Solvolytic Reactivity of 2,4‐Dinitrophenolates

Abstract

A series of X,Y‐substituted benzhydryl 2,4‐dinitrophenolates (DNP, 1–5) were subjected to solvolysis in various methanol/water, ethanol/water, and acetone/water mixtures at 25 °C. The linear free energy relationship (LFER) equation, log k = sf(Ef + Nf), was used to derive the nucleofuge‐specific parameters (Nf and sf) for an SN1‐type reaction. The magnitudes of nucleofugalities (Nf) are around zero, indicating that DNP falls in the middle of the established nucleofugality scale. The slope parameters (sf) and the Grünwald–Winstein mOTs parameters obtained demonstrate that benzhydryl DNPs solvolyze through a late transition state (TS) in which the negative charge delocalization causes considerably diminished solvation. Because of the late TS, the nucleofuge‐specific slope parameters, sf, are relatively high, i.e., the log k vs. Ef plots are steeper than for most of the previously investigated leaving groups. This may lead to intersection of the log k vs. Ef plots that correspond to DNP and to some other leaving groups of similar reactivity, i.e., inversion of the relative reactivities may occur. Such inversion is shown here for DNPs and phenyl carbonates.

Structural effects responsible for stability and solvolytic reactivity of sulfonium ions

“…For example, a very good linear relationship between log k values for solvolysis of the series of benzhydryl carboxylates (heptafluorobutyrate, trifluoroacetate, dinitrobenzoate, p‐nitrobenzoate, acetate) in 80% aq. ethanol and p K a values of the corresponding carboxylic acids has been obtained ( R = 0.997) …”

“…It is a general rule of thumb that a weaker base constitutes a better leaving group (LG). However, relative reactivities can be predicted reliably only for substrates in which variations in the structure of leaving groups are small., This is because the related groups (e.g., those with the same functionality) have similar stabilizing effects in a ground state and in a transition state (TS), so their net relative reactivities are greatly determined with the stability of the LGs. For example, a very good linear relationship between log k values for solvolysis of the series of benzhydryl carboxylates (heptafluorobutyrate, trifluoroacetate, dinitrobenzoate, p‐nitrobenzoate, acetate) in 80% aq.…”

Nucleofugality of Pentafluorophenolate in Various Solvents: Solvolytic Behavior of Phenolates

“…In order to develop a comprehensive leaving group ability scale, which would be limited by using a single type of substrate, [4] an electrofugality scale based on 39 benzhydrylium carbocations that covers 18 orders of magnitude in reactivity has been established. [5] Employing that scale and Equation (1), which represents LFER, abilities of structurally diverse leaving groups have been determined in the range of 12 orders of magnitude in a particular solvent.…”

“…[5] Employing that scale and Equation (1), which represents LFER, abilities of structurally diverse leaving groups have been determined in the range of 12 orders of magnitude in a particular solvent. [3][4][5][6][7] In this equation, the contributions of an electrofuge (alkyl/aryl part of a substrate that in heterolytic process gives carbocation) and a nucleofuge to the overall solvolytic reactivity of a substrate are separated. This approach enables the main practical application of the scales, the estimation of reactivities of substrates constituted from any combination of an electrofuge and a nucleofuge in a given solvent at 25 °C.…”

“…[5] The nucleofuge-specific parameters for the certain combination of a leaving group and a solvent can be determined from the log k vs. Ef correlation line using solvolytic rate constants in a given solvent of a series of benzhydrylium derivatives and the reference Ef values. [3][4][5][6][7] In this LFER model, influences of solvation effects are associated to nucleofugality only, defined by two nucleofuge-specific parameters (Nf and sf), which specify the reactivity of some leaving group in a given solvent. [5] In fact, the Nf parameter determines the basic reactivity of the certain leaving group/solvent combination, while the reaction constant (sf parameter) varies within a limited range and slightly decreases as reactivity of leaving groups increases.…”