Ethene elimination from CH3CH2NH=CH 2 + : Reaction pathways at the boundary between stepwise and concerted processes

The elimination of ethene from CH₃CH₂NH=CH ₂ ⁺ is characterized by ab initio procedures. This reaction occurs through several asynchronous stages, but without passing through formal intermediates. A potential energy barrier to hydrogen migration from the β carbon to N is largely determined by the energy required to cleave the CN bond, but is lowered slightly by H transfer from the β to the α carbon and then to N. The complex C₂H ₅ ⁺ NH=CH₂] is bypassed, even though that complex could exist at energies only slightly above that of the transition state for ethene elimination. Furthermore, conversion of a substantial reverse activation energy into energy of motion causes CH₂=NH ₂ ⁺ and CH₂=CH₂ to dissociate faster than they can form CH₂=NH ₂ ⁺ CH₂=CH₂]. Comparison of results for CH₃CH₂NH=CH ₂ ⁺ to ab initio ones for methane from CH₃CH₂CH ₃ ⁺ and elimination of ethene from CH₃CH₂O=CH ₂ ⁺ and CH₃CH₂CH=OH⁺ reveals that these dissociations occur in a similar but, in each case, a distinct series of asynchronous steps or stages, and that there is no sharp demarcation between concerted and stepwise eliminations as presently defined. In dissociations of CH₃CH₂NH=CH ₂ ⁺ , loss of electron density at the C in the breaking N bond leads the transfer of electron density to that carbon by migration of a hydrogen from the adjacent C. We attribute this to a requirement for the moving H to be close to Cα before the moving H can start to develop covalent bonding to Cα. It is also concluded that elimination of ethene from CH₃CH₂NH=CH ₂ ⁺ avoids a Woodward-Hoffmann symmetry-imposed barrier by H migrating sufficiently from the β to the α carbon on the way to N, so that the dissociation is essentially a 1,1 rather than a 1,2 elimination.