Which of the following reaction types involves the formation of a new π bond?

The formation of a new π bond is characteristic of elimination reactions. In these reactions, two atoms or groups are removed from a molecule, which typically results in the formation of a double bond (a new π bond) between adjacent carbon atoms.

For example, when an alcohol undergoes dehydration, water is eliminated, leading to the formation of an alkene where a new π bond is created between the two carbons that were previously bonded to the hydroxyl group and the leaving group. This results in a compound with a double bond, illustrating the essence of elimination reactions.

In contrast, addition reactions involve the breaking of a π bond to add new atoms or groups to a molecule, substitution reactions involve the replacement of one atom or group with another without forming new π bonds, and hydrogenation, a type of addition reaction, involves the addition of hydrogen across a double bond, resulting in the saturation of that bond rather than the formation of a new π bond.