Drawing reaction mechanisms with curved arrows

A reaction mechanism is a detailed step-by-step description of how reactants are converted to products. It consists of a sequence of steps showing the making of bonds and the breaking of bonds. It is a way to explain or rationalize how a reaction happens to change a reactant to a product.

When bonds are made or broken, there are movements of electrons from one molecule or a group of atoms to another.

In most reactions you meet in organic chemistry (except radical reactions), the electrons move in a pair (two). We draw the flow of this pair of electrons as a curved arrow like this:

Another important point to note is that these two electrons always move from a place of more electrons (or negatively charged) to a place with less electrons (positively charged):

Here are some common examples, which you meet in reactions you have learned:

Whichever a reaction may be, when drawing a mechanism, remember that a curved arrow means the flow of two electrons, and the electrons always flow from a place of more electrons to a place of less electrons, never the opposite direction.