Étude du Comportement D'Isomères Géométriques Sous L'Impact Électronique IV. Cyclopropane, méthylcyclopropane et diméthylcyclopropanes gem, cis et trans

Abstract

AbstractMass spectra of cyclopropane, methylcyclopropane, 1,1‐dimethylcyclopropane, cis‐1,2‐ and trans‐1,2‐dimethylcyclopropane have been recorded in the electron energy range 50‐10 volts, especially in order to study the effect of isomerism in the three dimethyl compounds.Surprisingly, only a slight effect of position isomerism shows up in 1,1 and 1,2 isomers and no effect of geometrical isomerism can be detected in cis and trans stereoisomers, contrarily to the published data. The identity of behavior of cis and trans isomers under electron impact seems to be related to the pseudoethylenic character of C‐C bonds in C3 ring which, in the present case, overcomes the expected steric effect observed in larger cyclanic isomers.Ionization potential of the three isomers and appearance potentials of main fragment ions are found to be the same within experimental uncertainties. Metastable transitions in the three isomers being identical, corresponding types of fragmentation are also the same. Fragment ions have therefore to be produced by a common molecular ion whose most probable structure is an isopentanic chain magnified image. Fragility of C‐C bonds close to substituting alkyl groups in branched hydrocarbons is confirmed by the present study of methylcyclopropane and gem‐dimethylcyclopropane.