Download alcohols (2013)

AN INTRODUCTION TO
THE CHEMISTRY
OF ALCOHOLS
THE CHEMISTRY OF ALCOHOLS
CONTENTS
• Structure of alcohols
• Nomenclature
• Isomerism
• Physical properties
• Chemical properties of alcohols
• Revision check list
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 (Not required for NCEA Level 3)
• 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 (Not required for NCEA Level 3)
• 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
CONSTITUTIONAL 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 intermolecular 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 instantaneous dipole-dipole interactions
+78
instantaneous dipole-dipole interactions +
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 instantaneous dipole-dipole interactions
+78
instantaneous dipole-dipole interactions +
hydrogen bonding
Boiling point is higher for “straight” chain isomers.
butan-1-ol
CH3CH2CH2CH2OH
butan-2-ol
CH3CH2CH(OH)CH3
2-methylpropan-2-ol (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
REACTIONS OF
ALCOHOLS
ELIMINATION OF WATER (DEHYDRATION)
Reagent/catalyst conc. sulfuric acid (H2SO4)
Conditions
reflux at 180°C
Product
alkene
Equation
e.g.
C2H5OH(l) ——> CH2 = CH2(g) + H2O(l)
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.
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 sulfur so there is less pollution
can be obtained from renewable resources
CHLORINATION OF ALCOHOLS
Reagent(s)
conc. hydrochloric acid HCl(aq) with ZnCl2 (Lucas Reagent)
Product
haloalkane
Equation
C2H5OH(l) + conc. HCl(aq)
Reagent(s)
thionyl chloride SOCl2
Product
haloalkane
Equation
ROH (l) + SOCl2 (l)
Reagent(s)
phosphorus pentachloride PCl5
Product
haloalkane
Equation
ROH (l) + PCl5 (l)
———>
C2H5Cl(l) + H2O(l)
RCl (l) + SO2(g) + HCl (g)
RCl (l) + POCl3(l) + HCl (g)
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 sulfuric 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 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
CAN YOU DO ALL OF THESE?
YES
NO
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relevant topic(s) again
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WELL DONE!
Try some past paper questions
AN INTRODUCTION TO
THE CHEMISTRY
OF ALCOHOLS
THE END