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.
Read our standard health & safety guidance
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.
Apparatus and chemicals
Eye protection: goggles
Each group of students will need:
Test-tubes, 2
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 mol dm–3 (Corrosive at this concentration)
Iodine solution, 0.5 mol dm–3 (dissolved in potassium iodide solution 0.2 mol dm–3) (Low hazard at this concentration)
Technical notes
Methanol (Highly flammable, Toxic) Refer to CLEAPSS Hazcard 40B
Ethanol (Highly flammable) and IDA (Highly flammable, Harmful) Refer to CLEAPSS Hazcard 40A
Sodium hydroxide (Corrosive at concentration used) Refer to CLEAPSS Hazcard 91 and Recipe card 65
Iodine solution (Low hazard at concentration used) Refer to CLEAPSS Hazcard 54B and Recipe card 39
Product: Triiodomethane (Harmful) Refer to CLEAPSS Hazcard 104
Procedure
HEALTH & SAFETY: Wear goggles throughout.
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?
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:
Second step
This occurs three times to give CI3CHO
Third step
The negative charge on the carbon in the I3C– ion is stabilised by the three electronegative iodine atoms.
Health & Safety checked, February 2008
Web Links
www.chemguide.co.uk/organicprops/carbonyls/iodoform.html gives details of the iodoform reaction, but no mechanism.
www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-Jodof-e.htm gives the mechanism.
(Websites accessed June 2008)
Updated 13 Nov 2008Average rating: 
4 out of 5
Your reviews
Thanks for this test. I do note a flaw, however. This test only shows how to prove the presence of ethanol, you have not actually given a test to show the presence of methanol, which is actually a greater concern. For instance, I have a wine that may have been poorly fermented and contain BOTH ethanol AND methanol; I can only prove the existence of ethanol and not methanol with the iodoform reaction. Can you also detail a similar test that would prove or disprove the existence of methanol in said wine. Thanks
Brian
Submitted by: brian vaitkus on 20 February 2009
Iodoform test also tests for terminal aldehydes. Yes, only for ethanal - Ed.
Submitted by: Ndahiriwe on 18 August 2009
This is almost exactly what I need in distillation. A similar test could be used for the wine since methyl alcohol has a lower specific heat than ethyl alcohol, it will evaporate more quickly in distillation tests. A little experimentation with different vintages should yield a fairly reliable result if properly conducted and closely observed.
Submitted by: Steve S. on 4 May 2010
I think the experiment should explain basically why ethanol is the only alcohol which undergoes triiodomethane reaction
Submitted by: Tinashe Blessing Sithole on 23 May 2010
Errors in Teaching Notes:-
Product of step 2 is CCl3CHO not Cl3CHO. Same error occurs in last line of the Mechanism for step 2
Submitted by: M J Hall on 11 November 2008