In a substitution reaction, what happens?

• A. A leaving group is replaced by a nucleophile or radical
• B. A polymer is formed
• C. A double bond is created
• D. Water is added to the molecule

Answer: (A)

In a substitution reaction, a leaving group leaves the molecule and is replaced by another atom or group, usually a nucleophile or a radical. Mechanistically, this replacement can happen in two main ways: SN2, where a nucleophile attacks and the leaving group leaves in one concerted step, and SN1, where the leaving group first forms a carbocation and then the nucleophile attacks. The key idea is that the atom or group that was attached at that carbon is swapped for the incoming nucleophile or radical, giving a product where the leaving group has been replaced.

This is different from the other scenarios listed. Forming a polymer is polymerization, not substitution. Creating a double bond results from elimination or addition reactions. Adding water to a molecule is hydration, not substitution.