2.10 Organic synthesis – Oxidation of alcohols
d. demonstrate an understanding of, and practise, the preparation of an organic liquid (reflux and
distillation), eg oxidation of alcohols.
Connector:
K2Cr2O7 is a mild oxidising agent oxidising agent.
a) Balance the ionic half equation below and state whether the Cr2072ion is being oxidised or reduced and justify your answer.
a) Describe the colour change for the above reaction.
Orange to green
b) Draw the skeletal structures of butan-1-ol, butan-2-ol and
2-methylpropan-2-ol, and their reaction products, if any, when they
are heated separately with acidified potassium dichromate(VI)
solution.
c) The oxidation of propan-1-ol to propanal can be written as:
CH3CH2CH2OH + [O]
 CH3CH2CHO + H2O
Write the corresponding equation for the complete oxidation of
propan-1-ol. How would you carry out these two reactions?
Crowe2013
Acidified potassium dichromate(VI) and alcohols
K2Cr2O7/H+
1o
OH
O
O
HEAT
H
K2Cr2O7/H+
H
aldehyde
K2Cr2O7/H+
2o
HEAT
O
O
ketone
OH
O
H
K2Cr2O7/H+
3o
NO REACTION
HEAT
OH
O
H
HEAT
O
OH
carboxylic acid
CH3CH2CH2OH + [O]
 CH3CH2CHO + H2O
Heat propan-1-ol with potassium dichromate (VI) and dilute
H2SO4 and immediately distil off the product.
CH3CH2CH2OH + 2[O]
 CH3CH2COOH + H2O
Heat propan-1-ol with potassium dichromate (VI) and dilute
H2SO4 under reflux. Then collect the propanoic acid by
distillation.
1. Draw labelled diagrams of the apparatus used for
heating under reflux and distillation.
2. Explain why each process is used, and describe what
happens in each process.
• The purpose is to thermally accelerate
the reaction by conducting it at an
elevated temperature (i.e. the solvent's
boiling point.).
• The reaction will proceed at a constant
temperature (i.e. the solvent's boiling
point.).
•Any vapours given off are cooled back
to liquid, and fall back into the reaction
vessel
• Useful for performing chemical
reactions under controlled conditions
that require substantial time for
completion.
heat
• The advantage of this technique is that
it can be left for a long period of time
without the need to add more solvent or
fear of the reaction vessel boiling dry as
any vapour is immediately condensed in
the condenser.
• Distillation is a physical separation process.
• Distillation is a method of separating mixtures based on
differences in their volatilities in a boiling liquid mixture.
• See exps 2.13 & 2.14 do reflux and dist
parts
Preparation of ethanal
3.
preventethanal
the ethanal
1. To
Prevents
vapour
vapour
from
escaping
by escaping.
condensing it.
2. The exothermic reaction
would be too vigorous /
hazardous / dangerous
3. Why is it necessary to close
1. Explain
whyseparating
the collection
the
tap of the
funnel
container
surrounded
when
all ofisthe
sodium by a
cooling mixture.
dichromate(VI)
and ethanol
solution
has
been
to the
2. Explain
why
it isadded
important
to
flask?
add the sodium dichromate(VI)
and ethanol solution to the acid
solution a few drops at a time.
4. Write an equation to show the oxidation of ethanol into
ethanal using [O] to represent the oxidizing agent.
CH3CH2OH + [O] → CH3CHO + H2O
5. Write an equation to show the oxidation of ethanol into
ethanoic acid using [O] to represent the oxidizing agent.
CH3CH2OH + 2[O] → CH3COOH + H2O