Rearrangement and Fragmentation Reactions
Sometimes a reaction occurs in which the final product is totally different in structure from the starting materials. This may have resulted from a rearrangement of the carbon skeleton of the original molecule. This chapter considers fragmentation reactions, that is, those reactions in which the molecule falls apart to yield smaller units. The rearrangement reactions are divided by the nature of the principal bond that is being broken in the first part of the reaction, for example, carbon/carbon, carbon/nitrogen or carbon/oxygen. An important and common rearrangement reaction involves the relocation of a double bond in an alkene derivative. Rearrangement reactions that involve the carbon skeleton isomerising usually involve a 1,2-shift, that is, the migrating group, M, goes from the migration origin, A, to the adjacent atom, which is called the migration terminus, B. In the Hofmann rearrangement, when an unsubstituted amide is treated with sodium hydroxide and bromine, the corresponding amine with one fewer carbon atom is produced.