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Redox reactions
Redox reactions of primary
alcohols
• Primary alcohols react with oxidising
agents such as potassium manganate(VII)
or sodium dichromate(VI), forming the
corresponding aldehyde
• For example, ethanol reacts forming
ethanal
• Ethanal is also formed in the metabolism
of ethanol in the human body
Reaction of ethanol with sodium
dichromate(VI)
Reaction of ethanol with sodium
dichromate(VI)
• This reaction is used in the preparation of
ethanal
• Reaction conditions: heat, excess ethanol,
acidified sodium dichromate(VI) solution
• The aldehyde is distilled off as it is formed
in order to prevent further oxidation to
ethanoic acid
Preparation of ethanal
Oxidation of primary alcohols
• Primary alcohols such as ethanol are
oxidised to the corresponding aldehydes,
which can be further oxidised to the
corresponding carboxylic acids.
Oxidation of ethanol
H
H
H
C
C
H
OH
H
+
H
Na2Cr2O7 / H
H
C
H
O
C
H
Na2Cr2O7 / H+
H
H
C
H
O
C
OH
Reaction of ethanol with sodium
dichromate(VI)
• This reaction is used in the preparation of
ethanoic acid
• Reaction conditions: heat, excess acidified
sodium dichromate(VI) solution
• The reaction mixture is refluxed in order to
bring about oxidation to ethanoic acid
Preparation of ethanoic acid
Reflux
followed by
Distillation
Oxidation of secondary alcohols
• Secondary alcohols such as propan-2-ol
are oxidised to the corresponding ketones,
such as propanone
• Unlike aldehydes, ketones are not easily
oxidised, and so no further oxidation takes
place
Oxidation of propan-2-ol
OH
O
+
H3C
C
CH3
H
Na2Cr2O7 / H
H3C
C
CH3
Combustion of organic
compounds
• Most organic compounds burn in air,
forming carbon dioxide and water
• The structure of the compounds’
molecules is completely destroyed, with
the carbon and hydrogen atoms in each
molecule being oxidised
• Combustion is exothermic, and ethanol is
used as a fuel where it can be produced
cheaply
Non-flammable organic
compounds
• Fully halogenated alkanes such as
bromochlorodifluoromethane are nonflammable
• Because of this they can be used in fire
extinguishers and as flame retardants
• For environmental reasons, the use of
many of these substances is being phased
out
Reduction of aldehydes and
ketones
• Aldehydes and ketones can be reduced to
the corresponding alcohols, using
hydrogen passed over the heated surface
of a nickel catalyst
• For example, ethanal is reduced to ethanol
Reduction of ethanal to ethanol
Reduction of propanone to
propan-2-ol