Hydrogen interchange prior to the fragmentation of protonated molecules

Protonated ethanol shows extensive interchange of hydrogen between the carbons and oxygen prior to fragmentation to either [C2H5]++H2O or to [H3O]++C2H4. Protonated isopropanol shows only a small amount of interchange of hydrogens between carbon and oxygen prior to fragmentation, while protonated alkyl amines show practically no interchange of hydrogens between carbon and nitrogen prior to fragmentation. These results, and earlier results (Terlouw et al., Org. Mass Spectrom. 21, 665 (1986)) showing hydrogen interchange between carbon and oxygen in protonated methyl isopropyl ether, arc discussed in relation to the energetics for fragmentation of the protonated species [RHXH]+. It is concluded that as long as the proton affinity of XH is greater than the proton affinity of the olefin R by no more than ∼50 kJ mol−1 hydrogen interchange between the alkyl group and the heteroatom will occur.