Teaching curly arrows at post-16

Students aged 11–14 see chemical processes as the rearrangement of elements. At 14–16, they start to see chemical processes as the rearrangement of atoms. However, post-16 students need to change their perspective to see these processes as the rearrangement of electrons.

For organic mechanisms, we use curly arrows to show the stepwise rearrangement of electrons in a reaction. Understanding mechanisms allows synthetic organic chemists to optimise reactions, for example to get predominantly the E or Z isomer around a double bond.

What students need to know

Hidden underneath the list of reagents, reaction conditions and things to recall, organic chemistry can be understood by applying a few fundamental principles about how electrons rearrange. For many reactions that students encounter, the reactant either accepts a new dative bond or makes one.

• Reactions are described in terms of electron movement. The movement of pairs of electrons are shown using curly arrows.
• Nucleophiles form a new (dative) bond to which they contribute both electrons. Nucleophiles mostly have a lone pair; some are negatively charged. Examples of nucleophiles are H₂O, OH^–, NH₃, ROH and CN^–. The curly arrow starts from the nucleophile.
• Electrophiles can accept a pair of electrons in the form of a new dative bond. Curly arrows go to electrophiles, which are sometimes positively charged or have a weak bond, like Br₂, HCl, H⁺ and NO₂⁺.
• Free radicals have an unpaired electron that tends to be quite reactive and contribute a single electron to a new bond, like Br∙, Cl∙ and ∙O∙.

Threshold concept

To understand an organic reaction and predict the products and possible side reactions, we need to understand the stepwise rearrangement of electrons and the electronic structure of any intermediates.

Where students go wrong

Students often apply the wrong kind of thinking to organic mechanisms, trying to recall curly arrow mechanisms rather than analyse, understand and apply the concepts of electron movement. Some recall is involved, because students need to know whether reagents are nucleophiles or electrophiles, and if they can recall the reaction product then that makes writing the mechanism easier.

Ideas for the classroom

• Focus on ions like OH^–, NH₄⁺ and CN^– and reagents such as NH_3.
• Revisit molecular shapes to raise awareness of lone pairs.
• Ask students to show all the implicit electrons in the structures of reactants.
• Show free radical substitution mechanisms through fully drawn dot and cross diagrams (see diagram).
• Emphasise how many electrons from the reagent contribute to the new bond.

There is great benefit derived from dual coding – putting curly arrows into words. Encourage students to talk about where the pair of electrons started and where they ended up. Avoid language like ‘the nucleophile attacks’ and instead use ‘the pair of electrons that started as a lone pair on the oxygen forms a new dative bond to the carbon’.

Assessment

It can be revealing to ask for mechanisms from past topics alongside a current topic; if students try to simply recall mechanisms, they will find this very challenging because this information will be abstract and not rely on understanding underlying concepts.

Ready for more tips on teaching curly arrows? Read End curly arrow anxiety and download the worksheets.