How to help students tackle organic mechanisms – and the internet

Have you ever had a student draw a curly arrow starting from an H⁺? Surely, nobody who had received any teaching in mechanistic chemistry would begin a curly arrow at a species that has no electrons? And yet, this misconception persists, sometimes right into higher education.

So where might it come from? Open up your web browser and search for ‘Brønsted-Lowry’, then click on ‘Images’. The internet is littered with incorrect representations of this fundamental concept in organic chemistry. Sometimes when students find a concept difficult they turn to the internet for help. And what do they see? Bad chemistry!

Through the lens of an expert

Take, for example, the reaction of ammonia with hydrogen chloride, correctly represented above. An expert might describe this process as the donation of a (lone) pair of electrons from the nitrogen atom of ammonia (here acting as a nucleophile) into the antibonding σ* orbital of hydrogen chloride, resulting in the formation of a new N-H bond, with associated breaking of the H-Cl σ-bond. Experts view reactivity as concerning the movement of electrons, and as relating to ‘bond-forming’ and ‘bond-breaking’ events, with a foundation in bonding models like valence bond and molecular orbital theory. The key to understanding these models lies in ideas about the combination of atomic orbitals to give bonding and antibonding orbitals. Students in the later stages of school education don’t have these tools to work with.

Language can help and hinder

Brønsted-Lowry theory defines an acid as a species capable of donating a proton and a base as a species capable of accepting a proton. A quick search throws up many images with incorrect curly arrows with descriptions such as: ‘HCl is donating (or ‘giving’) a proton to NH₃’. This is true, but an unfortunate use of language and easily misinterpreted. If I give you an apple, I hand it over to you, so perhaps it follows that students want to show a ‘handing over’ process with their arrows?

Could it be that the idea that the acid ‘gives’ the proton is the cause of the arrow going away from the acid (or proton)?

When examined only for atom connectivity, the outlined mechanism looks correct. After all, a curly arrow drawn the wrong way can, and often does, lead to the correct connection between atoms. However, in terms of an orbital description of reactivity (and indeed, electron accounting), it makes no sense at all.

Superficially reliable

The reliability of internet sources is something that is included in many PSHE courses, as well as academic subjects. Don’t trust the first source you see, evaluate the author, look for typographical errors and poor language. Some of these sources of bad chemistry look legitimate, and there are lots of them!

Part of the problem is that novice learners are not able to easily distinguish between the answers they find in search engines. When they come across a scheme that looks correct in terms of the motifs used – with representations of atoms, bonds and curly arrows leading to the right connections – but containing fundamental flaws, they don’t have the ability to adequately filter this information. Therefore they are much more likely to just copy what they see.

So how can we prevent our students from making these mistakes, as novice learners in organic chemistry?

• Encourage students to annotate their mechanisms with explanations, linking the background theory with the arrows.
• Teach the movement of curly arrows from first principles before tackling each of the advanced mechanisms (A-level or higher) in turn.
• Use ‘spot the error’ games as scaffolding exercises for students to evaluate representations.
• Provide students with a range of trusted sources and encourage study routines that use these when tackling homework exercises.
• Tell students that mechanisms aren’t just arbitrary representations; they have foundations in bonding theory. This is especially important for students who are progressing to chemistry courses in higher education.