How to draw reaction mechanisms

Reaction mechanisms are drawn using curly arrows to denote the movement of the electrons in each step of the reaction. It is very important that the arrow starts where the electrons are and points towards where the electrons go. A curly arrow with a whole arrowhead represents the movement of an electron pair, whereas a curly arrow with a half arrowhead represents the movement of a single electron, typically in a radical reaction. The two types of curly arrows are shown in figure 1.

Figure 1: The curly arrow with a whole arrowhead (left) represents the movement of an electron pair.
The curly arrow with half an arrowhead (right) represents the movement of a single electron.

Overall there are three different types of curly arrows:

• Arrows that break bonds. The arrows root is drawn to begin in the bond which is broken. The arrowhead points to the atom or part of the molecule which will accept the electron pair from the bond. In the example below, a bromine atom breaks away from tert-butyl bromide. The electrons that made up that bond leaves with the bromine atom, creating a negatively charged bromine ion and a carbocation.

  

• Arrows that make bonds. The arrows root is drawn to begin in the lone-pair or the negative charge of the nucleophile which will attack. The arrowhead is drawn to point at the electrophile where the nucleophile will attack or the new bond which will be formed as a result of the reaction. In the example below, a lone-pair on the oxygen atom of a water molecule attacks the electrophilic carbon atom of the carbocation. The result is a protonated tertiary alcohol.

  

• Arrows that make and break bonds at the same time. The arrows root is drawn to begin in the bond which is being broken and the arrowhead is drawn to point at the electrophile where the nucleophile will attack or the new bond which will be formed. In the example below, the bond between the carbon and hydrogen is being broken, and the electrons from that bond are used to create a new bond between that same carbon atom and the positively charged carbon atom. The result is an alkene and a proton.

  

Notice that for all the reaction mechanisms drawn, charge is conserved, just as when writing reaction schemes. Drawing reaction mechanisms is an accounting system, so no electrons or charges can suddenly appear or disappear.