A test to distinguish between ethanol and methanol
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.
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.
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)
Refer to Health & Safety and Technical notes section below for additional information.
Eye protection: goggles
Each group of students will need:
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.
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.
g After 2 min carefully observe the two test-tubes. What differences do you notice?
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.
Health & Safety checked, February 2008
Chemguide - gives details of the iodoform reaction, but no mechanism.
University of Regensburg - gives the mechanism.
Page last updated on 21 November 2011