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Chapter 13 Alcohols, Phenols, Ethers, and
Thiols
(Chapter 14, 1st Edition)
In this chapter, we are going to look at molecules that
contain single bonds to oxygen atoms or sulphur atoms.
Structure of Alcohols and Thiols.
In an alcohol, a hydroxyl group (-OH) replaces a
hydrogen atom in an alkane. In a phenol, a hydroxyl
group is attached to an aromatic ring. Thiols are similar
to alcohols except that thiols contain a sulfhydryl (-SH)
instead of a hydroxyl group.
O
H
H
O
O
H
H
methanol
water
phenol
S
S
H
H
methanethiol
thiophenol
Classification of Alcohols.
Recall how we classified the different carbon atoms in
alkanes. Carbon atoms bonded to only one other carbon
atom was designated as a 1° carbon atom. Carbon atoms
bonded to two other carbon atoms were designated as 2°
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and carbon atoms bonded to three or four other carbon
atoms were designated as 3° or 4° respectively. Alcohols
can be classified in a similar manner. Alcohols in which
the hydroxyl group is bonded to a 1° carbon is said to be a
1° alcohol. Similarly, alcohols bonded 2° and 3° carbon
atoms are said to be 2° alcohols and 3° alcohols
respectively. NOTE: The symbol R is used to represent
any alkyl group.
H
R
C
R'
OH
R
R'
C
OH
R
C
OH
H
H
R''
1° alcohol
2° alcohol
3° alcohol
e.g.
OH
OH
OH
OH
OH
OH
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Naming Alcohols.
Step 1.
Select the longest carbon chain containing the hydroxyl
group, and derive the parent name by replacing the –e of
the corresponding alkane with –ol.
Step 2.
Number the main chain beginning at the end nearer the
hydroxyl group. Molecules containing more than one
hydroxyl group are named as diols, triols, etc.
Step 3.
Name and number other substituents relative to the
hydroxyl groups.
Step 4.
Cyclic alcohols are named as cycloalkanols. The carbon
atom bearing the hydroxyl group is numbered as carbon
1. Number in the direction that gives the lowest series of
numbers.
e.g.
Name the following molecules, ignoring any chiral
centers.
OH
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CH3
Cl
OH
OH
Br
Br
OH
OH
Cl
Phenols are a family of molecules that have a hydroxyl
group bonded to a benzene ring.
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e.g.
OH
OH
OH
OH
F
2-fluorophenol
ortho-fluorophenol
phenol
3-ethylphenol
meta-ethylphenol
Br
4-bromo-3-methylphenol
Many disubstituted phenols have common names based
on historical uses. Some are listed below:
OH
OH
OH
CH3
OH
OH
OH
ortho-cresol
catechol
OH
resorcinol
hydroquinone
Naming Thiols.
Thiols are named by the same system used for alcohols,
with the suffix –thiol used in place of –ol. The most
obvious physical characteristic of thiols is their strong,
disagreeable, odour. For example, the scent of skunks is
caused primarily by the simple thiols, 3-methyl-1butanethiol and 2-butene-1-thiol.
CH3
SH
SH
3-methyl-1-butanethiol
2-butene-1-thiol
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e.g.
Name the following thiols:
SH
SH
F
SH
F
SH
Some Important Alcohols.
Methanol. Methanol, also known as ‘wood alcohol’, is
found in many solvents and paint removers. Methanol is
also used in the productions of plastics and medicines and
used as fuels. Methanol is highly toxic and, if ingested,
can cause blindness and possibly death.
O
CH3OH
methanol
[O]
H
H
formaldehyde
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Ethanol. The fermentation of grains, fruits, and starches
has been known for millennia. The intoxicating effect of
ethanol, for good or bad, is a part of our society.
Commercially, ethanol is used as a solvent for perfumes,
varnishes, and some medicines.
2-Propanol (isopropyl alcohol). Commonly referred to as
‘rubbing alcohol’, 2-propanol is used as an astringent and
to sterilize medical equipment because it destroys
bacteria.
1,2-Ethanediol (ethylene glycol). Often used as antifreeze
in heating and cooling systems. It is also used as a
solvent for paints, inks, and plastics. In the body, 1,2ethanediol is oxidized to oxalic acid, which can lead to
death.
O
OH
[O]
HO
OH
HO
O
1,2-ethanediol
oxalic acid
1,2,3-Propanetriol (glycerol or glycerin). The presence
of three hydroxyl groups attracts 1,2,3-propanetriol
strongly to water. For this reason, it is used as a skin
softener in skin lotions, cosmetics and shaving creams. It
is also used as antifreeze. When mixed with a strong
oxidizer, extremely explosive products are formed.
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Menthol. Menthol has a pleasant peppermint taste and
odour and is used in candy, throat lozenges, and nasal
inhalers.
OH
menthol
Ethers.
Ethers are molecules that contain an oxygen atom
attached to two carbon groups (alkyl or aryl) by single
bonds.
O
H
O
H
water
O
H
methanol
dimethyl ether
Naming Ethers.
Simple ethers are named by their common names. The
IUPAC names are used only when the ether is more
complex.
Step 1.
When naming ethers by their common names, write the
name of each alkyl or aryl group attached to the oxygen
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atom in alphabetical order followed by the word ether.
Step 2.
For more complex ethers, the ether is named as an alkoxy
(alkyl + oxygen) substituent similar to alkyl groups.
Step 3.
Number the main chain beginning at the end nearest the
alkoxy group and give the location of the alkoxy group on
the main chain.
e.g.
O
O
diethyl ether
(ethoxyethane)
3-ethoxypentane
O
O
anisole
(methoxybenzene)
diphenyl ether
(phenoxybenzene)
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Name the following molecules.
O
O
O
OH
OH
O
F
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Cyclic Ethers.
Carbon rings that contain one oxygen atom are known as
cyclic ethers. Such compounds are generally known as
heterocyclic compounds because one or more of the
atoms in the ring are not carbon. Most heterocycles are
given common names. Some cyclic ethers are listed
below.
4
CH3
3
4
3
O
5
2
5
O
O
O
1
ethylene oxide
2
1
furan
tetrahydrofuran (THF)
3-methylfuran
CH3
4
3
O
O
pyran
tetrahydropyran
2
O
1
4-methylpyran
Six membered rings containing two oxygen atoms are
known as dioxanes. The oxygen atoms are numbered
according to their positions.
1
O
1
2
3
O
O
2
O3
4
1,4-dioxane
1,3-dioxane
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Physical Properties of Alcohols, Phenols, and Ethers.
Consider the molecules and their physical properties
listed in Table 14.1. Observe how all the alcohols have
much higher boiling points than any alkanes or ethers of
comparable molar mass. Why is this? Alcohols, which
contain the polar hydroxyl group, can form hydrogen
bonds. Hydrogen bonds are very strong dipole-dipole
interactions between hydrogen atoms bonded to small,
strongly electronegative atoms (O, N, or F) and
nonbonding pairs on other such electronegative atoms.
Although hydrogen bonds are considered strong dipoledipole interactions, they are much weaker than ordinary
covalent bonds.
hydrogen bonds
O
H
H
O
H
O
Alcohols have higher boiling points because higher
temperatures are required to provide the energy to break
the hydrogen bonds. Although ethers contain oxygen
atoms, there is no hydrogen atom attached. Therefore,
ethers cannot hydrogen bond with each other, but can
hydrogen bond with water or alcohols. The ability to
hydrogen bond with water is what makes alcohols and
ethers with one to four carbon atoms very soluble in
water.
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O
H
O
H
H
H
O
H
CH3
O
H
H
O
O
H
H
H
H
O
CH3
CH3
When there are more than four carbon atoms in the alkyl
portion of an alcohol, the effect of the hydroxyl group is
diminished. The hydrogen atoms of the alkyl group do
not participate in hydrogen bonds and therefore these
longer chain alcohols are not very soluble in water.
nonpolar carbon chain
OH
OH
soluble in water
not soluble in water
Ethers are more soluble in water than alkanes of
comparable mass but are less soluble than alcohols.
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Reactions of Alcohols.
Just as the alkanes underwent combustion reactions, so
too can the alcohols. Alcohols react with oxygen to
produce carbon dioxide, water, and heat.
2C3H8O
9O2
+
6CO2
8H2O +
+
heat
We saw in Chapter 13 that alkenes undergo addition
reactions with water and a catalyst to form alcohols.
Alcohols can undergo the reverse reaction, called an
elimination reaction, to form alkenes. Because an
alcohol loses a molecule of water to form an alkene, this
type of elimination is termed a dehydration. During the
dehydration of an alcohol, a hydrogen atom and a
hydroxyl group are removed from adjacent carbon atoms
of the same alcohol. A double bond forms between the
same two carbon atoms to produce the alkene product.
This reaction is also catalyzed with an acid.
e.g.
H
H3C
OH
H
CH3
CH
CH2
H
C
+
C
H2O
heat
H
H
H
OH
H
heat
+
H2O
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The dehydration of a 2° or 3° alcohol can give two
different products. Consider the dehydration of 2-butanol
below:
b
a
H
H
H
+
H2O
+
H2O
A
heat
OH
B
Saytzeff’s rule states that the major product will be the
alkene that is more substituted. In the example above,
product A is a monosubstituted alkene while product B is
a disubstituted alkene. Thus, product B would be the
major product.
e.g.
Provide the product(s) for the following reactions. Where
possible, indicate the major product.
H
heat
OH
OH
H
heat
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Alcohols can also dehydrate to form ethers. In this
reaction, a hydrogen atom and a hydroxyl group are
removed from different alcohol molecules. This reaction
also requires an acid catalyst but is done at a lower
temperature then dehydration to the alkene.
H
CH3
O
H
+
HO
CH3
CH3
heat
O
H2O
+
CH3
H2SO4
CH2
180° C
CH2
CH3CH2OH
H2SO4
CH3CH2OCH2CH3
140° C
Oxidation of Alcohols.
Recall from Chapter 6 that oxidation can be defined as a
loss of hydrogen atoms or a gain of oxygen atoms.
Conversely, reduction can be defined as a gain of
hydrogen atoms or a loss of oxygen atoms. In organic
chemistry we can list different functional groups at
different levels of oxidation or reduction. Shown below
are the different stages of oxidation for ethane.
OH
oxidation
CH3
CH3
alkane
O
oxidation
CH3
reduction
O
CH2
alcohol
oxidation
CH3
reduction
C
aldehyde
H
CH3
reduction
C
OH
carboxylic acid
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Oxidation of 1° Alcohols.
Primary alcohols are oxidized to produce aldehydes and
water. Oxidation occurs by removing two atoms of
hydrogen, one from the hydroxyl group and a one from
the carbon atom that is bonded to the hydroxyl group.
The oxidation requires an oxidizing agent such as O2,
KMnO4, H2CrO4, or K2Cr2O7. The oxidizing agents are
often written with the symbol [O].
H
O
O
[O]
R
C
R
H
C
H
+
H2O
aldehyde
H
1° alcohol
As shown above, aldehydes can oxidize further to give
carboxylic acids. This requires the addition of oxygen.
This step occurs readily making it difficult to isolate the
aldehyde product.
O
O
[O]
R
C
aldehyde
H
R
C
OH
carboxylic acid
The toxicity of methanol has already been discussed. We
can now look more closely as to why methanol is so toxic
to humans. In the body, enzymes oxidize methanol to
formaldehyde and then to formic acid. Blindness occurs
because these products destroy the retina of the eye. As
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little as 30mL of formic acid can lower blood pH severely
enough and lead to coma or death.
O
O
[O]
CH3
[O]
OH
H
C
H
H
OH
formic acid
formaldehyde
methanol
C
Oxidation of 2° Alcohols.
The oxidation of secondary alcohols is similar to that of
primary alcohols except the product is a ketone and not an
aldehyde. Oxidation occurs by removing two atoms of
hydrogen, one from the hydroxyl group and a one from
the carbon atom that is bonded to the hydroxyl group.
H
O
O
[O]
R
C
R
R
C
R
+
H2 O
ketone
H
2° alcohol
Consider the tertiary alcohol below. Provide a reason
why tertiary alcohols resist oxidation.
H
O
[O]
R
C
R
R
3° alcohol
NO REACTION
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Oxidation of Thiols.
Oxidation of thiols occurs by removing a hydrogen atom
from two different thiol molecules. The oxidized product
is called a disulfide and is characterized by a sulfur-sulfur
single bond.
[O]
R
S
H
+
H
S
R
R
S
S
R
Provide the product(s) for the following reactions.
OH
[O]
[O]
OH
H
heat
OH
OH
[O]
+
H2O
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OH
H3C
H
heat
CH3
OH
H
heat
H
OH
+
OH
H
CH3OH
+
CH3CH2OH
[O]
2 CH3CH2SH
heat
heat
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Important Concepts from Chapter 13
• Classification of Alcohols - 1°, 2°, and 3°
• Nomenclature of Alcohols
• Nomenclature of Thiols
• Nomenclature of Ethers
• Physical Properties of Alcohols and Ethers
• Reactions of Alcohols:
Dehydration (alkene product)
Dehydration (ether product)
Oxidation
• Reaction of Thiols:
Oxidation (disulfides)