High-level ab initio molecular orbital calculations at the G2(+) level of theory have been carried out for the identity nucleophilic substitution reactions at saturated nitrogen, X– + NH2X → XNH2 + X–, for X = F, Cl, Br, and I, and the results compared with data for the analogous reactions at saturated carbon, X– + CH3X → XCH3 + X–. Central barriers ΔH‡ cent for substitution at nitrogen are found to lie within a relatively narrow range, decreasing in the following order: Cl (58.5 kJ mol–1) ≥ F (58.2 kJ mol–1) > Br (46.9 kJ mol–1) > I (39.1 kJ mol–1). They are surprisingly similar to those for substitution at carbon, the barriers at nitrogen being slightly higher than the corresponding barriers at carbon for X = F and Cl and slightly lower for X = Br and I. The overall bathers relative to the reactants (ΔH‡ ovr) are negative for all halogens: –55.8 (F), –9.3 (Cl), –13.7 (Br), and –10.9 kJ mol–1 (I), in contrast to the analogous reactions at carbon where the overall barrier is negative only for X = F. This suggests that nucleophilic substitution is likely to be more facile at nitrogen than at carbon. Stabilization energies of the ion-molecule complexes (ΔHcomp) decrease in the order F (114.0 kJ mol–1) > Cl (67.8 kJ mol–1) > Br (58.4 kJ mol–1) > I (50.0 kJ mol–1) and are found to correlate well with halogen electronegativities.
|Number of pages||7|
|Journal||Journal of the American Chemical Society|
|State||Published - 1 Jan 1995|
ASJC Scopus subject areas
- Chemistry (all)
- Colloid and Surface Chemistry