The oxidation of alcohols
Using a microscale well-plate, students add acidified dichromate(VI) to primary, secondary and tertiary alcohols to observe the difference in their oxidation reactions.
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.
Potassium dichromate(VI) solution, 0.1 M (TOXIC), 2 cm3
Sulfuric acid, 1 M (IRRITANT), 1 cm3
The following alcohols in dropping pipettes:
Ethanol (HIGHLY FLAMMABLE) or Industrial denatured alcohol, IDA (HIGHLY FLAMMABLE, HARMFUL)
Propan-1-ol (HIGHLY FLAMMABLE, IRRITANT)
Propan-2-ol (HIGHLY FLAMMABLE, IRRITANT)
2-Methylpropan-2-ol (HIGHLY FLAMMABLE, HARMFUL), or other tertiary alcohol
Refer to Health & Safety and Technical notes section below for additional information.
Each group of students will need:Measuring cylinder (5 or 10 cm3)
Well-plate (24 wells) – eg Sigma ref: M9655
Plastic dropping pipettes, 6
Health & Safety and Technical notes
Wear eye protection throughout.
Potassium dichromate(VI) solution, K2Cr2O7(aq), (TOXIC) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.
Dilute sulfuric acid, H2SO4(aq), (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.
Ethanol, C2H5OH(l), (HIGHLY FLAMMABLE or HIGHLY FLAMMABLE, HARMFUL if using IDA, Industrial Denatured Alcohol)- see CLEAPSS Hazcard.
Propan-1-ol, CH3CH2CH2OH(l), (HIGHLY FLAMMABLE, IRRITANT) - see CLEAPSS Hazcard.
Propan-2-ol, CH3CH(OH)CH3(l), (HIGHLY FLAMMABLE, IRRITANT) - see CLEAPSS Hazcard.
2-Methylpropan-2-ol, (CH3)3COH(l), (HIGHLY FLAMMABLE, HARMFUL) - see CLEAPSS Hazcard.
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:
e Observe the wells over the next 15 minutes and record any changes you see. Do not put any alcohol into well B2 – this well is used as a control.
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
Chemguide - covers the theory well.
MP Docker - provides useful “quizzes” on the work.
Page last updated on 29 December 2011