SN2 versus E2 Competition of F–and PH2–Revisited

We have quantum chemically analyzed the competition between the bimolecular nucleophilic substitution (S N 2) and base-induced elimination (E2) pathways for F - + CH 3 CH 2 Cl and PH 2 - + CH 3 CH 2 Cl 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 PH 2 - , i.e., E2 and S N 2, respectively, to a supposedly unfavorable shift in the polarity of the abstracted β-proton along the PH 2 - -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 CH 3 CH 2 Cl 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 PH 2 - has a weaker stabilizing interaction with CH 3 CH 2 Cl and is, therefore, unable to overcome the characteristic distortivity of the E2 pathway. Therefore, the mechanistic preference of PH 2 - is steered to the S N 2 reaction channel (less-destabilizing activation strain).