7525-21-5Relevant academic research and scientific papers
Ionizing power of aprotic solvents
Streidl, Nicolas,Mayr, Herbert
supporting information; experimental part, p. 2498 - 2506 (2011/06/10)
Rate constants for the heterolysis reactions (SN1) of a series of chloro-diarylmethanes in aprotic solvents (dimethyl sulfoxide (DMSO), acetonitrile, carboxamides, etc.) have been determined conductometrically in the presence of amines or triphenylphosphane, which trap the intermediate ion-pairs and suppress ion recombination. The operation of SN2 mechanisms can be excluded because the observed first-order rate constants become almost independent of the nature of the nucleophilic additive when a certain concentration of nucleophile is exceeded. The heterolysis rate constants are used to calculate nucleofugality parameters Nf and sf for chloride in aprotic solvents according to the linear free-energy relationship lgak (25 °C) = sf(Nf + Ef). Ionizing powers YBnCl of these solvents were calculated according to the Winstein-Grunwald equation. Because the heterolysis rate constants in aprotic solvents correlate only poorly with dielectric constants or empirical solvent parameters such as Gutmann's acceptor numbers or ETN values, the most common solvent polarity parameters do not reliably predict ionization rates in aprotic solvents. The kinetics of the heterolyses of chloro-diarylmethanes in aprotic solvents have been studied conductometrically by suppressing ion recombination with amine additives. In this way it became possible to determine the ionizing power of aprotic solvents and compare them to those of protic solvents. Copyright
Suppression of common-ion return by amines: A method to measure rates of fast SN1 reactions
Streidl, Nicolas,Antipova, Anna,Mayr, Herbert
supporting information; experimental part, p. 7328 - 7334 (2010/01/16)
(Chemical Equation Presented) Rate constants for solvolyses of benzhydryl chlorides, which take place on the 10 ms to minute time scale, have been determined in aqueous acetone and acetonitrile by conductometry, using conventional conductometers as well as stopped-flow techniques. Secondary and tertiary amines were used to suppress ion recombination (common-ion return) thus giving access to the ionization rate constants k1. The observed common-ion rate depressions can be rationalized by the correlation equation for electrophile-nucleophile combinations, log k(20 °C) = s(E + N), where electrophiles (here: carbocations) are characterized by the parameter Eand nucleophiles (here: chloride anions and solvents) are characterized by N and s. 2009 American Chemical Society.
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.
Reactions between nitrile oxides and carbenium ions: Synthesis of benzoxazines, oximes, and amides through intramolecular ortho or ipso attack
Auricchio, Sergio,Magnani, Caterina,Truscello, Ada M.
, p. 2411 - 2416 (2007/10/03)
Reactions between nitrile oxides and benzylic carbocations are described. Different results were obtained when the carbocations were generated from the corresponding chlorides with different Lewis acids, with addition products such as benzoxazines, oximes, and amides being produced. Primary, secondary, and tertiary carbocations showed different reactivities. The product ratios strongly depended on the substituents on the aromatic ring of the benzylic carbocations. Evidence for the proposed mechanism is reported. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.
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.
Thermochemical Study of the Addition of Carbenium Ions to Alkenes
Schade, Christian,Mayr, Herbert,Arnett, Edward M.
, p. 567 - 571 (2007/10/02)
Scales of carbenium ion stabilities have been obtained from 1H NMR equilibrium studies of mixtures of partly ionized para-substituted diarylmethyl chlorides in CD2Cl2/BCl3 at -70 deg C and via calorimetric determination of the heats of ionization (ΔHi) of the alkyl chlorides (Ar2CHCl + BCl3 -> Ar2CH+BCl4-) in CH2Cl2/BCl3 at -70 deg C.The heats of the reactions of diarylmethyl tetrachloroborates with 2-methyl-1-pentene (Ar2CH+BCl4- + CH2=C(CH3)C3H7 -> Ar2CHCH2C(CH3)ClC3H7 + BCl3), which were determined by low-temperature calorimetry, increase from -53.1 kJ/mol for (H3CC6H4)2CH+BCl4- to -33.0 kJ/mol for the better stabilized (H3COC6H4)(H3CC6H4)CH+BCl4-.In contrast, the heats (ΔHa) of the Lewis acid catalyzed additions of the corresponding para-substituted diarylmethyl chlorides to 2-methyl-1-pentene are independent of the para substituents (ΔHa=-86.5 +/-2.7 kJ/mol).Similar values of ΔHa were obtained for the addition of p-anisylphenylmethyl chloride to trimethylethylene, styrene, β-methylstyrene, and isoprene. ΔHa is predominantly determined by the conversion of a ?(C=C) into a ?(C-C) bond, and the heats of addition of diarylmethyl tetrachloroborates to alkenes in CH2Cl2 are given by the equation ΔHb=(9.5 +/- 6.3 pKR+) kJ/mol.
Ionisation and Dissociation of Diarylmethyl Chlorides in BCl3/CH2Cl2 Solution: Spectroscopic Evidence for Carbenium Ion Pairs
Schneider, Reinhard,Mayr, Herbert,Plesch, Peter H.
, p. 1369 - 1374 (2007/10/02)
The ionisation equilibria of diarylmethyl chlorides Ar2CHCl (Ar = p-CH3O-C6H4, p-PhO-C6H4, p-CH3-C6H4) reacting with BCl3 in CH2Cl2 to give ion-pairs Ar2CH(+)BCl4(-) (KI), and the dissociation of these (KD) were studied by conductimetry and spectro-photometry.The molar conductivities are almost independent of the nature of the aryl group (ca. 3.5E-3 Sm2/mol at -70 deg C).The ionisation constants KI increase strongly with increasing electron releasing ability of the p-substituents.The standard ionisation enthalpies and entropies for (p-CH3-C6H4)2CHCl and (p-PhO-C6H4)(Ph)CHCl, calculated from the KI at different temperatures are negative.The dissociation constants KD do not show a systematic dependence on the electron donating abilities of the substituents in the aryl groups.Small differences between the UV-vis absorption spectra of unpaired and paired ions were used to confirm the conductimetrically determined values of KD. - Key words: Carbocations / Electrochemistry / Ionisation / Spectroscopy, Visible / Thermodynamics
