Transcript Camp 1 - TypePad

Frederick A. Bettelheim
William H. Brown
Mary K. Campbell
Shawn O. Farrell
www.cengage.com/chemistry/bettelheim
Chapter 14
Alcohols, Ethers, and Thiols
William H. Brown • Beloit College
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
alphabetical order.
14-2
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)
2-Meth yl-1-propanol
(Is ob utyl alcoh ol)
OH
Cycloh exanol
(Cycloh exyl alcoh ol)
14-3
Nomenclature
Problem: Write the IUPAC name for each alcohol.
OH
(a)
OH
(b)
OH
(c)
(d)
OH
14-4
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
7
(d)
3
2
1
OH
2,2-D imeth yl-1-p ropanol
14-5
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 forth.
• 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-Ethanediol
(Ethylene glycol)
1,2-Propanediol
(Propylene glycol)
CH2 CHCH2
OH OHOH
1,2,3-Propanetriol
(Glycerol, Glycerin)
14-6
Physical Properties
Alcohols are polar molecules.
• The C-O and O-H bonds are both polar covalent.
14-7
Physical Properties
In the liquid state,
alcohols associate
by hydrogen
bonding.
14-8
Physical Properties
Alcohols have higher boiling points than hydrocarbons of
comparable molecular weight.
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
46
44
78
-42
infinite
CH3 CH2 CH3
ethan ol
propane
CH3 CH2 CH2 OH
1-propanol
bu tane
97
0
infinite
CH3 CH2 CH2 CH3
60
58
CH3 CH2 CH2 CH2 OH
1-bu tanol
74
117
8 g/100 g
CH3 CH2 CH2 CH2 CH3
pen tane
72
36
ins olub le
ins olub le
ins olub le
14-9
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 than phenols and do not
react in this manner.
14-10
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.
14-11
Dehydration
CH3 CH2 OH
Ethanol
OH
Cycloh exanol
H2 SO4
180°C
H2 SO4
140°C
CH2 =CH2 + H2 O
Ethylene
+ H2 O
Cyclohexene
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)
14-12
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-Butene
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)
14-13
Dehydration-Hydration
Acid-catalyzed hydration of alkenes to give alcohols
(Chapter 12) and acid-catalyzed dehydration of alcohols to
give alkenes are competing reactions.
• The following acid-catalyzed equilibrium exists.
C C
An alk ene
+ H2 O
hydration
dehydration
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.
14-14
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
commonly carried out using potassium dichromate,
K2Cr2O7, in aqueous sulfuric acid.
CH3 ( CH2 ) 6 CH2 OH
1-O ctanol
K2 Cr2 O 7
H2 SO 4
O
O K Cr O
2
2 7
CH3 ( CH2 ) 6 CH
CH3 ( CH2 ) 6 COH
H2 SO 4
O ctanoi c aci d
O ctanal
• It is sometimes 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 the carboxylic acid.
14-15
Oxidation
• Oxidation of a 2° alcohol gives a ketone.
K2 Cr 2 O7
OH
H2 SO 4
O
2-Is opropyl -5-me th yl - 2-Is opropyl -5-me th yl cycl oh e xan ol
cycl oh e xan on e
(Me n th ol)
(Me n th on e )
• Tertiary alcohols are resistant to oxidation.
CH3
OH
1-Methylcyclopentanol
K2 Cr2 O7
H2 SO4
(n o oxidation)
14-16
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
D imethyl ether
CH3 CH2 -O-CH2 CH3
Dieth yl eth er
14-17
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
Diethyl ether
OCH3
Cyclohexyl methyl ether
(Methoxycyclohexane)
14-18
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
O
Tetrahydrofu ran
(THF)
14-19
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 OCH3
CH3 CH2 OH
Ethanol
Dimethyl ether
bp 78°C
b p -24°C
14-20
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.
14-21
Thiols
Thiol: A compound containing an -SH (sulfhydryl) group.
• The most outstanding property of low-molecularweight 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-Bu ten e-1-thiol
3-Methyl-1-bu tanethiol
14-22
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 CH2 SH
Ethaneth iol
(Ethyl mercap tan)
CH3
CH3 CHCH2 SH
2-Methyl-1-propan ethiol
(Isobu tyl mercaptan)
14-23
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
bp (°C)
6
Alcoh ol
methanol
35
98
ethan ol
1-bu tanol
78
117
bp (°C)
65
14-24
Acidity of Thiols
Thiols are weak acids 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
Ethan ethiol
(pKa 10)
H2 O
-
+
CH3 CH2 S Na + H2 O
S od ium
ethaneth iolate
14-25
Oxidation 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 including H2 in the presence of
a transition metal catalyst.
2HOCH2 CH2 SH
A thiol
oxidation
reduction
HOCH2 CH2 S-SCH2 CH2 OH
A disulfide
14-26
Important Alcohols
Coal
[O]
2 H2
or
CO
meth ane
Carbon
monoxide
H2 O, H2 SO4
CH2 =CH2
Ethylene
O2
catalys t
CO
catalys t
CH3 OH
Meth anol
CH3 COOH
Acetic acid
O2
CH2 O
oxidation Formaldehyde
H2 SO4
CH3 CH2 OH
180°C
Eth anol
O
H2 O, H2 SO4
H2 C CH2
Eth ylen e
oxide
CH3 CH2 OCH2 CH3 + H2 O
D iethyl ether
HOCH2 CH2 OH
Ethylen e glycol
14-27
Important Alcohols
O
several
step s
H2 O, H2 SO4
CH3 CH=CH2
Propene
several s teps
ClCH2 CH
CH2
Ep ichlorohydrin
several steps
other reagents
Ep oxy resins
and glu es
OH
CH3 CHCH3
2-Propan ol
OH
HOCH2 CHCH2 OH
Glycerin, glycerol
14-28
Chapter 14 Alcohols, Ethers, and Thiols
End
Chapter 14
14-29