SN2 versus E2 Competition of F- and PH2- Revisited.

Abstract

We have quantum chemically analyzed the competition between the bimolecular nucleophilic substitution (SN2) and base-induced elimination (E2) pathways for F– + CH3CH2Cl and PH2– + CH3CH2Cl using the activation strain model and Kohn–Sham molecular orbital theory at ZORA-OLYP/QZ4P. Herein, we correct an earlier study that intuitively attributed the mechanistic preferences of F– and PH2–, i.e., E2 and SN2, respectively, to a supposedly unfavorable shift in the polarity of the abstracted β-proton along the PH2–-induced E2 pathway while claiming that ″...no correlation between the thermodynamic basicity and E2 rate should be expected.″ Our analyses, however, unequivocally show that it is simply the 6 kcal mol–1 higher proton affinity of F– that enables this base to engage in a more stabilizing orbital interaction with CH3CH2Cl and hence to preferentially react via the E2 pathway, despite the higher characteristic distortivity (more destabilizing activation strain) associated with this pathway. On the other hand, the less basic PH2– has a weaker stabilizing interaction with CH3CH2Cl and is, therefore, unable to overcome the characteristic distortivity of the E2 pathway. Therefore, the mechanistic preference of PH2– is steered to the SN2 reaction channel (less-destabilizing activation strain).