What is the characteristic feature of a substitution reaction?

A substitution reaction is characterized by the replacement of one functional group or substituent in a molecule with another. In this process, a new atom or group is introduced while an existing atom or group leaves, effectively "swapping" places. This can typically happen to alkyl halides, alcohols, and other organic compounds, allowing for diverse modifications in the molecular structure.

For example, when chloromethane (CH3Cl) undergoes a substitution reaction with sodium hydroxide (NaOH), the chlorine atom is replaced by a hydroxyl group (OH) to form methanol (CH3OH). This demonstrates the fundamental nature of substitution reactions—altering the molecular structure by exchanging specific groups.

While breaking bonds is a part of substitution reactions, as bonds must be broken to allow the new groups to take their place, it is the overall exchange that defines the reaction as a substitution rather than just bond-breaking or bond-forming. The involvement of a catalyst can occur in many reactions, but it is not a defining characteristic of substitution reactions specifically. Additionally, substitution reactions do not result in the formation of double bonds; that phenomenon is more typical of addition reactions, which involve the addition of atoms across a double bond rather than the replacement of one