How does an intramolecular carbene rearrangement differ from an intermolecular carbene reaction?

In the study of carbene chemistry, the distinction between intramolecular and intermolecular reactions lies in the number of molecular species involved.1

Intramolecular Carbene Rearrangements

Intramolecular reactions, often called rearrangements, occur entirely within a single carbene molecule without the involvement of outside substances. These processes typically lead to the formation of stable molecules where the carbon atom achieves a tetravalent state. A classic example is the rearrangement of methylcarbene into ethylene, which occurs through the shift of a hydrogen atom from one carbon to another. Such rearrangements are structurally limited and cannot occur in simple carbenes like methylene.1

Intermolecular Carbene Reactions

Intermolecular reactions involve the interaction of two or more separate molecules. These reactions are categorized into three primary classes:1
* Dimerization: The combination of two carbene molecules to form an olefin.1
* Addition: The reaction of a carbene with another molecule, such as an olefin or acetylene, to synthesize ring systems like cyclopropanes.1
* Insertion: A process where the carbene is incorporated into another molecular structure.1

While most addition reactions are intermolecular, intramolecular versions can also occur, such as the cyclization of allylcarbene to form bicyclobutane.1