)
Figure 15.4 Infrared Spectrum of Cyclohexanol
Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved.
Infrared Spectroscopy
S—H stretching: 2550-2700 cm–1 (weak)
1H
NMR
Chemical shift of O—H proton is variable; depends
on temperature and concentration.
O—H proton can be identified by adding D2O; signal
for O—H disappears (converted to O—D).
H—C—O signal is less shielded than H—C—H.
H
 3.3-4 ppm
C
O
H
 0.5-5 ppm
Figure 15.5
CH2CH2OH
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
Chemical shift (, ppm)
2.0
1.0
0
1H
NMR
Sulfur is less electronegative than oxygen, so is
less deshielding.
CH3CH2CH2CH2OH
 3.6
CH3CH2CH2CH2SH
 2.5
13C
NMR
Chemical shift of C—OH is  60-75 ppm.
Deshielding effect of O is much larger than S.
 14
 19
 35
 62
CH3 — CH2 — CH2 — CH2 — OH
CH3 — CH2 — CH2 — CH2 — SH
 13
 21
 36
 24
UV-VIS
Unless there are other chromophores in the
molecule, alcohols are transparent above
about 200 nm; max for methanol, for
example, is 177 nm.
Mass Spectrometry of Alcohols
Molecular ion peak is usually small.
R
A peak corresponding to loss of H2O
from the molecular ion (M - 18) is
usually present.
Peak corresponding to loss of an
alkyl group to give an oxygenstabilized carbocation is usually
prominent.
R
R•
CH2
••
OH
••
•+
CH2
OH
CH2
+
OH
••
••