Teaching optical isomers

I’ve just started to teach this topic to my students. It doesn’t take too long but I do find it an enjoyable topic to teach.

But how do you teach it?

I always warn the students that the topic is jargon heavy – the words / phrases used in the topic such as  chiral, racemic, racemate, optically active are all pretty specific to optical isomers and are not used outside of the topic.

In my teaching I have found that students understand the concept but sometime get held back with these technical terms.

I launch the topic by talking about hands – biologically they are the same for us all. In other words, four finders, two thumbs, five finger nails and so on. Then I ask the question are they identical. Most students will reply yes at this point.

I then ask them how they would describe their hands to an alien, something that has never seen humans before. Most go down the path mentioned above, ie, four fingers, two thumbs and so on. What they ALL forget to mention is that the hands are actually mirror images of each other.

When this is pointed out they usually disagree and say they are the same – this is when the idea of non superimposable is introduced. That usually keeps them quiet!

I then tend to go on an cover the technical side of things, drawing molecules, making molecular models introducing the technical terms above before introducing the idea of optical activity and a polarimeter. Most students seem to understand this but when they are challenged to explain the idea to the others, they do struggle.

Finally I bring in two real world examples – the first one, thalidomide, is one I am sure you have used yourself. But the other other I use is about ibuprofen. It turns out that ibuprofen contains a chiral carbon. It is worth getting students to make models of this to see the chiral carbon.

And only half of the enantiomers have the desired effect.

By Derrick Coetzee (User:Dcoetzee) – Bottle of Ibuprofen tablets with cap removed and tablets in frontUploaded by Dcoetzee, CC0, https://commons.wikimedia.org/w/index.php?curid=24740729

However, ibuprofen can be bought very cheaply in the UK (and all around the world). This because the other enantiomer is not removed as is often thought but is left in the tablet. However, in contrast to thalidomide the other enantiomer is harmless to the body.

How do you teach this topic? I’d love to hear any techniques or methods you have, so please post your ideas below.

 

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