15
Alcohols
●● TEST YOURSELF ON PRIOR KNOWLEDGE 1
1 The boiling point of the Group IV hydrides increases from H2S to H2Te.
The only intermolecular force between the molecules in these three
compounds is van der Waals’ forces. As the Mr increases the force of
attraction between the molecules due to van der Waals’ forces increases
thus increasing the boiling point. If water were to fit into this pattern
it would have a boiling point of approximately −100 °C as it has the
lowest Mr of the group. However, hydrogen bonding is also present
between water molecules. Hydrogen bonds are much stronger forces of
attraction than van der Waals’ forces and more energy must be supplied
to overcome these attractive forces increasing the boiling point. Hydrogen
bonds in water arise because hydrogen atoms are directly bonded to
oxygen which is a much more electronegative atom. The oxygen takes the
greater share of electron density in the bond leaving the hydrogen atom
δ+ and the oxygen atom δ−. The δ+ hydrogen atom accepts a lone pair
of electrons from an oxygen atom of another molecule forming what is
known as a hydrogen bond.
2 a)2-bromo-2-methylpropane
b) Reagent: potassium hydroxide
Conditions: dissolve the halogenoalkane and the reagent in ethanol
Mechanism: elimination
H
H
CH3 H
C
Cδ+ C
H
δ_
Br H
_
OH
CH3 H
H
H
C
H
C
C
H
H
_
+ H2O + Br
c)alcohol
d) Temperature, 600 oC; pressure, 60 atm
3 a)Fe2+ → Fe3+ + e−
b)MnO4− + 8H+ + 5e− → Mn2+ + 4H2O
c)Cr2O72− + 14H+ + 6e− → 2Cr3+ + 7H2O
1
●● TEST YOURSELF 2
1 a) ethanol, primary alcohol
b) 3-methyl-pentan-3-ol, tertiary alcohol
c) 2-methyl-propan-1-ol, primary alcohol
d) pentan-2-ol, secondary alcohol
2
H
H
H
H
H
H
C
C
C
C
C
H
H
H
H
H
O
H
pentan-1-ol
primary alcohol
H
H
H
H
H
H
C
C
C
C
C
H
H
O
H
H
H
H
pentan-3-ol
secondary alcohol
H
15ALCOHOLS
H
H
H H C H H
C
C
C
C
H
H
O
H
H
H
2-methyl-butan-2-ol
tertiary alcohol
2
3 a) heptan-1-ol, CH3CH2CH2CH2CH2CH2CH2OH
octane, CH3CH2CH2CH2CH2CH2CH2CH3
b) Octane and heptan-1-ol have similar Mr so the intermolecular
attractions due to van der Waals’ forces will be similar but heptan1-ol also has hydrogen bonding between the molecules. This is an
additional and stronger intermolecular force therefore more energy
must be supplied to overcome these forces and the boiling point
increases.
b) Hexan-1-ol is much less soluble in water than ethanol as it has a
longer hydrocarbon chain which dominates the physical properties of
the molecule.
●● TEST YOURSELF 3
TEST YOURSELF 4
4 a) Ethanol is soluble in water due to the polar OH group at the end of
the short hydrocarbon chain which hydrogen bonds with water. Most
polar molecules are soluble in water.
1 a) A: pentan-3-one, (CH3CH2)2CO
B: butanal, CH3CH2CH2CHO
C: hexanoic acid, CH3CH2CH2CH2CH2COOH
b)Cr2O72− + 14H+ + 6e− → 2Cr3+ + 7H2O
2 Without refluxing, the alcohol will evaporate before the reaction has
taken place. Refluxing continually returns the alcohol to the reaction flask
allowing it to react.
●● TEST YOURSELF 4
1 a)
Name of reagent
observation with
Fehling’s solution
propanal
hexan-2-one
orange-red precipitate
forms
solution remains blue
b) Fehling’s solution is freshly prepared in the laboratory. It is made
initially as two separate solutions which are added together
immediately before use.
c)Cu2+ + e− → Cu+
d) Tollens’ reagent can be used to distinguish between an aldehyde
and a ketone. It is a colourless solution of silver(I) ions, which when
warmed gently (using a water bath) with an aldehyde are reduced
to silver atoms, leaving a silver mirror on the interior of the reaction
vessel. When Tollens’ reagent is warmed gently in the presence of a
ketone the solution remains colourless.
2
H
H H
H
C C
C
H H
H
H
H H
C
C
H C C C
H
H
H H H
pent-1-ene
H
C
C H
H H
pent-2-ene
3 a) aluminium oxide or concentrated sulfuric acid
b) When using aluminium oxide the reaction should be carried out at
a temperature of 600 °C. When using concentrated sulfuric acid as
the catalyst the reaction should be carried out at a temperature of
170 °C.
3
●● TEST YOURSELF 5
1 a) A catalyst is a substance that increases the rate of a chemical reaction
and remains chemically unchanged at the end of the reaction. It
decreases the activation energy for the reaction by providing an
alternative pathway or mechanism for the reaction.
b) Hydration is the addition of water to a compound. (This is
sometimes confused with hydrolysis which is the decomposition of a
compound by reaction with water)
c)CH3CHCH2 + H2O → CH3CH(OH)CH3
d)propan-2-ol
●● Activity
Preparation of ethanal
1 density = mass/volume
0.79 × 6 = 4.74 g
2 Sodium dichromate is orange, if it was not in excess it would be green.
3 a) ethanoic acid
b) The ethanal is distilled off as soon as it is formed preventing further
reaction.
4 Ethanal has a low boiling point
5 Fractional distillation
6Cr2O72− + 14H+ + 6e− → 2Cr3+ + 7H2O
15ALCOHOLS
7 Moles = 4.74/46.0 = 0.103
4
0.103 × 44.0 = 4.53 g
1.32 × 100 = 29.1%
4.53
8 Ethanol and ethanal can hydrogen bond with water; they dissolve.
Ethanol and ethanal boil.