8.2.2 Product Stabilization by Rearrangement

• A rearrangement may occur to produce a small, stable molecule as the neutral product. Such rearrangements may involve only migration of a hydrogen atom, or more complex species may migrate (such as methyl groups). Example common neutral losses formed in rearrangement decompositions include H$_{\textrm{2}}$O, CH$_{\textrm{4}}$, C$_{\textrm{2}}$H$_{\textrm{4}}$, CO$_{\textrm{2}}$,NH$_{\textrm{3}}$, etc. Such neutrals are more 'more stable' than their radical counterparts, lowering the total product energy.
• A rearrangement may occur during or after an apparent simple cleavage to produce a product of lower energy. These rearrangements may be difficult to discern when trying to identify likely simple cleavage reactions a given structure may show. Consider the fragmentation of n-butane between C1 and C2. Such a simple cleavage, with no rearrangements, may result in the methyl radical and the n-propyl ion (a 1$^{\textrm{o}}$ cation). Rearrangement of a hydrogen atom in the n-propyl ion could yield the i-propyl cation; being a 2$^{\textrm{o}}$, we predict the i-propyl cation to have a smaller energy than n-propyl. In either case, ordinary analytical mass spectrometric data cannot be used to discern these products. An interesting fundamental question that arises is ``Does the H atom migrate during or after the cleavage of the C1-C2 bond?'' {insert figure}
• A good rule of thumb is that the net number of bonds broken (# broken - # formed) in a rearrangement reaction is almost always odd, and usually 1.