Formation of [M — C2H4]+· in O-d1-butyric acid

The mass spectrum of CH₃CH₂CH₂COOD shows no observable loss of C₂H₃D. This indicates for this compound that either the McLafferty rearrangement is concerted, or that the second step (olefin loss) is very fast in comparison to the reverse transfer of hydrogen back to the methylene radical.