429677-40-7Relevant academic research and scientific papers
Nucleofugality and nucleophilicity of fluoride in protic solvents
Nolte, Christoph,Ammer, Johannes,Mayr, Herbert
experimental part, p. 3325 - 3335 (2012/06/17)
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 k1(25 °C) were found to follow the correlation equation log k1(25 °C) = sf(Nf + Ef), which allowed us to determine the nucleofuge-specific parameters Nf and sf 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 = sN(N + E) to derive the nucleophilicity parameters N and sN for fluoride in various protic solvents. Complete Gibbs energy profiles for the solvolysis reactions of benzhydryl fluorides are constructed.
Kinetics of the reactions of halide anions with carbocations: Quantitative energy profiles for SN1 reactions
Minegishi, Shinya,Loos, Robert,Kobayashi, Shinjiro,Mayr, Herbert
, p. 2641 - 2649 (2007/10/03)
Rate constants for the reactions of Laser flash photolytically generated benzhydrylium ions (diarylcarbenium ions) with halide ions have been determined in various solvents, including neat and aqueous acetonitrile as well as some alcohols. Substitution of the rate constants into the correlation equation log k = s(N + E) yields the nucleophilicity parameters N for the halide ions in different solvents. Linear correlations with negative slopes are found between the nucleophilicity parameters N for Cl- and Br- in different solvents and the solvent ionizing powers Y of the corresponding solvents. Increasing halide solvation reduces the rates of carbocation/chloride combinations by approximately half as much as it increases the rates of ionizations of benzhydryl chlorides. Comparison of the solvent dependent nucleophilicity parameters N of halide anions and the nucleophilicity parameters N1 for solvents yields a quantitative prediction of common ion rate depression, as demonstrated by the analysis of a variety of literature reported mass-law constants α. Combination of the rate constants for the reactions of benzhydrylium ions with halide ions (k-1) reported in this work with the ionization constants of benzhydryl halides (k1) and the recently reported rate constants for the reactions of benzhydrylium ions with solvents (k2) yields complete quantitative free energy profiles for solvolysis reactions. The applicability of Hammond's postulate for interpreting solvolysis reactions can thus be examined quantitatively.
The nature of the transition state in diarylmethyl cation - Nucleophile combination reactions as probed by secondary α-deuterium isotope effects
Van Pham, Thuy,McClelland, Robert A.
, p. 1887 - 1897 (2007/10/03)
Transition-state structures for the carbocation-nucleophile combination reactions of (4-substituted-4′-methoxydiphenyl)methyl cations with water, chloride, and bromide ions in acetonitrile-water mixtures have been investigated by measuring the secondary α-deuterium kinetic and equilibrium isotope effects. Rate constants in the combination direction were measured with laser flash photolysis. Equilibrium constants were measured for the water reaction by a comparison method in moderately concentrated sulfuric acid solutions, for the bromide reaction via the observation of reversible combination, and for the chloride reaction from the ratio of the combination rate constant and the rate constant for the ionization of the diarylmethyl chloride product. The fraction of bond making in the transition state has been calculated as the ratio log (kinetic isotope effect):log (equilibrium isotope effect). For the water reaction, there is 50-65% bond making in the transition state; this is also true for cations that are many orders of magnitude less reactive. The same conclusions, 50-65% bond formation in the transition state independent of reactivity, have previously been made in corre-lations of log kw vs. log KR. Thus, two quite different measures of transition structure provide the same result. The kH:kD values for the halide combinations in 100% acetonitrile are within experimental error of unity. This is consistent with suggestions that these reactions are occurring with diffusional encounter as the rate-limiting step. Addition of water has a dramatic retarding effect on the halide reactions, with rate constants decreasing steadily with increased water content. Small inverse kinetic isotope effects are observed (in 20% acetonitrile:80% water) indicating that carbon-halogen bond formation is rate-limiting. Comparison of the kinetic and equilibrium isotope effects shows ～25 and ～40% bond formation in the transition states for the reactions with bromide and chloride, respectively.
