Neutral Molecules Eliminated in the Mass Spectrometer

Although the M-H,0 ion is represented here by a cyclobutane structure, this need not be its actual structure no evidence is available to confirm such structures, but it is felt that new bond formation in the product ion lowers the energy requirements of the reaction. The fact that hydrogen is abstracted from the 8-position has been conclusively danonstrated by extensive deuterium-labeling studies. That is, replacement of the hydrogens on the 5-carbon by deuterium results in an almost-exclusive 

Through what is most certainly a different transition state, the elimination of water is sometimes accompanied by the expulsion of a molecule of ethylene. An M-46 peak thereby results. The transition state in Equation 2.48 may be postulated, although the deuterium-labeling studies to confirm the location from which the hydrogen comes have not been performed. 

Fairly abundant ions appear at miz 84 in both spectra corresponding to M-H2O species. 

The deuterium-labeling experiments indicate that 1,4-eliminations are favored, a fact that is supported by the spectra of the isomeric 4-tert-butylcyclohexanols. Contrary to the previously held belief that a more crowded (generally axial) hydroxy group will be eliminated faster, cw-4-tert-butylcyclohexanol eliminates water more slowly than the trans isomer (Equation 2.52). This would appear to indicate that 1,4-eliminations from a boat conformation are preferred, and that the elimination is indeed cis. 

Rather surprisingly, if the hydroxy group is replaced by a halogen atom, elimination of HX (X = Cl or Br) appears to prefer a five-membered transition state (Equation 2.53). Deuterium-labeling studies with n-butyl and -pentyl chloride demonstrate that abstraction of the hydrogen from the y-carbon is the favored process. There are also indications that cyclic chlorides and bromides eliminate HX by the 1,3-mechanism. 

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