Exploring the reactivity trends in the E2 and S_N2 Reactions of X- + CH₃CH₂Cl (X = F, Cl, Br, HO, HS, HSe, NH₂ PH₂, AsH₂,CH₃, SiH₃, and GeH ₃)

The reactivity order of 12 anions toward ethyl chloride has been investigated by using the G2(+) method, and the competitive E2 and S _N2 reactions are discussed and compared. The reactions studied are X- + CH₃CH₂Cl - HX + CH₂=CH₂ + Cl- and X- + CH3CH₂Cl - CH₃CH₂X + Cl^-, with X = F, Cl, Br, HO, HS, HSe, NH₂ PH₂, AsH ₂,CH_3?, SiH₃, and GeH₃. Our results indicate that there is no general and straightforward relationship between the overall barriers and the proton affinity (PA) of X-; instead, discernible linear correlations only exist for the X's within the same group of the periodic table. Similar correlations are also found with the electronegativity of central atoms in X, deformation energy of the E2 transition state (TS), and the overall enthalpy of reaction. It is revealed that the electronegativity will significantly affect the barrier height, and a more electronegative X will stabilize the E2 and S_N2 transition states. Multiple linear regression analysis shows that there is a reasonable linear correlation between E2 (or S_N2) overall barriers and the linear combination of PA of X- and electronegativity of the central atom. © 2009 American Chemical Society.