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14
General, Organic, and
Biochemistry, 7e
Bettelheim,
Brown, and March
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14-1
14 Chapter 14
Alcohols, Ethers,
and Thiols
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14-2
14 Alcohols
• Alcohol: a compound that contains an -OH
(hydroxyl) group bonded to a tetrahedral carbon
• methanol, CH3OH, is the simplest alcohol
• Nomenclature
1.select the longest carbon chain that contains the -OH
group as the parent alkane and number it from the end
that gives the -OH the lower number
2.change the ending of the parent alkane from -e to -ol
and use a number to show the location of the -OH
group; for cyclic alcohols, the carbon bearing the -OH
group is carbon-1
3.name and number substituents and list them in
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alphabetical
order
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14-3
14 Nomenclature
OH
OH
Ethan ol
(Ethyl alcohol)
OH
1-Propan ol
(Prop yl alcohol)
2-Prop anol
(Is op ropyl alcohol)
OH
OH
1-Butan ol
(Bu tyl alcohol)
OH
2-Bu tanol
(sec-Bu tyl alcohol)
OH
2-Methyl-2-p ropan ol
(t ert -Butyl alcoh ol)
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2-Meth yl-1-propanol
(Is ob utyl alcoh ol)
OH
Cycloh exanol
(Cycloh exyl alcoh ol)
14-4
14 Nomenclature
• Problem: write the I UPAC name for each alcohol
OH
(a)
OH
(b)
OH
(c)
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(d)
OH
14-5
14 Nomenclature
• Solution:
(a)
5
4
OH
1
OH
(b )
2
2
4-Methyl-2-pentan ol
t rans-2-Meth ylcycloh exanol
OH
(c)
2
2-Heptanol
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7
(d)
3
2
1
OH
2,2-D imeth yl-1-p ropanol
14-6
14 Nomenclature
• in the IUPAC system, a compound containing two
hydroxyl groups is named as a diol, one containing
three hydroxyl groups as a triol, and so on
• IUPAC names for diols, triols, and so on retain the final
"-e" in the name of the parent alkane
• we commonly refer to compounds containing two
hydroxyl groups on adjacent carbons as glycols
CH2 CH2
OH OH
CH3 CHCH2
HO OH
1,2-Eth anediol
(Eth ylen e glycol)
1,2-Propan ediol
(Propylene glycol)
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CH2 CHCH2
OH OHOH
1,2,3-Propan etriol
(Glycerol, Glycerin)
14-7
14 Physical Properties
• Alcohols are polar molecules
• the C-O and O-H bonds are both polar covalent
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14-8
14 Physical Properties
• in the liquid state, alcohols associate by hydrogen
bonding
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14-9
14 Physical Properties
• bp increases as MW increases
• solubility in water decreases as MW increases
Molecular
Weigh t
bp
(°C)
Solubility
in Water
Structu ral Formula
N ame
CH3 OH
CH3 CH3
methanol
ethan e
32
30
65
-89
infinite
ins olub le
CH3 CH2 OH
ethan ol
propane
46
infinite
44
78
-42
1-propanol
bu tane
60
58
97
0
1-bu tanol
pen tane
74
117
72
36
CH3 CH2 CH3
CH3 CH2 CH2 OH
CH3 CH2 CH2 CH3
CH3 CH2 CH2 CH2 OH
CH3 CH2 CH2 CH2 CH3
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ins olub le
infinite
ins olub le
8 g/100 g
ins olub le
14-10
14 Acidity of Alcohols
• Alcohols have about the same pKa values as
water; aqueous solutions of alcohols have the
same pH as that of pure water
• alcohols and phenols both contain an OH group
• phenols are weak acids and react with NaOH and other
strong bases to form water-soluble salts
OH + NaOH
Phenol
H2 O
O- Na+ + H2 O
S od ium phenoxide
(a w ater-soluble salt)
• alcohols are weaker acids and do not react in this
manner
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14-11
14 Dehydration
• Dehydration: elimination of a molecule of water
from adjacent carbon atoms gives an alkene
• dehydration is most often brought about by heating an
alcohol with either 85% H3PO4 or concentrated H2SO4
• 1° alcohols are the most difficult to dehydrate and
require temperatures as high as 180°C
• 2° alcohols undergo acid-catalyzed dehydration at
somewhat lower temperatures
• 3° alcohols generally undergo acid-catalyzed
dehydration at temperatures only slightly above room
temperature
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14-12
14 Dehydration
CH3 CH2 OH
Ethan ol
OH
Cycloh exanol
H2 SO4
180°C
H2 SO4
140°C
CH2 =CH2 + H2 O
Ethylene
+ H2 O
Cyclohexen e
CH3
CH3
H2 SO4
CH3 CCH3
CH3 C=CH2 + H2 O
50°C
OH
2-Meth yl-2-p ropanol
2-Methylpropene
(Isobutylene)
(t ert -Butyl alcoh ol)
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14-13
14 Dehydration
• when isomeric alkenes are obtained, the alkene having
the greater number of alkyl groups on the double bond
generally predominates
• examples:
OH
CH3 CH2 CHCH3
2-Butanol
H3 PO4
-H2 O
CH3 CH=CHCH3 + CH3 CH2 CH=CH2
2-Bu tene
1-Butene
(80%)
(20%)
CH3
CH3
CH3
H2 SO4
CH3 CHCHCH3
CH3 C=CHCH3 + CH3 CHCH=CH2
-H2 O
OH
3-Meth yl-2-b utanol
2-Methyl-2-b utene 3-Methyl-1-bu tene
(major prod uct)
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14-14
14 Dehydration-Hydration
• Acid-catalyzed hydration of alkenes to give
alcohols and acid-catalyzed dehydration of
alcohols to give alkenes are competing reactions
• the following equilibrium exists
C C
+ H2 O
dehydration
h yd ration
An alk ene
C C
H OH
An alcoh ol
• in accordance with Le Chatelier's principle, large
amounts of water favor alcohol formation, whereas
removal of water from the equilibrium mixture favors
alkene formation
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14-15
14 Oxidation
• Oxidation of a 1° alcohol gives an aldehyde or a
carboxylic acid, depending on the experimental
conditions
• oxidation of a 1° alcohol to a carboxylic acid is carried
out using potassium dichromate, K2Cr2O7, in aqueous
sulfuric acid
CH3 ( CH2 ) 6 CH2 OH
1-Octanol
K2 Cr2 O7
H2 SO4
O K Cr O
2
2 7
CH3 ( CH2 ) 6 CH
H2 SO4
Octanal
O
CH3 (CH2 ) 6 COH
Octanoic acid
• it is often possible to stop the oxidation at the aldehyde
stage by distilling the mixture; the aldehyde usually
has a lower boiling point than either the 1° alcohol or
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the
carboxylic
acid
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14-16
14 Oxidation
• oxidation of a 2° alcohol gives a ketone
K2 Cr2 O7
OH
H2 SO4
2-Is opropyl-5-methylcyclohexanol
(Menthol)
O
2-Isopropyl-5-methylcyclohexan on e
(Menth on e)
• tertiary alcohols are resistant to oxidation
CH3
OH
1-Methylcyclopentanol
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K2 Cr2 O7
H2 SO4
(no oxidation)
14-17
14 Ethers
• The functional group of an ether is an oxygen
atom bonded to two carbon atoms
• the simplest ether is dimethyl ether
• the most common ether is diethyl ether
CH3 -O-CH3
CH3 CH2 -O-CH2 CH3
D imethyl ether
Dieth yl eth er
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14-18
14 Nomenclature
• Although ethers can be named according to the
IUPAC system, chemists almost invariably use
common names for low-molecular-weight ethers
• common names are derived by listing the alkyl groups
bonded to oxygen in alphabetical order and adding the
word "ether”
• alternatively, name one of the groups on oxygen as an
alkoxy group
CH3 CH2 OCH2 CH3
D iethyl eth er
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OCH3
Cyclohexyl meth yl eth er
(Methoxycycloh exane)
14-19
14 Nomenclature
• Cyclic ether: an ether in which one of the atoms
in a ring is oxygen
• cyclic ethers are also known by their common names
• ethylene oxide is an important building block for the
organic chemical industry; it is also used as a fumigant
in foodstuffs and textiles, and in hospitals to sterilize
surgical instruments
• tetrahydrofuran is a useful laboratory and industrial
solvent
O
Eth ylene oxide
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O
Tetrahydrofu ran
(THF)
14-20
14 Physical Properties
• Ethers are polar compounds in which oxygen
bears a partial negative charge and each carbon
bonded to it bears a partial positive charge
• however, only weak forces of attraction exist between
ether molecules in the pure liquid
• consequently, boiling points of ethers are close to
those of hydrocarbons of similar molecular weight
• ethers have lower boiling points than alcohols of the
same molecular formula
CH3 CH2 OH
Ethanol
bp 78°C
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CH3 OCH3
D imethyl ether
b p -24°C
14-21
14 Reactions of Ethers
• Ethers resemble hydrocarbons in their resistance
to chemical reaction
• they do not react with oxidizing agents such as
potassium dichromate
• they do not react with reducing agents such as H2 in
the presence of a transition metal catalyst
• they are not affected by most acids or bases at
moderate temperatures
• Because of their general inertness and good
solvent properties, ethers such as diethyl ether
and THF are excellent solvents in which to carry
out organic reactions
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14-22
14 Thiols
• Thiol: a compound containing an -SH (sulfhydryl)
group
• the most outstanding property of low-molecular-weight
thiols is their stench
• they are responsible for smells such as those from
rotten eggs and sewage
• the scent of skunks is due primarily to these two thiols
CH3
CH3 CH=CHCH2 SH
CH3 CHCH2 CH2 SH
2-Butene-1-thiol
3-Methyl-1-butanethiol
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14-23
14 Thiols - Nomenclature
• IUPAC names are derived in the same manner as
are the names of alcohols
• to show that the compound is a thiol, the final -e of the
parent alkane is retained and the suffix -thiol added
• Common names for simple thiols are derived by
naming the alkyl group bonded to -SH and adding
the word "mercaptan"
CH3
CH3 CH2 SH
Ethaneth iol
(Ethyl mercap tan)
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CH3 CHCH2 SH
2-Methyl-1-propan ethiol
(Isobu tyl mercaptan)
14-24
14 Physical Properties
• Because of the small difference in
electronegativity between sulfur and hydrogen
(2.5 - 2.1 = 0.4), an S-H bond is nonpolar covalent
• thiols show little association by hydrogen bonding
• thiols have lower boiling points and are less soluble in
water and other polar solvents than alcohols of similar
molecular weight
Thiol
methanethiol
ethan ethiol
1-bu tanethiol
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bp (°C)
6
35
98
Alcoh ol
methanol
ethan ol
1-bu tanol
bp (°C)
65
78
117
14-25
14 Reactions of Thiols
• Thiols are weak acids (pKa 10), and are
comparable in strength to phenols
• thiols react with strong bases such as NaOH to form
water-soluble thiolate salts
CH3 CH2 SH + NaOH
Ethanethiol
(pKa 10)
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H2 O
CH3 CH2 S-Na+ + H2 O
Sodium
ethanethiolate
14-26
14 Reactions of Thiols
• The most common reaction of thiols in biological
systems is their oxidation to disulfides, the
functional group of which is a disulfide (-S-S-)
bond
• thiols are readily oxidized to disulfides by O2
• they are so susceptible to oxidation that they must be
protected from contact with air during storage
• disulfides, in turn, are easily reduced to thiols by
several reducing agents.
2HOCH2 CH2 SH
A thiol
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oxidation
reduction
HOCH2 CH2 S-SCH2 CH2 OH
A disulfide
14-27
14 Important Alcohols
Coal
or
meth ane
2 H2
CO
Carbon
monoxide
H2 O, H2 SO4
CH2 =CH2
Ethylen e
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O2
catalyst
CO
catalys t
CH3 OH
Meth anol
CH3 CH2 OH
Ethanol
CH3 COOH
Acetic acid
O2
CH2 O
oxidation Formaldehyde
H2 SO4
180°C
O
H2 O, H2 SO4
H2 C CH2
Ethylene
oxide
CH3 CH2 OCH2 CH3
Dieth yl eth er
HOCH2 CH2 OH
Ethylene glycol
14-28
14 Important Alcohols
H2 O, H2 SO4
CH3 CH=CH2
Propene
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several s teps
OH
CH3 CHCH3
2-Propan ol
OH
HOCH2 CHCH2 OH
Glycerin, glycerol
14-29
14 Alcohols, Ethers, and Thiols
End
Chapter 14
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14-30