A Stereochemical Approach toward a More Detailed Understanding of Electron Impact Induced Elimination Reactions^1,2

Electron impact induced elimination reactions involving the loss of small molecules, such as water and hydrogen chloride from alcohols and chlorides, possess considerable potential for the application of mass spectrometry to stereochemical problems. In the present work the elimination of water and hydrogen chloride has been found to obey identical steric driving forces in acyclic systems following the observation of equivalent propensity for the abstraction of the pro-R over the pro-S hydrogen at C-4 in (S)-2-pentyl alcohol and chloride. This similarity is not extended entirely in cyclohexanol and cyclohexyl chloride. Diastereotopic deuterium labeling has revealed that 1,4 elimination in both compounds is stereospecific while the 1,3 process is only stereospecific in the chloride. Ring opening has been proposed as a prerequisite to 1,3 elimination in cyclohexanol and this hypothesis allows correlation of the apparently dissimilar behavior of cyclic and acyclic alcohols and as well explains such anomalous results as a 90/1 difference for loss of water from the stereoisomers of 4-t-butylcyclohexanol while the derived chlorides exhibit stereoinsensitive loss of hydrogen chloride. Deuterium scrambling is found not to intervene in both low and high voltage measurement in these systems and thus offers a counterpoint to recent examples in ketones. A general theory is proposed connecting site selective hydrogen abstracting elimination reactions to the size of the heteroatom involved and findings on the model compounds studied here suggest that ground-state interatomic distances may be extrapolated to the ionized states in the mass spectrometer and thereby used to predict the positions of hydrogen abstraction.