Curly arrows represent the movement of electrons when bonds are broken, made, or moved. The tail of the arrow shows where the electrons have come from, and the head of the arrow shows where the electrons are going to. When a pair of electrons is moving, a double-headed arrow is used. When only one electron is moving (e.g. in radical reactions), a single-headed arrow is used.
This curly arrow indicates that the pair of electrons that form the A-B bond are leaving A and becoming the exclusive property of B. The products are an A+ cation and a B– anion. The arrow should start in the centre of the bond and the head should lie on atom B itself, because both the electrons will end up in an orbital on the B– ion.
This curly arrow indicates that an electron pair that was the exclusive property of B– is now shared in the bond formed between A and B. Again, the arrow starts at the source of the moving electrons and the head is drawn in the place where the new bond will be formed. The bond is formed between a nucleophile and an electrophile. The nucleophile is B–, which is electron-rich and donates a pair of electrons. The electrophile is A+, which is electron-poor and accepts a pair of electrons.
How to practice
The best way to get to grips with drawing reaction mechanisms is to practice as much as you can. If you just look at a mechanism in a text book or your lecture notes, it’ll make sense at the time (hopefully!) but you probably won’t remember it. It’s important that you test yourself. It’s far more rewarding than just reading and you’ll develop a better understanding of the reactions. To practice drawing curly arrows, go to curlyarrows.org.
It’s quite a self-explanatory website, but if you’re unsure what to do, you can read the instructions here.
If you want to read more about organic mechanisms and curly arrows, a good start would be chapter 5 of Organic Chemistry (second edition) – Clayden, Greeves and Warren.
Acknowledgement: Notes provided by Sarah Piggott, KLabs Student Author