The oxidation of alcohols

Read our standard health & safety guidance

Lesson organisation

This experiment can be done by students in 20 minutes. The colour change of the dichromate(VI) indicates where reaction is occurring. Primary, secondary and tertiary alcohols can be distinguished by the rate of reaction, though no attempt is made to identify the products.

Students acidify the dichromate for themselves to emphasise the need for acid.

The alcohols 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.

Apparatus and chemicals

Each group of students will need:

Eye protection

Measuring cylinder (5 or 10 cm³)
Well-plate (24 wells) – eg Sigma ref: M9655.
Glass rod
Plastic dropping pipettes, 6

Access to:

The following alcohols in dropping pipettes:

Ethanol (Highly flammable) or Industrial denatured alcohol, IDA (Highly flammable, Harmful)
Propan-1-ol (Irritant, Highly flammable)
Propan-2-ol (Irritant, Highly flammable)
2-Methylpropan-2-ol (Harmful, Highly flammable), or other tertiary alcohol

Potassium dichromate(VI) solution, 0.1 mol dm^–3 (Toxic at this concentration), 2 cm³
Sulfuric acid, 1 mol dm^–3 (Irritant at this concentration), 1 cm³

Technical notes

Ethanol (Highly flammable) or Industrial denatured alcohol, IDA (Highly flammable, Harmful). Refer to CLEAPSS Hazcard 40A
Propan-1-ol (Irritant, Highly flammable) Refer to CLEAPSS Hazcard 84A
Propan-2-ol (Irritant, Highly flammable) Refer to CLEAPSS Hazcard 84A
2-Methylpropan-2-ol (Harmful, Highly flammable) Refer to CLEAPSS Hazcard 84B
Potassium dichromate(VI) solution (Toxic at concentration used) Refer to CLEAPSS Hazcard 78 and Recipe card 53
Sulfuric acid (Irritant at concentration used) Refer to CLEAPSS Hazcard 98A and Recipe card 69

Procedure

HEALTH & SAFETY: Wear eye protection throughout.

a Take approximately 2 cm³ of potassium dichromate(VI) solution in a measuring cylinder and add about 1 cm³ of dilute sulfuric acid. Stir with a glass rod.

b Put 10 drops of the acidified potassium dichromate(VI) solution into each of the wells
A1 – A4 and B2 (see diagram below).

c Practice (over a sink) producing single drops of ethanol from the pipette.

d Add two drops of the alcohols to the wells as follows:

Do not put any alcohol into well B2 – this well is used as a control.

3 Observe the wells over the next 15 minutes and record any changes you see.

Teaching notes

For the primary alcohols (ethanol and propan-1-ol), the dichromate turns green after a few minutes. The secondary alcohol (propan-2-ol) is slower. The tertiary alcohol (2-methylpropan-2-ol) is not oxidised at all. (Methanol could have been used in another well but, since this is slower than the other two primary alcohols, the differences are then not so obvious.)

Discussion could continue to the products of oxidation (aldehydes, then carboxylic acids for primary alcohols; ketones for secondary alcohols), and students could draw out the relevant alcohol and oxidation product structures.

Macro-scale experiments involving distillation and reflux of primary alcohols, to get aldehydes and acids respectively, may be useful to demonstrate the techniques involved, but seldom yield very satisfactory results in terms of identifying the products.

Health & Safety checked, May 2008

Web Links

www.chemguide.co.uk/organicprops/alcohols/oxidation.html covers the theory well.

www.mp-docker.demon.co.uk/as_a2/topics/oxidation_of_alcohols/index.html provides useful “quizzes” on the work

(Websites accessed June 2008)