Learn how to test for various organic functional groups, differentiating between isomers or unknown organic compounds

There are four key functional groups that crop up quite often in organic chemistry tests: halogens (haloalkanes), the double bond (alkenes), the hydroxy group (alcohols) and carbonyls (aldehydes, ketones and carboxylic acids).

Students familiar with the test for halide ions will recognise the overlap with the following test for haloalkanes as shown by Chemistry Channel. A nucleophilic substitution occurs to form an alcohol on heating a mixture of the haloalkane with sodium hydroxide (and an ethanol solvent). The liberated halide ions can be tested as for any inorganic metal halide solution. A white (silver chloride), cream (silver bromide) or yellow (silver iodide) precipitate is formed on adding acidified silver nitrate solution.

The haloalkane precipitation test can also be used to explore the rate of nucleophilic substitution as shown in the following video from FranklyChemistry.

The following video from Chemistry Channel demonstrates two tests for alkenes using bromine water and potassium permanganate respectively. In the first case, the alkene undergoes electrophilic addition by the bromine molecule to form a dibromo organic compound. In the second instance, the manganate is reduced to form an MnO2 suspension while the alkene forms a diol.

This second video from FranklyChemistry looks at how acidified potassium dichromate can be used to distinguish between primary, secondary and tertiary alcohols.

Potassium dichromate can also conveniently be used to distinguish between aldehydes and ketones with only aldehydes being oxidised to carboxylic acids. The accompanying colour change from orange to green is one that students will need to become familiar with. Draw their attention to the labelling of test tubes, as it’s very easy to lose track of which compound is where when carrying out multiple tests. Encourage your students to approach these test tube experiments in an orderly fashion.

A second test to distinguish aldehydes from ketones is the ‘silver mirror’ test using Tollen’s reagent. This video from chemistry edb also demonstrates how to make Tollen’s reagent in the test tube.

And, if two tests weren’t enough, students can also be introduced to using Brady’s reagent (2,4-dinitrophenylhydrazine in a mixture of sulfuric acid and methanol or ethanol).

It would be easy to overwhelm students with the different tests that are available. Spending time with the chemistry involved in each test is likely to be more beneficial than simply learning colour changes and positive/negative results. Why not consider how you can link these tests to the wider specification and reaction mechanisms? Students will find it easier to remember the test for haloalkanes, for example, if they are familiar and comfortable with nucleophilic substitution.

Carboxylic acids are the final functional group for students to test. Before introducing them to the tests, ask students to draw on their knowledge of acids to develop some possible tests. The video below from amritacreate explores three tests: litmus paper, reaction with a carbonate and esterification.

You can view a video of the iodoform experiment here, which can be used to identify ethanal, any methyl ketones or their alcohol precursors. What is unique about this experiment is that it is particularly useful for distinguishing between compounds which cannot be differentiated through other tests. For example, 2-butanone and 3-pentanone both give a negative result with Tollen’s reagent but only 2-butanone will give a yellow precipitate with the iodoform test. You can find a full experimental procedure here.

The following video from SpaceyScience brings all of these tests together, providing a context in which students can plan a sequence of tests – the less the better! – to match six unlabelled organic compounds to a list of the six compounds. This experiment requires a solid understanding of the tests and a logical approach to deduce the identities through positive/negative results and a process of elimination.

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