Transcript 8 - Wiley

8
Introduction to
Organic
Chemistry
2 ed
William H. Brown
8-1
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8
Alcohols,
Ethers, and
Thiols
Chapter 8
8-2
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Structure - Alcohols
• The functional group of an alcohol
is an -OH group bonded to an sp3
hybridized carbon
• bond angles about the hydroxyl
oxygen atom are approximately 109.5°
• Oxygen is also sp3 hybridized
• two sp3 hybrid orbitals form sigma
bonds to carbon and hydrogen
• the remaining two sp3 hybrid orbitals
each contain an unshared pair of
electrons
8-3
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Structure - Ethers
• The functional group of an ether is an oxygen
atom bonded to two carbon atoms
• Oxygen is sp3 hybridized with bond angles of
approximately 109.5°. In dimethyl ether, the C-OC bond angle is 110.3°
H
H
••
H
C
H
O
••
C
H
H
8-4
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Structure - Thiols
• The functional group of a thiol is an
-SH (sulfhydryl) group bonded to an
sp3 hybridized carbon
• The bond angle about sulfur in
methanethiol is 100.3°, which
indicates that there is considerably
more p character to the bonding
orbitals of divalent sulfur than there
is to oxygen
8-5
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Nomenclature-Alcohols
• IUPAC names
• the longest chain that contains the -OH group is taken
as the parent.
• the parent chain is numbered to give the -OH group the
lowest possible number
• the suffix -e is changed to -ol
• Common names
• the alkyl group bonded to oxygen is named followed
by the word alcohol
8-6
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Nomenclature - Alcohols
OH
CH3 CH 2 CH 2 OH
CH 3 CHCH 3
CH3 CH2 CH2 CH 2 OH
1-Propanol
(Propyl alcohol)
2-Propanol
(Isopropyl alcohol)
1-Butanol
(Butyl alcohol)
CH3
CH 3
OH
CH3 CH 2 CHCH3
CH 3 COH
CH 3 CHCH 2 OH
CH3
2-Butanol
2-Methyl-1-propanol 2-Methyl-2-propanol
(sec-Butyl alcohol) (Isobutyl alcohol)
(tert-Butyl alcohol)
8-7
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Nomenclature - Alcohols
• Problem: write IUPAC names for these alcohols
CH 3
OH
(a) CH 3 CHCH 2 CHCH 3
OH
(b)
CH3
CH 2 OH
(c) CH 3 ( CH 2 ) 6 CH 2 OH
(d)
H3 C
C
CH 2 CH 3
H
8-8
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Nomenclature - Alcohols
• Compounds containing more than one -OH group
are named diols, triols, etc.
CH 2 CH 2
HO OH
1,2-Ethanediol
(Ethylene glycol)
CH 3 CHCH 2
CH 2 CHCH 2
HO OH
1,2-Propanediol
(Propylene glycol)
HO HO OH
1,2,3-Propanetriol
(Glycerol, Glycerin)
8-9
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Nomenclature - Alcohols
• Unsaturated alcohols
• the double bond is shown by the infix -en• the hydroxyl group is shown by the suffix -ol
• number the chain to give OH the lower number
1
2
5
HOCH2 CH 2
3
4
6
CH2 CH 3
C C
H
H
trans-3-Hexen-1-ol
8-10
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Nomenclature - Ethers
• IUPAC: the longest carbon chain is the parent. Name the
OR group as an alkoxy substituent
• Common names: name the groups attached to oxygen
followed by the word ether
CH 3 CH 2 OCH 2 CH 3
Ethoxyethane
(Diethyl ether)
CH 3
OH
CH3 OCCH 3
OCH 2 CH 3
CH 3
2-Methoxy-2-methylpropane
trans-2-Ethoxycyclohexanol
(methyl tert-butyl ether, MTBE)
8-11
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Nomenclature - Ethers
• Although cyclic ethers have IUPAC names, their
common names are more widely used
O
O
Ethylene
oxide
O
Tetrahydrofuran, THF
O
Tetrahydropyran
O
1,4-Dioxane
8-12
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Nomenclature - Thiols
• IUPAC names:
• the parent is the longest chain that contains the -SH
group
• change the suffix -e to -thiol
• Common names:
• name the alkyl group bonded to sulfur followed by the
word mercaptan
SH
CH3 CH 2 CH2 CH 2 SH
1-Butanethiol
(Butyl mercaptan)
CH3 CH2 CHCH3
2-Butanethiol
(sec-Butyl mercaptan)
8-13
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Physical Prop - Alcohols
• Alcohols are polar compounds
H
+
C
O
H
H
H
+
• Alcohols are associated in the liquid state by
hydrogen bonding
8-14
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Physical Prop - Alcohols
• Hydrogen bonding: the attractive force between a
partial positive charge on hydrogen and a partial
negative charge on a nearby oxygen, nitrogen, or
fluorine atom
• the strength of hydrogen bonding in water is
approximately 5 kcal/mol
• hydrogen bonds are considerably weaker than
covalent bonds
• nonetheless, they can have a significant effect on
physical properties
8-15
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Physical Prop - Alcohols
• Ethanol and dimethyl ether are constitutional
isomers.
• Their boiling points are dramatically different
• ethanol forms intermolecular hydrogen bonds which
increases attractive forces between its molecules,
which results in a higher boiling point
CH 3 CH2 OH
Ethanol
bp 78°C
CH 3 OCH 3
Dimethyl ether
bp -24°C
8-16
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Physical Prop. - Alcohols
• In relation to alkanes of comparable size and
molecular weight, alcohols
• have higher boiling points
• are more soluble in water
• The presence of additional -OH groups in a
molecule further increases boiling points and
solubility in water
8-17
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Physical Prop. - Alcohols
Formula
Name
CH 3 OH
methanol
CH 3 CH 3
ethane
CH 3 CH 2 OH
CH 3 CH 2 CH 3
ethanol
propane
CH 3 CH 2 CH 2 OH 1-propanol
CH 3 CH 2 CH 2 CH 3 butane
HO ( CH 2 ) 4 OH
CH 3 ( CH 2 ) 4 OH
CH 3 ( CH 2 ) 4 CH 3
1,4-butanediol
1-pentanol
hexane
MW
32
30
bp
(°C)
65
-89
Solubility
in Water
46
44
78
-42
infinite
insoluble
60
58
97
0
infinite
insoluble
90
88
86
230
138
69
infinite
2.3 g/100 g
insoluble
infinite
insoluble
8-18
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Physical Prop. - Ethers
• Ethers are polar molecules;
• the difference in electronegativity
between oxygen (3.5) and carbon
(2.5) is 1.0
• each C-O bond is polar covalent
• oxygen bears a partial negative
charge and each carbon a partial
positive charge
8-19
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Physical Prop. - Ethers
• Ethers are polar molecules, but because of steric
hindrance, only weak attractive forces exist
between their molecules in the pure liquid state
• Boiling points of ethers are
• lower than alcohols of comparable MW and
• close to those of hydrocarbons of comparable MW
• Ethers hydrogen bond with H2O and are more
soluble in H2O than are hydrocarbons
8-20
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Physical Prop. - Thiols
• Low-molecular-weight thiols have a STENCH
• the scent of skunks is due primarily to these two thiols
CH3
CH 3 CHCH 2 CH 2 SH
CH3 CH= CH CH 2 SH
3-Methyl-1-butanethiol
2-Butene-1-thiol
8-21
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Physical Prop. - Thiols
• The difference in electronegativity between S
(2.5) and H (2.1) is 0.4. Because of the low
polarity of the S-H bond, thiols
• show little association by hydrogen bonding
• have lower boiling points and are less soluble in water
than alcohols of comparable MW
Thiol
bp (°C)
methanethiol
6
ethanethiol
35
1-butanethiol 98
Alcohol bp (°C)
methanol
65
ethanol
78
1-butanol 117
8-22
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Acidity of Alcohols
• In dilute aqueous solution, alcohols are weakly
acidic
CH 3 O-H
+
O H
CH 3 O
-
+
+ H O H
H
H
[CH 3 O - ] [H 3 O + ]
Ka =
= 15.5
[CH 3 OH]
8-23
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Acidity of Alcohols
Compound
Formula
pKa
hydrogen chloride
HCl
-7
acetic acid
CH 3 CO2 H
methanol
CH 3 OH
15.5
water
H2 O
15.7
ethanol
CH 3 CH2 OH
15.9
2-propanol
(CH 3 ) 2 CHOH
17
2-methyl-2-propanol
(CH 3 ) 3 COH
18
4.8
Stronger
acid
Weaker
acid
8-24
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Basicity of Alcohols
• In the presence of strong acids, the oxygen atom
of an alcohol behaves as a weak base
• proton transfer from the strong acid forms an oxonium
ion
CH 3
••
O
••
H
An acid
+
O H +
••
CH 3
H
An oxonium ion
••
••
A base
H + H
+
H2 SO 4
O H
••
O
H
H
8-25
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Reaction with Metals
• Alcohols react with Li, Na, K, and other active
metals to liberate hydrogen gas and form metal
alkoxides
2 CH 3 OH +
2 Na
Methanol
2 CH 3 O - Na +
+
H2
Sodium
methoxide
8-26
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Conversion of ROH to RX
• Conversion of an alcohol to an alkyl halide
involves substitution of halogen for -OH at a
saturated carbon
• the most common reagents for this purpose are the
halogen acids, HX, and thionyl chloride, SOCl2
8-27
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Reaction with HX
• Water-soluble 3° alcohols react very rapidly with
HCl, HBr, and HI. Low-molecular-weight 1° and 2°
alcohols are unreactive under these conditions
CH 3
CH 3 COH +
CH 3
HCl
2-Methyl-2propanol
25°C
CH 3
CH 3 CCl +
H2 O
CH 3
2-Chloro-2methylpropane
8-28
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Reaction with HX
• Water-insoluble 3° alcohols react by bubbling
gaseous HX through a solution of the alcohol
dissolved in diethyl ether or THF
OH
+
HCl
CH 3
1-Methylcyclohexanol
0 oC
ether
Cl
+
CH 3
1-Chloro-1-methylcyclohexane
H2 O
8-29
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Reaction with HX
• 1° and 2° alcohols require concentrated HBr and
HI to form alkyl bromides and iodides
H2 O
CH3 CH 2 CH 2 CH 2 OH + HBr
reflux
1-Butanol
CH3 CH 2 CH 2 CH 2 Br + H 2 O
1-Bromobutane
8-30
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Mechanism: 3° ROH + HCl
• An SN1 reaction
• Step 1: rapid, reversible acid-base reaction that
transfers a proton to the OH group
CH 3
CH 3 - C
••
O- H
••
+ H
H
CH 3
H
••
••
CH 3 H
+
+
CH 3 - C O
••
CH 3
2-Methyl-2-propanol
(tert-Butyl alcohol)
+
O H
••
rapid and
reversible
O H
H
An oxonium ion
8-31
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Mechanism: 3° ROH + HCl
• Step 2: loss of H2O to give a carbocation intermediate
CH 3
rate-limiting
step
H
An oxonium ion
CH 3
CH 3 -C
+
CH 3
+
••
••
••
CH 3 H
+
CH 3 -C O
O H
H
A 3° carbocation
intermediate
8-32
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Mechanism: 3° ROH + HCl
• Step 3: reaction of the carbocation intermediate (a
Lewis acid) with halide ion (a Lewis base)
CH 3
CH 3
Cl
••
CH 3
rapid
••
CH 3 - C- Cl
••
••
••
+
••
CH 3 - C +
••
CH 3
2-Chloro-2-methylpropane
(tert-Butyl chloride)
8-33
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Mechanism: 1°ROH + HBr
• An SN2 reaction
• Step 1: rapid and reversible proton transfer
+
CH 3 CH 2 CH2 CH 2 - O-H + H O H
••
H
••
••
rapid and
reversible
An oxonium ion
H
+
••
••
••
+
CH 3 CH 2 CH2 CH 2 -O
H
O H
H
8-34
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Mechanism: 1° ROH + HX
• Step 2: displacement of HOH by halide ion
Br
••
+
+ CH3 CH2 CH2 CH2 -O
••
••
••
••
H
H
rate-limiting
step
S N2
H
+
••
••
••
CH3 CH2 CH2 CH2 -Br
••
••
O
H
8-35
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Mechanisms: SN1 vs SN2
• Reactivities of alcohols for SN1 and SN2 are in
opposite directions
SN 1
Increasing stability of cation intermediate
3° alcohol
2° alcohol
1° alcohol
Increasing ease of access to the reaction site
SN 2
8-36
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Reaction with SOCl2
• Thionyl chloride is the most widely used reagent
for the conversion of 1° and 2° alcohols to alkyl
chlorides
• a base, most commonly pyridine or triethylamine, is
added to neutralize the HCl
CH 3 (CH 2 ) 5 CH 2 OH + SOCl 2
1-Heptanol
Thionyl
chloride
pyridine
CH 3 (CH 2 ) 5 CH 2 Cl + SO 2 + HCl
1-Chloroheptane
8-37
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Dehydration of ROH
• An alcohol can be converted to an alkene by
elimination of H and OH from adjacent carbons (a
b-elimination)
• 1° alcohols must be heated at high temperature in the
presence of an acid catalyst, such as H2SO4 or H3PO4
• 2° alcohols undergo dehydration at somewhat lower
temperatures
• 3° alcohols often require temperatures only at or
slightly above room temperature
8-38
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Dehydration of ROH
CH 3 CH 2 OH
OH
H2 SO 4
o
180 C
CH 2 =CH 2 + H 2 O
H2 SO 4
+ H 2O
140oC
Cyclohexanol
CH3
CH 3 COH
CH3
Cyclohexene
H2 SO 4
o
50 C
CH 3
CH 3 C= CH 2
+
H2 O
2-Methylpropene
(Isobutylene)
8-39
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Dehydration of ROH
• Where isomeric alkenes are possible, the alkene
having the greater number of substituents on the
double bond generally predominates (Zaitsev
rule)
OH
CH 3 CH 2 CHCH3
2-Butanol
85% H 3PO4
heat
CH 3 CH= CH CH 3 + CH 3 CH 2 CH= CH2
2-Butene
1-Butene
(80%)
(20%)
8-40
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Dehydration of ROH
• A three-step mechanism
• Step 1: proton transfer from H3O+ to the -OH group to
form an oxonium ion
••
••
HO
CH 3 CHCH 2 CH 3 + H
+
O H
••
rapid and
reversible
H
H
+ H
O
••
An oxonium ion
••
CH 3 CHCH 2 CH3 +
••
O H
H
8-41
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Dehydration of ROH
• Step 2: the C-O bond is broken and water is lost,
giving a carbocation intermediate
H
+ H
O
••
CH 3 CHCH 2 CH 3
slow and
rate limiting
••
••
+
CH3 CHCH 2 CH3
+ H2 O
A 2o carbocation
8-42
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Dehydration of ROH
• Step 3: proton transfer of H+ from a carbon adjacent to
the positively charged carbon to water. The sigma
electrons of the C-H bond become the pi electrons of
the carbon-carbon double bond
H
••
••
+
CH 3 - CH - CH - CH 3 +
O
H
rapid
H
CH 3 - CH = CH - CH 3
+
+ H O H
••
H
8-43
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Dehydration of ROH
• Acid-catalyzed alcohol dehydration and alkene
hydration are competing processes
C
C
+
H2 O
An alkene
acid
catalyst
C
C
H OH
An alcohol
• large amounts of water favor alcohol formation
• scarcity of water or experimental conditions where
water is removed favor alkene formation
8-44
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Oxidation: 1° ROH
• A primary alcohol can be oxidized to an aldehyde
or a carboxylic acid, depending on the oxidizing
agent and experimental conditions
OH
CH 3 -CH 2
A primary
alcohol
[O]
O
CH 3 -C- H
An aldehyde
[O]
O
CH 3 -C- OH
A carboxylic
acid
8-45
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Oxidation: chromic acid
• Chromic acid is prepared by dissolving
chromium(VI) oxide or potassium dichromate in
aqueous sulfuric acid
CrO 3 +
Chromium(VI)
oxide
K2 Cr 2 O7
Potassium
dichromate
H2 O
H2 SO 4
H2 SO 4
H2 Cr 2 O7
H2 CrO 4
Chromic acid
H2 O
2 H2 CrO 4
Chromic acid
8-46
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Oxidation: 1° ROH
• Oxidation of 1-octanol by chromic acid gives
octanoic acid
• the aldehyde intermediate is not isolated
CrO3
CH 3 ( CH 2 ) 6 CH 2 OH
H2 SO4 , H 2 O
1-Octanol
O
O
CH 3 ( CH 2 ) 6 CH
CH3 ( CH2 ) 6 COH
Octanal
(not isolated)
Octanoic acid
8-47
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 PCC
• Pyridinium chlorochromate (PCC): a form of
Cr(VI) prepared by dissolving CrO3 in aqueous
HCl and adding pyridine to precipitate PCC
N+
H
CrO 3 Cl -
• PCC is selective for the oxidation of 1° alcohols to
aldehydes; it does not oxidize aldehydes further to
carboxylic acids
8-48
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Oxidation: 1° ROH
• PCC oxidation of a 1° alcohol to an aldehyde
O
CH 2 OH
Geraniol
PCC
CH
Geranial
8-49
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Oxidation: 2° ROH
• 2° alcohols are oxidized to ketones by both
chromic acid and and PCC
CH(CH 3 ) 2
OH
CH(CH 3 ) 2
H2 CrO 4
acetone
CH 3
2-Isopropyl-5-methylcyclohexanol
(Menthol)
O
CH 3
2-Isopropyl-5-methylcyclohexanone
(Menthone)
8-50
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Reactions of ethers
• Ethers, R-O-R, resemble hydrocarbons in their
resistance to chemical reaction
• they do not react with strong oxidizing agents such as
chromic acid, H2CrO4
• they are not affected by most acids and bases at
moderate temperatures
• Because of their good solvent properties and
general inertness to chemical reaction, ethers are
excellent solvents in which to carry out organic
reactions
8-51
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Epoxides
• Epoxide: a cyclic ether in which oxygen is one
atom of a three-membered ring
• Common names are derived from the name of the
alkene from which the epoxide is formally
derived
CH 2
CH 2
O
Ethylene oxide
CH 2
CHCH 3
O
Propylene oxide
8-52
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Synthesis of Epoxides-1
• Ethylene oxide, one of the few epoxides
manufactured on an industrial scale, is prepared
by air oxidation of ethylene
2 CH 2 =CH 2
+ O 2
Ag
2 CH 2
CH 2
O
Oxirane
(Ethylene oxide)
8-53
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Synthesis of Epoxides-2
• The most common laboratory method for the
synthesis of epoxides is oxidation of an alkene
using a peroxycarboxylic acid (a peracid) such
as peroxyacetic acid
O
CH3 COOH
Peroxyacetic acid
(Peracetic acid)
8-54
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Synthesis of Epoxides-2
• Epoxidation of cyclohexene
O
+
Cyclohexene
RCOOH
A peroxycarboxylic
acid
CH 2 Cl 2
H
O
O
+
RCOH
H
1,2-Epoxycyclohexane A carboxylic
(Cyclohexene oxide)
acid
8-55
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Hydrolysis of Epoxides
• In the presence of an acid catalyst, an epoxide is
hydrolyzed to a glycol
CH2
CH2
+
H2 O
O
Ethylene oxide
H
+
HOCH2 CH2 OH
1,2-Ethanediol
(Ethylene glycol)
8-56
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Hydrolysis of Epoxides
• Step 1: proton transfer to the epoxide to form a
bridged oxonium ion intermediate
H2 C
CH2
O
H2 C
+
H O H
CH 2
O+
H
H
8-57
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Hydrolysis of Epoxides
• Step 2: attack of H2O from the side opposite the
oxonium ion bridge
H
H2 C
O
CH 2
O+
H
H
H
H
+O
H2 C
CH 2
O
H
8-58
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Hydrolysis of Epoxides
• Step 3: proton transfer to solvent to complete the
hydrolysis
H
O
H
H
H
+O
H2 C
CH 2
H
H2 C
O
CH 2 +
O
O
H
H
+
H O H
H
8-59
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Hydrolysis of Epoxides
• Attack of the nucleophile on the protonated
epoxide shows anti stereoselectivity
• hydrolysis of an epoxycycloalkane gives a trans-diol
H
O
OH
+
H
1,2-Epoxycyclopentane
(Cyclopentene oxide)
H2 O
H+
OH
trans -1,2-Cyclopentanediol
8-60
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Hydrolysis of Epoxides
• Compare the stereochemistry of the glycols
formed by these two methods
H
+
RCO3 H
OH
H
O
H2 O
OH
H
trans-1,2-Cyclopentanediol
OH
Os O4
ROOH
OH
cis-1,2-Cyclopentanediol
8-61
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Thiols
• Thiols are stronger acids than alcohols
CH 3 CH 2 SH + H2 O
pKa = 8.5
pKa = 15.9
CH 3 CH 2 OH + H 2 O
+
CH 3 CH 2 S + H3 O
-
CH 3 CH 2 O + H 3 O
+
8-62
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Thiols
• When dissolved an aqueous NaOH, they are
converted completely to alkylsulfide salts
CH 3 CH 2 SH + Na + OH pKa = 8.5
Stronger
Stronger
+
acid
base
CH 3 CH 2 S Na + H2 O
pKa = 15.7
Weaker base
Weaker
acid
8-63
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8 Thiols
• Thiols are oxidized to disulfides by a variety of
oxidizing agents, including O2.
• they are so susceptible to this oxidation that they must
be protected from air during storage
2 RSH +
A thiol
1
2
O2
RSSR +
A disulfide
H2 O
• The most common reaction of thiols in biological
systems in interconversion between thiols and
disulfides, -S-S8-64
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
8
Alcohols,
Ethers, and
Thiols
End of Chapter 8
8-65
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