Teaching functional groups: how to distinguish between isomeric alcohols

This is an effective practical procedure to illustrate how simple chemical tests can distinguish between primary, secondary and tertiary alcohols.  One of the tests is the iodoform reaction, which has not been on examination specifications for a while.  We’ve made this video in case you’re not familiar with it.

What are the merits of doing the practical first to establish theoretical concepts, or doing the theory first so that the practical makes sense?

Have you considered doing the practical on a microscale to save on chemicals and increase safety?

The iodoform reaction is relevant to aldehydes and ketones, but this resource suggests it’s all about alcohols – so when would it be used?

• The title of the resource could be a little misleading because students would ideally need to have covered aldehydes and ketones if they’re going to appreciate the significance of the iodoform reaction. This is usually taught after they’ve been introduced to alcohols. It would be a good way for them to revise the relationship of alcohols to aldehydes and ketones.
• The iodoform reaction has not been on examination specifications for a few years now; that just means students don’t need to answer exam questions on it.  It’s still an interesting reaction.

What other related practicals are there?

• There are other characteristic reactions which aldehydes and/or ketones undergo; this exercise could form an introduction to these. Common examples are the silver mirror test (using Tollen’s reagent), Fehling’s test, and the reaction with acidified 2,4-dinitrophenylhydrazine (Brady’s reagent).  

What is special about the iodoform reaction?

• It’s a reaction in which iodoform (or triiodomethane, to use its systematic name), a yellow crystalline product, is formed in the presence of certain compounds. Basically these are ethanal, any methyl ketones or their alcohol precursors. The first two of these contain the acetyl group, CH₃C=O.

How could we approach this practical to maximise engagement?

• This depends on what students already know and understand. For example, if they have good theoretical knowledge already, you could make this a task in which the three alcohols are not labelled and they have to identify which is which.  

Could we do this as a demonstration instead?

• If students are less confident with handling chemicals and equipment then doing this as a demonstration would be a good idea. Showing them the two different reactions first, performed by you as the expert, would be good modelling of skills. Demonstrations like this also save time and chemicals.

The resource says a risk assessment is required – what exactly does this involve?

• Risk assessment basically consists of three steps: identify the hazards, consider the risk of them causing harm, and if necessary take action to minimise this risk. Although the quantities used in these reactions are small, there is a risk of damage to eyes from some of the chemicals, in particular 2 mol.dm^–3 sodium hydroxide solution.