Alcohols, Diols, and Thiols

1
Chapter 15

• Alcohols, Diols, and Thiols

2
Methanol
Methanol is an industrial chemical end uses
solvent, antifreeze, fuel principal use
preparation of formaldehyde prepared by
hydrogenation of carbon monoxide
CO 2H2 ? CH3OH
3
Ethanol

• Ethanol is an industrial chemical
• Most ethanol comes from fermentation
• Synthetic ethanol is produced by hydrationof
  ethylene
• Synthetic ethanol is denatured (madeunfit for
  drinking) by adding methanol, benzene,pyridine,
  castor oil, gasoline, etc.

4
Other alcohols
Isopropyl alcohol is prepared by hydration of
propene. All alcohols with four carbons or fewer
are readily available. Most alcohols with five
or six carbons are readily available.
5
Sources of alcohols
Reactions discussed in earlier chapters (Table
15.1)
Hydration of alkenes Hydroboration-oxidation of
alkenes Hydrolysis of alkyl halides Syntheses
using Grignard reagents organolithium reagents
6
Sources of alcohols
New methods in Chapter 15
Reduction of aldehydes and ketones Reduction of
carboxylic acids Reduction of esters Reaction of
Grignard reagents with epoxides Diols by
hydroxylation of alkenes
7
Reduction of Aldehydes Gives Primary Alcohols
8
Reduction of Ketones Gives Secondary Alcohols
R
C
O
R'
9
Example Catalytic Hydrogenation
H
OH
Pt

H2
ethanol
(93-95)

10
Metal Hydride Reducing Agents
Sodiumborohydride
act as hydride donors
11
Examples Sodium Borohydride
Aldehyde
NaBH4
methanol
Ketone
NaBH4
ethanol
12
Fig. 15.2
13
Lithium aluminum hydride
more reactive than sodium borohydride cannot use
water, ethanol, methanol etc.as solvents diethyl
ether is most commonly used solvent
14
Selectivity
neither NaBH4 or LiAlH4reduces
carbon-carbondouble bonds
15
Reduction of Carboxylic Acids Gives Primary
Alcohols (LAH) is only effective reagent
1. LiAlH4diethyl ether
2. H2O
(78)
16
Reduction of EstersGives Primary Alcohols
Lithium aluminum hydride preferred forlaboratory
reductions Sodium borohydride reduction is too
slowto be useful Catalytic hydrogenolysis used
in industrybut conditions difficult or dangerous
to duplicate in the laboratory (special
catalyst, hightemperature, high pressure
17
Example Reduction of an Ester
18
Reaction of Grignard Reagentswith Epoxides

19
Example
CH2
H2C
CH3(CH2)4CH2MgBr


O
1. diethyl ether 2. H3O
20
Diols are prepared by...
reactions used to prepare alcohols hydroxylation
of alkenes
21
Example reduction of a dialdehyde
H2 (100 atm)
Ni, 125C
22
Example reduction of a dialdehyde
H2 (100 atm)
Ni, 125C
23
Osmium Tetraoxide is Key Reagent
OsO4
24
Example
25
Table 15.2 Review of Reactions of Alcohols
reaction with hydrogen halides reaction with
thionyl chloride reaction with phosphorous
tribromide acid-catalyzed dehydration conversion
to p-toluenesulfonate esters
26
New Reactions of Alcohols in This Chapter
conversion to ethers esterification esters of
inorganic acids oxidation cleavage of vicinal
diols
27
Conversion of Alcohols to Ethers
2CH3CH2CH2CH2OH
CH3CH2CH2CH2OCH2CH2CH2CH3
(60)
28
Intramolecular Analog
HOCH2CH2CH2CH2CH2OH
H2SO4
130
(76)
reaction normally works well only for5- and
6-membered rings
29
Esterification
H


ROH
H2O
condensation Fischer esterification acid
catalyzed reversible
30
Example of Fischer Esterification
0.1 mol
0.6 mol

H2O
70 yield based on benzoic acid
31
Reaction of Alcohols with Acyl Chlorides


ROH
HCl
high yields not reversible when carried outin
presence of pyridine
32
Esters of Inorganic Acids
ROH HOEWG
ROEWG H2O
EWG is an electron-withdrawing group
33
Esters of Inorganic Acids
ROH HOEWG
ROEWG H2O
EWG is an electron-withdrawing group
34
Aqueous Cr(VI)
FCH2CH2CH2CH2OH
H2SO4
K2Cr2O7
H2O
FCH2CH2CH2COH
(74)
35
Nonaqueous Sources of Cr(VI)
All are used in CH2Cl2 Pyridinium dichromate
(PDC) (C5H5NH)2 Cr2O72 Pyridinium
chlorochromate (PCC) C5H5NH ClCrO3
36
Example Oxidation of a primary alcohol with PDC
PDC
CH2Cl2
(94)
37
Biological Oxidation of AlcoholEnzyme Catalized
CH3CH2OH
alcohol dehydrogenase
38
Cleavage of Vicinal Diols by Periodic Acid
39
Cleavage of Vicinal Diols by Periodic Acid
HIO4
40
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
41
Properties of Thiols
1. hydrogen bonding is much weaker in thiols
than in alcohols (SH bond is less polar than
OH) 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
42
Thiols are stronger acids than alcohols
have pKas of about 10 can be deprotonated in
aqueous base
43
RS and HS are weakly basic and good
nucleophiles
44
Oxidation of thiols take place at sulfur
thiol
disulfide
45
Infrared Spectroscopy

• OH stretching 3200-3650 cm1 (broad)
• CO stretching 1025-1200 cm1 (broad)
• SH stretching 2550-2700 cm1 (weak)

46
1H NMR
chemical shift of OH proton is variable
dependson temperature and concentration OH
proton can be identified by adding D2O signal
for OH disappears (converted to OD) HCO
signal is less shielded than HCH
H
H
C
O
? 3.3-4 ppm
? 0.5-5 ppm
47
Figure 15.9 (page 653)
Chemical shift (?, ppm)
48
13C NMR
chemical shift of COH is ? 60-75 ppm deshielding
effect of O is much larger than S
49
Mass Spectrometry of Alcohols
molecular ion peak is usually small a peak
corresponding to loss of H2Ofrom the molecular
ion (M - 18) isusually present peak
corresponding to loss of analkyl group to give
an oxygen-stabilized carbocation is
usuallyprominent