Microscale Chemistry
173
THE ROYAL
SOCIETY OF
CHEMISTRY
45. A test to distinguish
between methanol and ethanol
Topic
Organic alcohols.
Level
Post-16.
Timing
20 min.
Description
These two important alcohols can be chemically distinguished by using the iodoform
reaction. Students do this reaction at microscale using a well-plate, one advantage
being that no heating is required.
I2
I2
NaOH
C2H5OH → CH3CHO → Cl3CHO → CHI3 (s)+HCOONa
Apparatus (per group)
▼
One plastic well-plate (24 well size, eg Sigma ref: M 9655).
Chemicals (per group)
Solutions contained in plastic pipettes, see p. 2
▼
Methanol
▼
Ethanol
▼
Iodine solution (0.05 mol dm–3) dissolved in potassium iodide (0.2 mol dm–3)
▼
Sodium hydroxide
1 mol dm–3.
Note
Both methanol and ethanol (and other alcohols and organic liquids) can be stored in
plastic pipettes and 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 is required before proceeding with this experiment.
Observations
After a short time a cloudiness is seen in the ethanol well while the methanol well
remains clear. After a few minutes a yellow precipitate forms which has a distinctive
antiseptic smell. Compounds that contain a methyl group adjacent to a carbonyl
group (methyl ketone) give a positive result with this test, as do secondary alcohols
which can be oxidised to ketones.
Safety
Students must wear eye protection.
It is the responsibility of the teacher to carry out a risk assessment.
Microscale Chemistry
81
THE ROYAL
SOCIETY OF
CHEMISTRY
45. A test to distinguish
between methanol and ethanol
These two important alcohols may be chemically distinguished by using the iodoform
reaction. You will be doing this reaction in microscale using a well-plate.
Instructions
1.
Add 10 drops of methanol to well A1 in your well-plate (see diagram).
2.
Add 10 drops of ethanol to well A2.
3.
Add 25 drops of iodine solution to each alcohol in the wells.
4.
Add 10 drops of sodium hydroxide solution to each alcohol.
5.
Gently swirl the well-plate a few times. The dark colour of the iodine should
start to fade.
6.
After 2 min carefully observe the two wells. What differences do you notice?
Can you explain these differences?
1
A
B
C
D
2
3
4
5
6