AN INTRODUCTION TO
THE CHEMISTRY
OF ALCOHOLS
KNOCKHARDY PUBLISHING
KNOCKHARDY PUBLISHING
THE CHEMISTRY OF ALCOHOLS
INTRODUCTION
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THE CHEMISTRY OF ALCOHOLS
CONTENTS
• Structure of alcohols
• Nomenclature
• Isomerism
• Physical properties
• Chemical properties of alcohols
• Identification using infra-red spectroscopy
• Industrial preparation and uses of ethanol
• Revision check list
THE CHEMISTRY OF ALCOHOLS
Before you start it would be helpful to…
• Recall the definition of a covalent bond
• Recall the difference types of physical bonding
• Be able to balance simple equations
• Be able to write out structures for simple organic molecules
• Understand the IUPAC nomenclature rules for simple organic compounds
• Recall the chemical properties of alkanes and alkenes
CLASSIFICATION OF ALCOHOLS
Aliphatic • general formula CnH2n+1OH - provided there are no rings
• the OH replaces an H in a basic hydrocarbon skeleton
CLASSIFICATION OF ALCOHOLS
Aliphatic • general formula CnH2n+1OH - provided there are no rings
• the OH replaces an H in a basic hydrocarbon skeleton
Aromatic • in aromatic alcohols (or phenols) the OH is attached directly to the ring
• an OH on a side chain of a ring behaves as a typical aliphatic alcohol
The first two compounds are
classified as aromatic alcohols
(phenols) because the OH group
is attached directly to the ring.
CLASSIFICATION OF ALCOHOLS
Aliphatic • general formula CnH2n+1OH - provided there are no rings
• the OH replaces an H in a basic hydrocarbon skeleton
Aromatic • in aromatic alcohols (or phenols) the OH is attached directly to the ring
• an OH on a side chain of a ring behaves as a typical aliphatic alcohol
The first two compounds are
classified as aromatic alcohols
(phenols) because the OH group
is attached directly to the ring.
Structural
differences • alcohols are classified according to the environment of the OH group
• chemical behaviour, eg oxidation, often depends on the structural type
PRIMARY 1°
SECONDARY 2°
TERTIARY 3°
NAMING ALCOHOLS
Alcohols are named according to standard IUPAC rules
• select the longest chain of C atoms containing the O-H group;
• remove the e and add ol after the basic name
• number the chain starting from the end nearer the O-H group
• the number is placed after the an and before the ol ... e.g butan-2-ol
• as in alkanes, prefix with alkyl substituents
• side chain positions are based on the number allocated to the O-H group
e.g.
CH3 - CH(CH3) - CH2 - CH2 - CH(OH) - CH3
is called 5-methylhexan-2-ol
STRUCTURAL ISOMERISM IN ALCOHOLS
Different structures are possible due to...
A Different positions for the OH group and
B Branching of the carbon chain
butan-1-ol
2-methylpropan-2-ol
butan-2-ol
2-methylpropan-1-ol
BOILING POINTS OF ALCOHOLS
Increases with molecular size due to increased van der Waals’ forces.
Alcohols have higher boiling points than
similar molecular mass alkanes
This is due to the added presence of
inter-molecular hydrogen bonding.
More energy is required to separate the molecules.
propane C3H8
ethanol C2H5OH
Mr
44
46
bp / °C
-42
just van der Waals’ forces
+78
van der Waals’ forces + hydrogen bonding
BOILING POINTS OF ALCOHOLS
Increases with molecular size due to increased van der Waals’ forces.
Alcohols have higher boiling points than
similar molecular mass alkanes
This is due to the added presence of
inter-molecular hydrogen bonding.
More energy is required to separate the molecules.
propane C3H8
ethanol C2H5OH
Mr
44
46
bp / °C
-42
just van der Waals’ forces
+78
van der Waals’ forces + hydrogen bonding
Boiling point is higher for “straight” chain isomers.
butan-1-ol
butan-2-ol
2-methylbutan-2-ol
CH3CH2CH2CH2OH
CH3CH2CH(OH)CH3
(CH3)3COH
bp / °C
118
Greater branching =
100
lower inter-molecular forces
83
SOLVENT PROPERTIES OF ALCOHOLS
Solubility
Low molecular mass alcohols are miscible with water
Due to hydrogen bonding between the two molecules
Heavier alcohols are less miscible
Solvent
properties
Alcohols are themselves very good solvents
They dissolve a large number of organic molecules
CHEMICAL PROPERTIES OF ALCOHOLS
The OXYGEN ATOM HAS TWO LONE PAIRS; this makes alcohols...
BASES
Lewis bases are lone pair donors
Bronsted-Lowry bases are proton acceptors
The alcohol uses one of its lone pairs to form a co-ordinate bond
NUCLEOPHILES
Alcohols can use the lone pair to attack electron deficient centres
ELIMINATION OF WATER (DEHYDRATION)
Reagent/catalyst
Conditions
conc. sulphuric acid (H2SO4) or conc. phosphoric acid (H3PO4)
reflux at 180°C
Product
alkene
Equation
e.g.
C2H5OH(l) ——> CH2 = CH2(g) + H2O(l)
Mechanism
Step 1
Step 2
Step 3
Alternative
Method
protonation of the alcohol using a lone pair on oxygen
loss of a water molecule to generate a carbocation
loss of a proton (H+) to give the alkene
Pass vapour over a heated alumina (aluminium oxide) catalyst
ELIMINATION OF WATER (DEHYDRATION)
MECHANISM
Step 1
Step 2
Step 3
protonation of the alcohol using a lone pair on oxygen
loss of a water molecule to generate a carbocation
loss of a proton (H+) to give the alkene
Note 1
There must be an H on a carbon atom adjacent the carbon with the OH
Note 2
Alcohols with the OH in the middle of a chain
can have two ways of losing water.
In Step 3 of the mechanism, a proton can be lost
from either side of the carbocation. This gives a
mixture of alkenes from unsymmetrical alcohols...
OXIDATION OF ALCOHOLS
All alcohols can be oxidised depending on the conditions
Oxidation is used to differentiate between primary, secondary and tertiary alcohols
The usual reagent is acidified potassium dichromate(VI)
Primary
Easily oxidised to aldehydes and then to carboxylic acids.
Secondary
Easily oxidised to ketones
Tertiary
Not oxidised under normal conditions.
They do break down with very vigorous oxidation
PRIMARY 1°
SECONDARY 2°
TERTIARY 3°
OXIDATION OF PRIMARY ALCOHOLS
Primary alcohols are easily oxidised to aldehydes
e.g.
CH3CH2OH(l) + [O]
ethanol
——>
CH3CHO(l) + H2O(l)
ethanal
it is essential to distil off the aldehyde before it gets oxidised to the acid
CH3CHO(l) + [O]
ethanal
——>
CH3COOH(l)
ethanoic acid
OXIDATION OF PRIMARY ALCOHOLS
Primary alcohols are easily oxidised to aldehydes
e.g.
CH3CH2OH(l) + [O]
ethanol
——>
CH3CHO(l) + H2O(l)
ethanal
it is essential to distil off the aldehyde before it gets oxidised to the acid
CH3CHO(l) + [O]
ethanal
——>
CH3COOH(l)
ethanoic acid
Practical details
•
•
•
•
the alcohol is dripped into a warm solution of acidified K2Cr2O7
aldehydes have low boiling points - no hydrogen bonding - they distil off immediately
if it didn’t distil off it would be oxidised to the equivalent carboxylic acid
to oxidise an alcohol straight to the acid, reflux the mixture
compound
formula
intermolecular bonding
boiling point
ETHANOL
C2H5OH
HYDROGEN BONDING
78°C
ETHANAL
CH3CHO
DIPOLE-DIPOLE
23°C
ETHANOIC ACID
CH3COOH
HYDROGEN BONDING
118°C
OXIDATION OF PRIMARY ALCOHOLS
Controlling the products
e.g.
CH3CH2OH(l) + [O]
——>
CH3CHO(l) + H2O(l)
then
CH3CHO(l) + [O]
——>
CH3COOH(l)
OXIDATION TO ALDEHYDES
DISTILLATION
OXIDATION TO CARBOXYLIC ACIDS
REFLUX
Aldehyde has a lower boiling point so
distils off before being oxidised further
Aldehyde condenses back into the
mixture and gets oxidised to the acid
OXIDATION OF SECONDARY ALCOHOLS
Secondary alcohols are easily oxidised to ketones
e.g.
CH3CHOHCH3(l) + [O]
propan-2-ol
——>
CH3COCH3(l) + H2O(l)
propanone
The alcohol is refluxed with acidified K2Cr2O7. However, on prolonged treatment
with a powerful oxidising agent they can be further oxidised to a mixture of acids
with fewer carbon atoms than the original alcohol.
OXIDATION OF TERTIARY ALCOHOLS
Tertiary alcohols are resistant to normal oxidation
ESTERIFICATION OF ALCOHOLS
Reagent(s)
carboxylic acid + strong acid catalyst (e.g conc. H2SO4 )
Conditions
reflux
Product
ester
Equation
Notes
e.g. CH3CH2OH(l) + CH3COOH(l)
ethanol
ethanoic acid
CH3COOC2H5(l) + H2O(l)
ethyl ethanoate
Concentrated H2SO4 is a dehydrating agent - it removes water
causing the equilibrium to move to the right and increases the yield
ESTERIFICATION OF ALCOHOLS
Reagent(s)
carboxylic acid + strong acid catalyst (e.g conc. H2SO4 )
Conditions
reflux
Product
ester
Equation
e.g. CH3CH2OH(l) + CH3COOH(l)
ethanol
ethanoic acid
CH3COOC2H5(l) + H2O(l)
ethyl ethanoate
Notes
Concentrated H2SO4 is a dehydrating agent - it removes water
causing the equilibrium to move to the right and increases the yield
Uses of esters
Esters are fairly unreactive but that doesn’t make them useless
Used as flavourings
Naming esters
Named from the alcohol and carboxylic acid which made them...
CH3OH + CH3COOH
from ethanoic acid
CH3COOCH3 + H2O
CH3COOCH3
METHYL ETHANOATE
from methanol
OTHER REACTIONS OF ALCOHOLS
OXYGEN
Alcohols make useful fuels
C2H5OH(l)
+
3O2(g) ———>
2CO2(g) + 3H2O(l)
Advantages have high enthalpies of combustion
do not contain sulphur so there is less pollution
can be obtained from renewable resources
OTHER REACTIONS OF ALCOHOLS
OXYGEN
Alcohols make useful fuels
C2H5OH(l)
+
3O2(g) ———>
2CO2(g) + 3H2O(l)
Advantages have high enthalpies of combustion
do not contain sulphur so there is less pollution
can be obtained from renewable resources
SODIUM
Conditions
room temperature
Product
sodium alkoxide and hydrogen
Equation
2CH3CH2OH(l) + 2Na(s)
Notes
alcohols are organic chemistry’s equivalent of water
water reacts with sodium to produce hydrogen and so do alcohols
the reaction is slower with alcohols than with water.
——> 2CH3CH2O¯ Na +
sodium ethoxide
Alkoxides are white, ionic crystalline solids
+ H2(g)
e.g. CH3CH2O¯ Na+
BROMINATION OF ALCOHOLS
Reagent(s)
conc. hydrobromic acid HBr(aq) or
sodium (or potassium) bromide and concentrated sulphuric acid
Conditions
reflux
Product
haloalkane
Equation
C2H5OH(l) + conc. HBr(aq)
Mechanism
The mechanism starts off similar to that involving dehydration
(protonation of the alcohol and loss of water) but the carbocation
(carbonium ion) is attacked by a nucleophilic bromide ion in step 3
Step 1
Step 2
Step 3
———>
C2H5Br(l) + H2O(l)
protonation of the alcohol using a lone pair on oxygen
loss of a water molecule to generate a carbocation (carbonium ion)
a bromide ion behaves as a nucleophile and attacks the carbocation
INFRA-RED SPECTROSCOPY
Chemical bonds vibrate at different frequencies. When infra red (IR) radiation is passed
through a liquid sample of an organic molecule, some frequencies are absorbed. These
correspond to the frequencies of the vibrating bonds.
Most spectra are very complex due to the large number of bonds present and each
molecule produces a unique spectrum. However the presence of certain absorptions
can be used to identify functional groups.
BOND
O-H
COMPOUND
alcohols
ABSORBANCE
RANGE
broad
3200 cm-1 to 3600 cm-1
O-H
carboxylic acids
medium to broad
2500 cm-1 to 3500 cm-1
C=O
ketones, aldehydes
esters and acids
strong and sharp
1600 cm-1 to 1750 cm-1
INFRA-RED SPECTROSCOPY
IDENTIFYING ALCOHOLS USING INFRA RED SPECTROSCOPY
Differentiation
Compound
O-H
C=O
ALCOHOL
YES
NO
ALDEHYDE / KETONE
NO
YES
CARBOXYLIC ACID
YES
YES
ESTER
NO
YES
ALCOHOL
BUTAN-1-OL
O-H absorption
CARBOXYLIC ACID
PROPANOIC ACID
O-H + C=O absorption
ESTER
ETHYL ETHANOATE
C=O absorption
INDUSTRIAL PREPARATION OF ALCOHOLS
FERMENTATION
Reagent(s)
GLUCOSE - produced by the hydrolysis of starch
Conditions
yeast
warm, but no higher than 37°C
Equation
C6H12O6
Advantages
LOW ENERGY PROCESS
USES RENEWABLE RESOURCES - PLANT MATERIAL
SIMPLE EQUIPMENT
Disadvantages
SLOW
PRODUCES IMPURE ETHANOL
BATCH PROCESS
——>
2 C2H5OH
+
2 CO2
INDUSTRIAL PREPARATION OF ALCOHOLS
HYDRATION OF ETHENE
Reagent(s)
ETHENE - from cracking of fractions from distilled crude oil
Conditions
catalyst - phosphoric acid
high temperature and pressure
Equation
C2H4 +
H2O
——>
2 C2H5OH
INDUSTRIAL PREPARATION OF ALCOHOLS
HYDRATION OF ETHENE
Reagent(s)
ETHENE - from cracking of fractions from distilled crude oil
Conditions
catalyst - phosphoric acid
high temperature and pressure
Equation
C2H4 +
Advantages
FAST
PURE ETHANOL PRODUCED
CONTINUOUS PROCESS
Disadvantages
HIGH ENERGY PROCESS
EXPENSIVE PLANT REQUIRED
USES NON-RENEWABLE FOSSIL FUELS TO MAKE ETHENE
Uses of ethanol
ALCOHOLIC DRINKS
SOLVENT - industrial alcohol / methylated spirits
FUEL - petrol substitute in countries with limited oil reserves
H2O
——>
2 C2H5OH
USES OF ALCOHOLS
ETHANOL
DRINKS
SOLVENT
FUEL
industrial alcohol / methylated spirits (methanol is added)
used as a petrol substitute in countries with limited oil reserves
METHANOL
PETROL ADDITIVE
SOLVENT
RAW MATERIAL
FUEL
Health warning
improves combustion properties of unleaded petrol
used as a feedstock for important industrial processes
Methanol is highly toxic
LABORATORY PREPARATION OF ALCOHOLS
from haloalkanes - reflux with aqueous sodium or potassium hydroxide
from aldehydes
- reduction with sodium tetrahydridoborate(III) - NaBH4
from alkenes
- acid catalysed hydration using concentrated sulphuric acid
Details of the reactions may be found in other sections.
REVISION CHECK
What should you be able to do?
Recall and explain the physical properties of alcohols
Recall the different structural types of alcohols
Recall the Lewis base properties of alcohols
Recall and explain the chemical reactions of alcohols
Write balanced equations representing any reactions in the section
Understand how oxidation is affected by structure
Recall how conditions and apparatus influence the products of oxidation
Explain how infrared spectroscopy can be used to differentiate between functional groups
CAN YOU DO ALL OF THESE?
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NO
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AN INTRODUCTION TO
THE CHEMISTRY
OF ALCOHOLS
THE END
© JONATHAN HOPTON & KNOCKHARDY PUBLISHING