Kinetics and mechanisms of nucleophilic displacements with heterocycles as leaving groups. Part 15. Dependence of mechanism of solvolysis of pyridinium cations on solvent character: evidence from rates, ρ* values, and product analysis

Abstract

ρ* Values are reported for solvolyses of a series of 1-(s-alkyl) pyridinium cations in 2,2,2-trifluoroethanol or 1,1,1,3,3,3-hexafluoropropan-2-ol as solvents in the absence of nucleophiles. The corresponding reactions with pyridine, piperidine, or morpholine as nucleophile in chloroform, acetonitrile, 2,2,2-trifluoroethanol, or 1,1,1,3,3,3-hexafluoropropan-2-ol are kinetically first-order (independent of nucleophile concentration); these allow ρ* to be calculated. ρ* Values correlate with solvent E_T Values except for pentanol and acetic acid, in which cases they are smaller than expected.

Product analysis discloses rearrangement products (derived by hydride shifts) only for solvolysis in non-nucleophilic solvents (trifluoroacetic acid and 1,1,1,3,3,3-hexafluoropropan-2-ol) in the absence of added nucleophilic solvents. The results are interpreted in terms of (a) solvent-assisted solvolysis in nucleophilic solvents, (b)S_N1 cleavage of the C–N bond in the rate-determining step both for the unimolecular reactions in the presence of external nucleophiles, and for solvolysis in 2,2,2-trifluoroethanol, and (c) rate-determining ion-molecule pair dissociation for solvolysis in trifluoroacetic acid or 1,1,1,3,3,3-hexafluoropropan-2-ol.