Protonation of C3H6 Isomers: Isotope Exchange and Isomerization

Article abstract of DOI:10.1021/j100211a042

Isotope exchange processes of the type (MH⁺ + C3D6 -> MD⁺ + C3HD5) or (MD⁺ + C3H6 -> MH⁺ + C3H5D) (where C3(H,D)6 is propylene or cyclopropane) have been studied in an ICR spectrometer.It is shown that, for both reactant molecules, the efficiency of the exchange reaction can be correlated with the exothermicity of the formation of the complex C3(H,D)7⁺> from separated M(H,D)7⁺ and C3(H,D)6 reactants.In the case of propylene, all of the M(H,D)⁺ reactants are chosen so that proton (deuteron) transfer is endothermic.For cyclopropane reactant molecules, some of the reactant pairs have available an exothermic channel leading to the formation of a sec-propyl ion product, but this process is shown to compete poorly with the isotope exchange process ( which may or may not involve isomerization of the neutral cyclopropane to propylene in the collision complex).Isotope exchange reactions involving the partially deuterated propylenes CD3CH=CH2, CH3CH=CD2, and CH3CH=CHD provide evidence that, when M is a nitrile, there is statistical scrambling of the H and D atoms in the C3(H,D)7⁺ in the sec-C3(H,D)7⁺> complex; when M is an aldehyde or a formate, the reaction involves only the H and D atoms on the methyl groups of the sec-C3(H,D)7⁺ species in that complex.Experiments with 1,1-C3D2H4 indicate that the isotope exchange process involves a single C(H,D)2 group in the cyclopropane molecule, except when proton transfer is exothermic.Rate constants are reported for a number of proton-transfer reactions involving cyclopropane.