Journal article

Publication year: 1995

Pages: 453-461

Journal: Journal of the American Chemical Society

Volume number: 117

Issue number: 1

ISSN: 0002-7863

DOI Link: https://doi.org/10.1021/ja00106a052

Publisher: American Chemical Society

Abstract: 
The reactions of ethane with the nitrosonium cation, a model electrophile, were investigated computationally at the Hartree-Fock as well as the correlated MP2(frozen core) and CISD levels of theory. In agreement with the experimentally observed electrophilic reactions of alkanes, the substitution (yielding protonated nitrosoethane) and C-C bond cleavage pathways (leading to the loss of methane) are competitive energetically. Standard basis sets (6-31G(d), 6-31G(dp), and 6-31+G(dp)) were used for geometry optimizations and triple-zeta plus polarization (TZP, and with the inclusion of diffuse functions: TZP+) for single point energies on the MP2/6-31G(dp) optimized structures. Vibrational frequencies were computed through the MP2/6-31G(d) level. A complex of ethane and the nitrosonium cation forms first [the complexation energy is 4.0 kcal mol(-1) at CISD+Q/TZP+//MP2/6-31G(dp) + ZPVE(MP2/6-31(d))]. Although several plausible pathways leading to nitrosomethane, to nitrosoethane, to loss of hydrogen after addition of NO+ to ethane, as well as insertion of the electrophile into the C-C or a C-H bond of ethane were examined, only two viable pathways were located. These lead to (a) the nitrosomethylene cation and methane through C-C bond cleavage and to (b) protonated nitrosoethane via abstraction of a hydride followed by addition of HNO to the ethyl cation. The computed activation energies of the two reactions are similar (33.5 and 31.1 kcal mol(-1), respectively). We found no evidence for a mechanism involving direct insertion of the electrophile into a C-H or a C-C bond of ethane. Computations of the vibrational frequencies for the H/D isotopically substituted complex and the transition structures yield relatively small primary H/D isotope effects for both mechanisms (2.6 and 2.9, respectively), due to nonlinearity of the transition structures. These results, like those we have reported earlier for methane, do not support the generally discussed mechanisms for the electrophilic substitution of alkanes. Our evidence suggests that some electrophiles may attack carbon directly, rather than C-H or C-C bonds, and that three-center transition structures with the electrophile need not be involved.

Harvard Citation style: Schreiner, P., von Schleyer, P. and Schaefer, H. (1995) The Electrophilic Reactions of Aliphatic Hydrocarbons: Substitution and Cleavage of Ethane by NO+, Journal of the American Chemical Society, 117(1), pp. 453-461. https://doi.org/10.1021/ja00106a052

APA Citation style: Schreiner, P., von Schleyer, P., & Schaefer, H. (1995). The Electrophilic Reactions of Aliphatic Hydrocarbons: Substitution and Cleavage of Ethane by NO+. Journal of the American Chemical Society. 117(1), 453-461. https://doi.org/10.1021/ja00106a052