Oxidation reactions of
alcohols
Reactions with sodium dichromate.
Sodium dichromate is an oxidising agent, ie it oxidises
other chemicals, being reduced in the process.
Orange
chromium (vi) is
reduced to
green
chromium (iii)
when heated
with acid.
Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H20
Oxidation of primary alcohols.
Primary alcohols
are oxidised as
orange
chromium (vi) is
reduced to
green
chromium (iii)
when heated
with acid.
Two organic products are possible;
Aldehydes or Carboxylic Acids.
Aldehydes vs Carboxylic Acids
Aldehydes have a
carbonyl group C=O, at
one end.
Eg Propanal
Carboxylic acids have the
functional group;
-CO2H
Eg; Ethanoic acid
The alcohol is oxidised;
CH3CH2OH → CH3HC=O + 2H+ + 2eEthanol
Ethanal
Dichromate is reduced;
Cr2O7 2- + 14H+ + 6e- → 2Cr3+ + 7H20
The aldehyde is then distilled as fast as it
is formed so that it is unable to react
further to form the carboxylic acid.
Oxidising alcohols to aldehydes using
distillation.
Aldehydes
evaporate
Oxidation of aldehydes.
Aldehydes are
oxidised as
orange
chromium (vi) is
reduced to
green
chromium (iii)
when heated
with acid.
Only one organic product is possible;
a Carboxylic Acids.
Aldehyde
condenses
Reflux
produces the
carboxylic
acid.
Aldehyde
evaporates.
So the aldehyde
can be oxidised
to a carboxylic
acid.
The carboxylic acid is then obtained by
distillation.
Carboxylic acid
evaporates
The aldehyde is oxidised;
CH3CHO + H2O→ CH3CO2H + 2H++ 2eEthanal
Ethanoic acid
Dichromate is reduced.
Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H20
NB Instead of H+ [H] or ‘H’ can be used in
writing half equations.
Distinguishing between aldehydes and
carboxylic acids.
Aldehydes
are neutral.
Aldehydes turn blue Cu(ii)
orange on heating.
Carboxylic
acids are acidic.
Carboxylic acids do not
react.
Aldehydes give a silver mirror when heated with
ammonical silver nitrate.
Carboxylic acids do not.
Effects of ethanol on the body.
• Ethanol depresses the activity of the
central nervous system.
• Reducing stress, tension and anxiety
and inducing a feeling of relaxation.
• Unfortunately it also reduces reaction
times and impairs judgement.
• Blood Alcohol Concentration (BAC) is
measure in mg/100ml of blood.
• The legal limit is 80mg/100ml.
Detecting alcohol
Alcohol is not
digested on
absorption nor
chemically changed
in the blood.
The UK legal limit
for drivers is 80 mg
of alcohol per 100
ml of blood
But it takes time to
analyse blood…
Breathalysers
It is much easier to
analyse exhaled air.
The oxidation reaction
of ethanol is the basis
of the breathalyser.
As the blood flows through the lungs some of
the alcohol moves across the alveoli into the
air, because it is volatile.
The concentration of alcohol in the alveolar is
directly proportional to the concentration of
alcohol in the blood.
As the alveolar air is exhaled the alcohol can
be detected by a breathalyzer, giving an
instant result.
How Breathalyzers Work
Amplifier
Breath
sample
with alcohol
Biological
recognition
layer
Photosystem cell
Enzyme catalyses the reaction of the alcohol
with potassium dichromate to chromium
sulphate, potassium sulphate, acetic acid and
water. The reddish-brown dichromate ion
changes to a green chromium ion.
The reacted mixture
is compared to a vial
of unreacted mixture
in a photosystem cell
that produces an
electric current,
displayed in a meter.
•
•
•
•
Modern breathalysers use a fuel cell.
At one electrode ethanol is oxidised;
CH3CH2OH+ H2O → CH3CO2H+ 2H++ 2eAt the other oxygen is reduced;
• ½O2 + 2H++ 2e- → H2O
• The voltage produced at varying
concentrations of ethanol can then be
measured to calibrate the machine.
• A green light will then indicate no
ethanol, amber a little and red on or
near the limit.
Oxidation of secondary alcohols.
Secondary
alcohols are also
oxidised as
orange
chromium (vi) is
reduced to
green
chromium (iii)
when heated
with acid.
Only one organic product ispossible;
a Ketone.
• Eg; CH3CH(OH)CH3
Propan 2ol
↓
• CH3C=OCH3 +
+
2H
+
•
Propanone
• Ketones cannot react further, so
oxidation stops.
2e
Aldehydes vs Ketones
Aldehydes have a
carbonyl group C=O, at
one end.
Ketones also have a
carbonyl group, but in the
interior of the molecule.
Oxidation of Ketones
Ketones cannot be
oxidised by acidified
potassium
dichromate.
To oxidise them a
much stronger
oxidising agent is
needed that can
break C/C bonds.
Oxidation of tertiary alcohols
Tertiary alcohols
cannot be oxidised
by acidified
potassium
dichromate.
To oxidise them a
much stronger
oxidising agent is
needed that can
break C/C bonds.
Ethanol as a renewable fuel
• Ethene is derived from
crude oil, a nonrenewable resource.
• But sugars are
renewable.
• Ethanol derived from
fermentation can be
burnt as a renewable
fuel.
• In some countries
ethanol, from sugar
cane, is mixed with
petrol to form gasohol.
Carbon footprints of biodiesel and
ethanol vs diesel.
• The carbon in diesel was taken out of
circulation hundreds of millions of years ago.
• When it is burnt it therefore results in a net
increase of carbon dioxide in the
atmosphere.
• But the carbon in biodiesel or ethanol from
fermentation was only recently taken up, so
its release does not result in an increase in
atmospheric carbon dioxide, ie it is carbon
neutral.
Carbon neutrality
• Carbon dioxide is taken up during
photosynthesis.
• 6CO2 + 6H2O→C6H12O6 + 6O2
• But released upon fermentation.
• C6H12O6 → 2C2H5OH + 2CO2
• And when ethanol is burnt;
• 2[C2H5OH → 2CO2 + 3H2O]
Carbon budget
Carbon
uptake
= 6CO2
Carbon production
= 2CO2 +2[ 2CO2]
= 6CO2
So there is no net change, but there are hidden costs
in ethanol production and distribution which mean
that ethanol is in fact not completely carbon neutral.