Mn. Glukhovtsev et al., GAS-PHASE IDENTITY S(N)2 REACTIONS OF HALIDE ANIONS AND METHYL HALIDES WITH RETENTION OF CONFIGURATION, Journal of the American Chemical Society, 118(45), 1996, pp. 11258-11264
High-level ab initio molecular orbital calculations at the G2(+) level
of theory have been carried out on the identity front-side nucleophil
ic substitution reactions with retention of configuration, X(-)+CH(3)X
, for X=F, Cl, Br, and I. Overall gas-phase barrier heights do not sho
w a strong variation with halogen atom and are calculated to be 184.5
(X=F), 193.8 (X=Cl), 178.9(X=Br), and 171.4 kJ mol(-1) (X=I), substant
ially higher than the corresponding barriers for back-side attack (-8.
0 for X=F, 11.5 for X=Cl, 5.8 for X=Br, and 6.5 kJ mol(-1) for X=I). T
he difference between the overall barrier for back-side attack and fro
nt-side attack is smallest for X=I (164.9 kJ mol(-1)). Central barrier
heights for front-side attack decrease in the following order: 241.0
(X=F), 237.8 (X=Cl), 220.0 (X=Br), and 207.4 kJ mol(-1) (X=I). The min
imum energy pathways for both back-side and front-side S(N)2 reactions
are found to involve the same ion-molecule complex (X(-)... H(3)CX),
with the front-side pathway becoming feasible at higher energies. Inde
ed, our results suggest that the chloride exchange in CH3Cl, which has
been found in gas-phase experiments at high energies, may be the firs
t example of a front-side S(N)2 reaction with retention of configurati
on at saturated carbon. Analysis of our computational data in terms of
frontier orbital theory suggests that elongation of the bond between
the central atom and X could be a significant factor in decreasing the
unfavorable nature of the front-side S(N)2 reaction with retention of
configuration in going from X=F to X=I. Ion-molecule complexes CH3-X
... X(-), which may be pre-reaction complexes in direct collinear halo
philic attack, were found for X=Br and I but not for X=F and Cl. The c
alculated complexation energies (Delta H-comp) for halophilic complexe
s are considerably smaller (7.3 and 19.4 kJ mol(-1) for X=Br and I, re
spectively) than those for the corresponding pre-reaction complexes fo
r S(N)2 attack at carbon (41.1 and 36.0 kJ mol(-1) for X=Br and I, res
pectively). Nucleophilic substitution reactions at the halogen atom in
CH(3)X (X=F-I) (halophilic reactions) are highly endothermic and appe
ar to represent an unlikely mechanistic pathway for identity halide ex
change.