Curly Arrows Without Fear: Reading Organic Mechanisms
Mechanisms aren't a list of drawings to learn. Every arrow tells the story of an electron pair — learn to read that story and you can draw ones you've never seen.
Students often try to memorise every mechanism in the syllabus as a picture. It works until the exam shows a slightly different molecule. The better approach is to understand what a curly arrow actually means.
An arrow is a moving electron pair
A curly arrow always starts at electrons — a lone pair or a bond — and ends where those electrons go: an atom that can accept them, or the space between two atoms where a new bond forms. If your arrow starts at a positive charge, it's wrong.
Find the electron-rich site, find the electron-poor site, and draw the arrow from rich to poor. Every mechanism is a variation on that one idea.
A checklist for any mechanism
- Label the nucleophile (electron-rich) and electrophile (electron-poor).
- Show partial charges (δ+ and δ−) on polar bonds.
- Draw arrows from the start of the electron pair to its destination.
- Check that charges balance on both sides of every step.
Practise on nucleophilic substitution of halogenoalkanes first, then electrophilic addition to alkenes. You'll notice the same logic everywhere.
