Nucleofugality and Nucleophilicity of Fluoride in Protic Solvents

Abstract

A series of p-substituted benzhydryl fluorides (diarylfluoromethanes) were prepared and subjected to solvolysis reactions, which were followed conductometrically. The observed first-order rate constants k(1)(25 °C) were found to follow the correlation equation log k(1)(25 °C) = s(f)(N(f) + E(f)), which allowed us to determine the nucleofuge-specific parameters N(f) and s(f) for fluoride in different aqueous and alcoholic solvents. The rates of the reverse reactions were measured by generating benzhydrylium ions (diarylcarbenium ions) laser flash photolytically in various alcoholic and aqueous solvents in the presence of fluoride ions and monitoring the rate of consumption of the benzhydrylium ions by UV-vis spectroscopy. The resulting second-order rate constants k(-1)(20 °C) were substituted into the correlation equation log k(-1) = s(N)(N + E) to derive the nucleophilicity parameters N and s(N) for fluoride in various protic solvents. Complete Gibbs energy profiles for the solvolysis reactions of benzhydryl fluorides are constructed.

Photogeneration of carbocations: applications in physical organic chemistry and the design of suitable precursors

“…Because of the fast background reaction of the carbocation with the solvent (10 5 s À1 ), however, the reaction of the carbocation with the fluoride anion can only be followed by laser flash photolysis using high concentrations of fluoride anions. [45] Rate constants for the reactions of some nucleophiles, for example, tertiary amines or dialkylsulfides, with carbocations cannot be studied with conventional methods at all. This is the case when the reactions of these nucleophiles with stabilized carbocations are thermodynamically unfavorable, while their reactions with less stabilized carbocations are very fast.…”

“…In 1972, Ivanov et al reported the flash photolytic generation of methoxysubstituted tritylium ions from the corresponding triarylacetonitriles Scheme 3. Reaction of the di(p-anisyl)methyl cation with fluoride in water [45] 0.0 t / µs Figure 1. Decays of the absorbance of the bis[4-(dimethylamino)phenyl] methyl cation at 605 nm after the irradiation of the substituted benzhydryl triphenylphosphonium salt (1 Â 10 À5 M) in the presence of different concentrations of 1,1-dimethyl hydrazine in CH 3 CN at 20 C [51] (Scheme 5(b)).…”

“…[25][26][27][28][29][30][31][32][33] More recently, we have also employed tertiary phosphines for this purpose. [43][44][45] The respective combinations of electrofuges and nucleofuges for these experiments are located in the blue area of Fig. 3.…”

“…Common precursors include alkyl halides R-Hal, [18][19][20][21][22][23][24] acetates R-OAc, [24][25][26][27][28][29][30][31][32][33] aryl ethers R-OAr, [24][25][26][27][28][29][30][31][32][33] ammonium salts R-NR 0 3 + , [18,34,37] and phosphonium salts R-PR 0 3 + . [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52] For the sake of simplicity, neutral and charged precursors will both be denoted as "R-PLG", and the corresponding anionic (X À ) and neutral (X) photo-leaving groups will be denoted as "PLG À " in the following.…”

“…[24] In recent years, we have employed different precursors in laser flash photolysis experiments for the purpose of generating carbocations and studying their reactivities in bimolecular reactions on the 10 ns to 1 ms time scale. [21][22][23][30][31][32][33][34][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][68][69][70][71][72] Whereas the original papers mostly focus on the chemistry of the photogenerated carbocations, this review article summarizes those aspects of our work that are relevant for the rational design of precursors for the photogeneration of carbocations and offers some guidelines for the use of laser flash photolysis in kinetic experiments.…”