Solvolytic reactivity of heptafluorobutyrates and trifluoroacetates

A series of X,Y-substituted benzhydryl heptafluorobutyrates (1-6-HFB) and trifluoroacetates (1-6-TFA) were subjected to solvolysis in various methanol/water, ethanol/water, and acetone/water mixtures at 25 degrees C. The LFER equation log k = s(f)(E(f) + N(f)) was used to derive the nucleofuge-specific parameters (N(f) and s(f)) for S(N)1-type reaction. In comparison with TFA, the HFB leaving group is a better nucleofuge for less than 0.5 unit of N(f). X,Y-Substituted benzhydryl trifluoroacetates solvolyze by way of S(N)1 reactions unless electron-withdrawing groups are attached to aromatic rings. In such cases the substrates solvolyze faster than predicted for the S(N)1 route because of the change in mechanism. X,Y-Substituted benzhydryl heptafluorobutyrates examined here (E(f) > or = -7.7) solvolyze according to the S(N)1 pathway. The almost parallel log k vs. E(f) lines in various solvents for HFBs and TFAs, and the corresponding slope parameters (s(f) are in the range of 0.91 and 0.83), indicate early TS with moderately advanced charge separation. NBO charges of HFB and TFA anions and the affinities obtained, all calculated at the PCM-B3LYP/6-311+G(2d,p)//PCM-B3LYP/6-311+G(2d,p) level, revealed that the HFB anion slightly better delocalizes the developing negative charge than TFA, and that the affinity of the benzhydrylium ion is slightly larger toward TFA than toward the HFB anion, which is in accordance with the greater solvolytic reactivity of HFB.