This tutorial introduces the concept of drawing curved arrows for reaction mechanisms, and explains how they are used to represent the movement of electrons.
Curved arrows are used as a way of showing how electrons move during a reaction. These arrows show how bonds are formed and how they are broken.
Polar reactions involve the movement of pairs of electrons, and radical reactions involve the movement of individual electrons. A double-headed curved arrow is used to show the movement of two electrons (polar reaction), while a single-headed curved arrow shows the movement of one electron (radical reaction):
This tutorial focuses on polar reactions; radical reactions will be discussed elsewhere.
Drawing Curved Arrows for Polar Reactions
Double-headed curved arrows show that two electrons, at the start of the arrow, move to the position at the end of the arrow. An arrow can start from a lone pair or from a bond (σ and π), and can end on an atom (as a lone pair or a σ bond), or on a bond (as a π bond).
Lone pairs and bonds are both made up of two electrons, so double-headed curved arrows represent movement of both electrons in the lone pair or the bond.
Don’t worry about the specific atoms in the following examples; for now, it’s more important to understand how the curved arrow represents the making and breaking of bonds.
If an arrow starts from a lone pair, it can be drawn to:
- Another atom (in the same molecule, or in a different molecule), making a σ bond.
- An adjacent bond, making a new π bond.
If an arrow starts from a bond, it can be drawn to:
- One of the bond atoms, breaking the bond, and placing a lone pair on that atom.
- An adjacent bond, breaking the bond at the start of the arrow, and making a π bond where the arrow ends.
- An (unattached) atom, making a new σ bond. Note that this can either be intramolecular or intermolecular. The bond at the start of the arrow is typically a π bond. While the example below shows a bond being formed between Y and Z, the arrow that is drawn could also show a bond being formed between X and Z.
The examples above show how double-headed curved arrows represent the movement of electron pairs in a reaction mechanism, and how this results in changes to the bonds, lone pairs, and formal charge in a molecule.
An arrow starting on a lone pair can end on another atom (making a σ bond), or on an adjacent bond (making a π bond). An arrow starting on a bond can end on an adjacent atom (breaking the bond and giving the atom a lone pair), on an adjacent bond (breaking the original bond, and making a new π bond), or on an unattached atom (breaking the bond, and making a new σ bond).
The examples above show that polar reactions involve changes in formal charge. The next tutorial, Formal Charges and Mechanisms (Part 1), explains how these changes in formal charge are related to curved arrows showing movement of lone pairs and bonds. After this, Drawing Curved Arrows (Part 2) provides examples, showing how curved arrows are used to represent the mechanisms of simple organic reaction mechanisms.