We have quantum chemically analyzed the competition between the bimolecular nucleophilic substitution (S_N2) 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 S_N2, 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 S_N2 reaction channel (less-destabilizing activation strain).

Maxime Grasser

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