Illustrated Glossary of Organic Chemistry

Stepwise reaction: A chemical reaction whose mechanism includes two or more steps. Compare with a concerted reaction, whose mechanism involves just one step.

The SN1hydrolysis of tert-butyl chloride with water has a stepwise mechanism involving three steps. The reaction is not concerted because all of the bond changes do not occur in the same mechanism step...

Step 1: The carbon-chlorine bond is ionized.

Step 2: The carbocation captures a molecule of water, and the carbon-oxygen bond is formed.

Step 3: The oxonium ion is deprotonated by water. An oxygen-hydrogen bond is broken, and separate oxygen-hydrogen bond is formed.

...whereas the SN2 reaction of methyl chloride with hydroxide ion is a concerted reaction because all bond changes occur in a single mechanism step. The carbon-chlorine bond is broken at the same time the carbon-oxygen bond is formed: