In this experiment you will be testing various alcohols to see whether they can be oxidised by a solution of acidified potassium dichromate.

Class practical

Using a microscale well-plate, students add acidified dichromate(VI) to primary, secondary and tertiary alcohols to observe the difference in their oxidation reactions.

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 Chemicals

Eye protection

Each group of students will need:

Measuring cylinder (5 or 10 cm3)

Well-plate (24 wells) – eg Sigma ref: M9655

Glass rod

Plastic dropping pipettes, 6

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.

Health & Safety and Technical notes

Read our standard health & safety guidance

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. 

Procedure

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:

Well number:

Alcohol

A1

Ethanol

A2

Propan-1-ol

A3

Propan-2-ol

A4

2-methylpropan-2-ol

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.

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, July 2016 

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

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Page last updated July 2016