The consecutive loss of two H2O molecules from protonated 1,2‐ and 1,3‐ cyclohexanediols in the gas phase: An example for the incidence of skeletal rearrangement in chemical ionization mass spectrometry

The consecutive dehydration of protonated molecules [MH]+ of 1,2‐ and 1,3‐cyclohexanediols (cis and trans isomers) by loss of two H2O molecules has been investigated. Analysis of 2H labelled compounds showed that loss of the first H2O molecule represents a simple heterolysis, i.e. a dissociation without exchange of hydrogens between O—H and C—H bonds. Subsequent elimination of the second H2O molecule in the process [MH–H2O]+→[MH–2H2O]+ followed several competing paths. The two major ones corresponded formally (with reference to an intact 6‐ring skeleton) to 1,3‐ and 1,4‐eliminations; in comparison, the alternative 1,2‐elimination is only a minor route at most. At least for the 1,3‐elimination, water loss from the [MH–H2O]+ ions is not direct, but is associated with skeletal rearrangement, most probably of the Wagner‐Meerwein‐type, effecting contraction of the 6‐ to a 5‐membered ring.