Help students draw curly arrows in unfamiliar reactions faced in exams, not just ones they’ve learned

Chemistry students encounter between five and ten different types of arrow through their education. They all have different meanings, but how many of us actually point out to pupils explicitly what each arrow represents?

The curly or curved arrow, used as a model to represent the movement of electrons in organic reaction mechanisms, is a key concept for advanced students in schools and universities. Curly arrows actually have no basis in evidence beyond some basic kinetic studies. For pupils who may go on to study chemistry beyond school it is useful to stress that electrons are rarely as neatly packaged and definable as the arrow model suggests. They should not consider a curved arrow as a trajectory.

Pupils from 16–18 preparing for their exams tend to learn the arrows as patterns or pictures without appreciating the underlying principles. While this will lead to some success, pupils can no longer rely on this strategy as exam boards get increasingly creative in their question design. For example, in 2014 the awarding organisation AQA asked A-level candidates to use their knowledge of organic reaction mechanisms to complete a mechanism by drawing two curly arrows. The mechanism given was not one they studied, so candidates couldn’t simply recall arrows from a memorised pattern. Such questions are good discriminators between candidates so expect to see more of them.

In a 2014 A-level exam students had to apply their knowledge of mechanisms to this reaction they had not seen before

There is understandable anxiety about drawing curly arrows in exams but clear guidance is given. Exam boards have sections in their mark schemes dedicated to what is and isn’t allowed. One thing that isn’t mentioned is the curve of the arrow. Theoretically, a candidate could draw a straight arrow, or even a loop-the-loop and still be credited, not that such representations should be encouraged.

Curly arrows in a mechanism

According to exam mark schemes, how curved a curly arrow is doesn’t matter, athough the above representations shouldn’t be encouraged

Mark scheme appendices will have several pictorial examples of wrong arrows. They are worth photocopying and sharing with pupils so they know what not to do as well as the correct approach.

More teaching tips and resources for curly arrows

  • Set mechanism exercises that use unfamiliar examples and rely on good knowledge of electron movements. Download this set of exercises (MS Word or pdf) and answers (pdf) as an example.
  • Check that pupils are making accurate copies of mechanisms given on the board. If necessary give them perfect copies (once they have had several attempts themselves).
  • Get pupils into the habit of annotating molecules with the significant dipoles and lone pairs. This will help focus their thinking.
  • Probe pupils’ understanding of what they have drawn by asking open questions. ‘Tell me about this arrow…’ is a good one.
  • Consider approaching organic mechanisms from first principles, outlining key terms such as nucleophile and electrophile.
  • Get students to mark their own work using a talking mark scheme; these are videos of experts explaining how to get the correct answer and are a great way of supporting focused independent study. A set of free resources around organic mechanisms with talking mark schemes is available from the University of Southampton. 
  • Get pupils to take a short video of themselves approaching a mechanism question and justifying their thinking as part of their homework. This also promotes oracy.