A test to distinguish between ethanol and methanol
Class practical
This experiment shows how methanol
and ethanol behave differently when treated with iodine solution
and sodium hydroxide solution. Ethanol reacts to form a yellow solid, triiodomethane,
often called iodoform.
Lesson organisation
This experiment takes about 20 minutes. It is eminently suitable for
students who have the iodoform reaction in their specification. For others, it
is a good observational exercise and an extension for more able students.
Both methanol and ethanol (and other
alcohols and organic liquids) can be stored in
plastic pipettes, from which they can be easily dispensed. The main difficulty for students arises from the fact that if the pipettes are squeezed too hard, the alcohols come out of the pipette in a stream (because of their low surface tension). Students must handle the pipettes very carefully, and some practice will be required before proceeding with this experiment.
plastic pipettes, from which they can be easily dispensed. The main difficulty for students arises from the fact that if the pipettes are squeezed too hard, the alcohols come out of the pipette in a stream (because of their low surface tension). Students must handle the pipettes very carefully, and some practice will be required before proceeding with this experiment.
Chemicals
Solutions in plastic dropping pipettes (one of each):
Methanol (HIGHLY FLAMMABLE, TOXIC)
Ethanol (HIGHLY FLAMMABLE) or Industrial denatured alcohol, IDA (HIGHLY
FLAMMABLE, HARMFUL)
Sodium hydroxide, 1 M (CORROSIVE)
Iodine solution, 0.5 M (dissolved in potassium iodide solution 0.2 M)
Apparatus
Eye protection: goggles
Each group of students will need:
Test-tubes, 2
Health & Safety and Technical notes
Wear goggles throughout.
Methanol, CH3OH(l), (HIGHLY FLAMMABLE, TOXIC) - see CLEAPSS Hazcard.
Ethanol, CH3CH2OH(l), (HIGHLY FLAMMABLE) and IDA
(HIGHLY FLAMMABLE, HARMFUL) - see CLEAPSS Hazcard.
Sodium hydroxide, NaOH(aq), (CORROSIVE at concentration used) - see
CLEAPSS Hazcard and CLEAPSS Recipe Book.
Iodine solution, I2(aq), dissolved in potassium iodide solution,
KI(aq), - see CLEAPSS Hazcard and CLEAPSS Recipe Book.
Product: Triiodomethane, CHI3(s), (HARMFUL) - see CLEAPSS Hazcard.
Procedure
a Practice (over a sink) producing single drops of ethanol from the
pipette.
b Add 10 drops of methanol to one test-tube.
c Add 10 drops of ethanol to the other test-tube.
d Add 25 drops of iodine solution to each alcohol.
e Add 10 drops of sodium hydroxide solution to each alcohol.
f Gently swirl the test-tubes a few times. The dark colour of the iodine
should
start to fade.
start to fade.
g After 2 min carefully observe the two test-tubes. What differences do
you notice?
Teaching notes
The solution in the ethanol test-tube should go cloudy and then a yellow
precipitate of triidomethane (iodoform) should be seen. This has a distinct
‘antiseptic’ smell. The methanol test-tube should remain clear.
The iodoform reaction is given by
compounds with a methyl group next to a carbonyl group. Secondary alcohols with
a CH3 on the carbon carrying the OH (eg propan-2-ol) that
can be oxidised to carbonyl compounds of this type, also give a positive
iodoform test. (NB carboxylic acids do not)
Ethanol is the only primary alcohol which will give the reaction and
ethanal the only aldehyde. A simple explanation of the reaction is:
The full mechanism for the second and third steps are:
This occurs three times to give CI3CHO
The negative charge on the carbon in
the I3C– ion is
stabilised by the three electronegative iodine atoms.
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