Solvolysis of benzylic chlorides with extended charge delocalization. ?-tert-butyl(2-naphthyl)methyl, 9-fluorenyl and monosubstituted benzhydryl chlorides

Abstract

Rate constants of solvolysis of a-tert-butyl(2-naphthyl)methyl chloride (1), 9-fluorenyl chloride (2) and a series of monosubstituted benzhydryl chlorides (3) in a wide range of solvents were measured. Grunwald-Winsteintype correlation analysis of log k for 2 and 3 against Y BnCl , with or without nucleophilicity N, yielded less satisfactory linear correlations than that against log k(1). A new scale of solvent ionizing power, Y xBnCl , for the correlation of solvolytic reactivities of benzylic chlorides with extended charge delocalization based on log k(1) was developed. Application to the mechanistic study suggested the solvolysis of 2 and 4-nitrobenzhydryl chloride were non-limiting. Hammett plots against ' constants exhibited more negative r values in less nucleophilic solvents. In a benzhydryl chloride containing a strong deactivating substituent, such as 4-nitro, the positive charge delocalizes mainly over the unsubstituted ring in the cationic transition state. The uneven charge distribution was also confirmed by Mulliken population analysis at the level of the RHF/6-31G * //RHF/3-21G

How Fast Do RX Bonds Ionize? A Semiquantitative Approach

“…Nucleofugalities of chloride and bromide in different solvents : Solvolysis rates of benzhydrylium chlorides and bromides have been investigated in a manifold of solvents,7, 11, 12 and the corresponding rate constants are given in the Supporting Information. As depicted in Figure 1, the isopropanolysis rate constants of benzhydrylium chlorides, as well as the corresponding solvolysis rate constants in 90 % aqueous methanol (90M) and 70 % aqueous acetone (70A), correlate well with the corresponding electrofugality parameters E f reported in the preceding article 10.…”

“…methanol (90M), 70 % aq. acetone (70A), and isopropanol7, 11, 12 with the electrofugalities E f of the benzhydrylium ions from ref. 10.…”

“…While Equation (3) was initially assumed to be applicable to all types of S N 1 solvolyses, it was soon realized that deviations from Equation (3) were not only due to nucleophilic solvent participation in the rate‐determining step, but were also due to variable solvation, particularly of alkyl and aryl groups, of the developing carbocations 13–20. As a consequence, a number of Y scales of solvent‐ionizing power have been established,5–7, 14a, 15, 21, 22 each of which was developed in an attempt to render reliable predictions of solvolysis rate constants for structurally related compounds.…”

Sixty Years of the Grunwald–Winstein Equation: Development and Recent Applications

“…43 Consistent with the early finding that substrates with two aromatic rings at the α-carbon are not satisfactorily correlated by a similarity model scale involving only one aromatic ring in this position, 35,36 Liu has also developed similarity model scales using substrates with the required two aromatic rings, these scales are designated as Y xBnX scales. The Y xBnX scales are available for several leaving groups: chloride, 44 bromide, 45 p-nitrobenzoate, 46 and tosylate. 47 Presumably, additional scales would be required for situations with three or more aromatic rings at the α-carbon.…”

Mechanistic Studies of the Solvolyses of Carbamoyl Chlorides and Related Reactions

“…Earlier, this research group had developed Y BnCl [78] and Y xBnCl scales [79] for application to the solvolyses of benzyl derivatives and benzhydryl (or 9-fluorenyl) derivatives, respectively. These scales differ somewhat, but not enormously, from the original Y Cl scale [28,56].…”