The McLafferty rearrangement is an organic reaction seen in mass spectrometry. A mass spectrometer breaks apart the molecule being studied. The molecule breaks apart in consistent ways that chemists can predict. Most of the time, a carbon-carbon bond breaks and the atoms do not jump across the break between the fragments. The McLafferty rearrangement is an example of a hydrogen atom jumping to the other fragment as a part of the process of the bond breaking. It happens in an organic molecule containing a keto-group.