Transcript Slide 1

Chapter 14: Alcohols, Ethers, and Thiols
-OH
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.
Chapter 14: Alcohols, Ethers, and Thiols
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)
Chapter 14: Alcohols, Ethers, and Thiols
Problem: Write the IUPAC name for each alcohol.
OH
(a)
OH
(b)
OH
(c)
(d)
OH
Chapter 14: Alcohols, Ethers, and Thiols
• 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)
Chapter 14: Alcohols, Ethers, and Thiols
Alcohol Properties:
• Alcohol molecules are polar
• Alcohol molecules can create hydrogen bonds
• Alcohols usually have a neutral pH, but phenols are weak acids
Properties: Polarity and H-bonds
Alcohols are polar molecules.
• The C-O and O-H bonds are both polar covalent.
Chapter 14: Alcohols, Ethers, and Thiols
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
Properties: Acidity
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.
Chapter 14: Alcohols, Ethers, and Thiols
Alcohol Reactions:
• Dehydration: Alcohol (-OH)  Alkene (C=C)
• Oxidation:
Primary Alcohol  Aldehide + Carboxylic acid
2. Secondary Alcohol  Ketone
3. Tertiary Alcohol  … Not possible
1.
Chapter 14: Alcohols, Ethers, and Thiols
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.
Chapter 14: Alcohols, Ethers, and Thiols
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)
Chapter 14: Alcohols, Ethers, and Thiols
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)
Chapter 14: Alcohols, Ethers, and Thiols
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.
Chapter 14: Alcohols, Ethers, and Thiols
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.
Chapter 14: Alcohols, Ethers, and Thiols
• 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)
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.
-OCH3 -O-CH3
D imethyl ether
CH3 CH2 -O-CH2 CH3
Dieth yl eth er
Chapter 14: Alcohols, Ethers, and Thiols
Although ethers can be named according to the IUPAC system,
chemists almost invariably use common names for low-molecularweight 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)
Chapter 14: Alcohols, Ethers, and Thiols
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)
Ethers: 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
Ethers: Reactions
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.
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-Bu ten e-1-thiol
3-Methyl-1-bu tanethiol
-SH
Chapter 14: Alcohols, Ethers, and Thiols
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)
Thiols 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
Thiols Properties
Thiols are weak acids (pKa 10) and are comparable in strength to
phenols (pKa 10).
• Thiols react with strong bases such as NaOH to form watersoluble thiolate salts.
CH3 CH2 SH + NaOH
Ethan ethiol
(pKa 10)
H2 O
-
+
CH3 CH2 S Na + H2 O
S od ium
ethaneth iolate
Thiols reactions: oxidation
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