Chemistry 20
Chapters 5 & 6
Alcohols, Ethers, Thiols and Chirality
Alcohols: the functional group of an alcohol is an hydroxyl group “-OH”.
CH3OH
CH3CH2CH2OH
Naming alcohols: 1. Select the longest chain that contains the –OH group as the parent
alkane. 2. Number chain from the end that gives –OH the lower number. 3. Change the ending
of the parent alkane from “-e” to “-ol” and use a number to show the location of the –OH
group. 4. Give the location and number of each substituent and list them in alphabetical order
as a prefix to the name of the main chain.
CH3
CH3-CH2-CH-CH2-OH
4
3
2 1
2-methyl-1-butanol
Common name: name the alkyl group bonded to –OH and then add the word “alcohol”.
CH3OH
IUPAC name: Methanol (Common name: Methyl alcohol)
CH3-CH2-OH
IUPAC name: Ethanol (Common name: Ethyl alcohol)
Diol and Triol: a compound containing two hydroxyl groups is named as a diol. A compound
containing three hydroxyl groups is named as a triol, and so on.
Naming Diols and Triols: the final –e in the name of the parent alkane is retained and we
add “-diol” or “-triol”.
Note: as a common name, a compound containing two hydroxyl groups on adjacent carbons
is often referred to as “glycol”. We add the word “glycol” at the end of the name of the
alkene.
CH2-CH2
OH OH
1,2-Ethanediol
(Ethylene glycol)
CH3-CH-CH2
CH2-CH-CH2
OH OH
OH OH OH
1,2-Propanediol
(Propylene glycol)
1,2,3-Propanetriol
(Glycerol, Glycerin)
Phenol: a compound that contains an hydroxyl group (-OH) bonded to a benzene ring.
Phenols are weak acids, with pKa values of approximately 10. Most phenols are insoluble in
water, but they react with strong bases, such as NaOH and KOH, to form water-soluble salts.
OH + NaOH
Phenol
Dr. Behrang Madani
H2 O
-
O Na
+
+ H2 O
S od ium phenoxide
(a w ater-soluble salt)
Chemistry 20
Mt SAC
Classification of alcohols: alcohols are classified as primary (1), secondary (2), or tertiary
(3), depending on number of carbon atoms bonded to the carbon bearing the –OH group.
CH3 -C-OH
H
CH3 -C-OH
CH3
CH3 -C-OH
H
A 1° alcohol
CH3
A 2° alcohol
CH3
A 3° alcoh ol
H
Physical properties of alcohols: 1. Because of the large difference in electronegativity
between oxygen and carbon (3.5 - 2.5 = 1.0) and between oxygen and hydrogen (3.5 - 2.1 =
1.4), alcohols are polar molecules. 2. There is hydrogen bonding between the alcohol
molecules (because of –OH). 3. Alcohols have higher boiling points than do alkans, alkenes,
and alkynes of similar molecular weight (hydrogen bonding and dipole-dipole interactions are
stronger than London dispersion forces). 4. The larger alcohol molecules (higher molecular
weight) have higher boiling points. 5. Alcohols are much more soluble in water than are
alkanes, alkenes, and alkynes (solubility decreases when molecular weight increases). They
are weak acids (weaker than phenol).
Chemical properties of alcohols:
1. Acidity of alcohols: Phenols are weak acids and react with aqueous sodium hydroxide to
form water-soluble salts. Alcohols are considerably weaker acids than phenols and do not
react in this matter.
2. Acid-Catalyzed Dehydration alkenes: we can convert an alcohol to an alkene by
eliminating a molecule of water from adjacent carbon atoms in a reaction called dehydration.
The dehydration of an alcohol is most often brought about by heating it with either 85%
phosphoric acid or concentrated sulphuric acid.
CH3 CH2 OH
Ethan ol
H2 SO4
180°C
CH2 =CH2 + H2 O
Ethylene
Note: Primary alcohols-the most difficult to dehydrate- generally require heating in
concentrated sulphuric acid at temperature as high as 180ºC. Secondary alcohols undergo
dehydration at somewhat lower temperature. Tertiary alcohols undergo dehydration at
temperature only slightly above room temperature.
Note: Dehydration is reversible and by adding water to an alkene we can produce an alcohol
(hydration).
C C
+ H2 O
dehydration
de h yd ration
An alk ene
Dr. Behrang Madani
Chemistry 20
C C
H OH
An alcoh ol
Mt SAC
3. Oxidation of alcohols:
3.1. Oxidation of primary alcohols (1): in the oxidation of a primary alcohol (1°), one H is
removed from the –OH group and another H from the C bonded to the –OH (oxidation is
either the loss of hydrogens or the gain of oxygens). A primary alcohol can be oxidized first
to an aldehyde and then to a carboxylic acid. In this reaction, Potassium dichromate
(K2Cr2O7) will be used as the oxidizing agent and concentrated sulfuric acid solution is used
as a catalyst.
O
OH
CH3-C-H
CH3-C-H + H2O
H
Ethanol
(Ethyl alcohol)
Ethanal
(Acetaldehyde)
3.2. Oxidation of secondary alcohols: it is similar to the oxidation of primary alcohols.
Secondary alcohols may be oxidized to ketones by using potassium dichromate as the
oxidizing agent.
OH
O
CH3-C-CH3
CH3-C-CH3 + H2O
H
2-propanol
2-propanone
Note: tertiary alcohols resist oxidation because the carbon bearing the -OH is bonded to three
carbon atoms and, therefore, cannot form a carbon-oxygen double bond.
Note: in our body, enzymes in the liver oxidize ethanol and the aldehydes can be produced.
The blood alcohol over 0.4% can be fatal.
Ethers: the functional group of an ether is an atom of oxygen bonded to two carbon atoms
(C-O-C).
Naming of ethers: ethers are named by listing the alkyl groups bonded to oxygen in
alphabetical order and adding the word “ether”.
CH3-O-CH3
CH3-CH2-O-CH2-CH3
CH3-CH2-O-CH3
Dimethyl ether
Diethyl ether
Ethyl methyl ether
Physical properties of ethers: 1. They are polar compounds (oxygen bears a partial negative
charge and each attached carbon bears a partial positive charge). 2. Only weak dipole-dipole
interactions exist between ether molecules (between carbon atom of a molecule and oxygen
atom from another molecule). 3. The boiling points of ethers are higher than hydrocarbons of
similar molecular weight (lower than alcohols). 4. They are more soluble in water than
hydrocarbons of similar molecule weight (because they are polar).
Dr. Behrang Madani
Chemistry 20
Mt SAC
δ+
δ+
δ+
δ-
Chemical properties of ethers: ethers resemble hydrocarbons in their resistance to chemical
reaction. They do not participate in oxidation and reduction reactions. Because of their
general inertness to chemical reactions, ethers are excellent solvents in which to carry out
many organic reactions.
Thiol: the functional group of a thiol is an –SH (sulfhydryl) group bonded to a tetrahedral
carbon atom.
CH3-SH
methanethiol
CH3-CH2-SH
Ethanethiol
Naming of thiols: we name them by selecting the longest carbon chain that contains the –SH
group. We add the word “-thiol” as suffix to the name of the parent alkane. The parent chain
is numbered in the direction that gives the –SH group the lower number.
CH3
CH3-CH-CH2-CH2-CH2-SH
4 3
1
5
2
4-methyl-1-pentanethiol
Physical properties of thiols: 1. They have unpleasant odors. 2. They are nonpolar
compounds (because of the small difference in electronegativity between sulfur and hydrogen
(2.5 – 2.1 = 0.4)). 3. They have low boiling points (London dispersion forces). 4. They are
almost insoluble in water (because they are nonpolar compounds).
Chemical properties of thiols:
1. They are weak acids and react with strong bases such as NaOH to form a salt (ther are
comparable in strength to phenols).
CH3CH2SH + NaOH
H2O
CH3CH2S-Na+ + H2O
2. Oxidation of thiols: thiols are readily oxidized to disulfide (-S-S-) by molecular oxygen.
They must be protected from contact with air during their storage. This reaction is reversible
and disulfides can be reduced by several reducing agents.
2CH3CH2SH + O2
oxidation
reduction
HOCH2CH2S-SCH2CH2OH
Chiral: an object that is not superposable on its mirror image.
Achiral: an object that lacks chirality; an object that is superposable on its mirror image.
Dr. Behrang Madani
Chemistry 20
Mt SAC
Stereocenter: a tetrahedral carbon atom that has four different groups bonded to it
Enantiomers: isomers that are nonsuperposable mirror images. The most common cause of
enantiomerism in organic molecules is the presence of a carbon with four different groups
bonded to it (stereocenter).
Isomers
same different
connectivity connectivity
Stereoisomers
without
stereocenters
Constitutional Isomers
with
stereocenters
Chiral
Achiral
Enantiomers
Cis-Trans Isomers
Diastereomers
Note: enantiomers have different physical and chemical properties.
Example: 2-butanol is a chiral compound and it is not superposable on its mirror. If we hold
the mirror image by the C-OH bond and rotate the bottom part of molecule by 180° about this
bond, you can see the orientation of H and -CH2CH3 is different between original molecule
and the mirror image rotated. You notice that we have two different compounds.
OH
180°
C H
CH2 CH3
Original molecule
H3 C
OH
H C CH
3
CH3 CH2
Mirror image
rotate by 180°
about the
C-OH b on d
OH
C CH CH
2
3
H
The mirror image
rotated b y 180°
H3 C
Example: 2-propanol is an achiral compound and it is superposable on its mirror. If we hold
the mirror image by the C-OH bond and rotate the bottom part of molecule by 120° about this
bond, you can see that the original molecule is similar to the mirror image rotated.
Consequently, they are identical compounds.
OH
C H
CH3
Origin al molecu le
H3 C
Dr. Behrang Madani
120° OH
H C CH
3
H3 C
Mirror image
rotate by 120°
about th e
C-OH bond
Chemistry 20
OH
H3 C
C H
CH3
The mirror image
rotated b y 120°
Mt SAC