Mass Spectra of Some Chemical Classes

To undergo a McLafferty rearrangement, a molecule must possess an appropriately located heteroatom (e.g., O), a 77 system (usually a double bond), and an abstractable hydrogen atom y to the C=0 system. 

Such rearrangements often account for prominent characteristic peaks and are consequently very useful for our purpose. They can frequently be rationalized on the basis of low-energy transitions and increased stability of the products. Rearrangements resulting in elimination of a stable neutral molecule are common (e.g., the alkene product in the McLafferty rearrangement) and will be encountered in the discussion of mass spectra of chemical classes. 

---