The carbanion mechanism of olefin-forming elimination. VIII. Theoretical analysis of substituent and isotope effects in proton transfers from 2,2-Diaryl-1,1,1-trichloroethanes

1976 ◽  
Vol 29 (4) ◽  
pp. 787 ◽  
Author(s):  
DJ McLennan ◽  
RJ Wong
Keyword(s):  
Proton Transfer ◽  
Rate Constants ◽  
Isotope Effects ◽  
Reaction Series ◽  
Free Energy Of Activation ◽  
Proton Transfers ◽  
Parabolic Relationship ◽  
Equilibrium Data ◽  
Pass Through ◽  
Experimental Values

Rate constants for the dehydrochlorination of the title compounds by anionic bases in alcoholic solvents are those for simple proton transfer. Rate and estimated equilibrium data for these reactions can be approximately analysed in terms of the Marcus theory of proton transfer. In one reaction series comprising the reactions of para-substituted Ar2CHCC13 compounds with NaOMe in MeOH, the work term (wr) and the intrinsic free energy of activation (ΔG?) are comparable and large. The change in kH/kD with variation in substituent can be quantitatively rationalized, and the fact that kH/kD does not pass through a maximum even though ΔpK = 0 within the series can be understood. Although the required rate-equilibrium parabolic relationship for a second series involving the reactions of (p-ClC6H4)2CHCCl3 with a series of bases is obtained, anomalous Marcus parameters are derived, and reasons for the anomalies are given. A revised set of approximate pKa values for Ar2CHCCl3 compounds is presented, and more precise experimental values of rate constants for the reactions of (p-NO2C6H4)2CHCC13 and (p-NO2C6H4)2CDCCl3 with NaOMe in MeOH are provided.

Isotope effects and activation parameters for the proton transfer reaction from 1-(4-nitrophenyl)-1-nitroethane to free anions and ion pairs of cesium n-propoxide in n-propanol solvent

1986 ◽  
Vol 64 (6) ◽  
pp. 1021-1025 ◽  
Author(s):  
Arnold Jarczewski ◽  
Grzegorz Schroeder ◽  
Przemyslaw Pruszynski ◽  
Kenneth T. Leffek
Keyword(s):  
Proton Transfer ◽  
Rate Constants ◽  
Isotope Effects ◽  
Ion Pairs ◽  
Activation Parameters ◽  
Solvation Shell ◽  
Ion Pair ◽  
Initial State ◽  
Free Ion ◽  
Deuteron Transfer

Rate constants for the proton and deuteron transfer from 1-(4-nitrophenyl)-1-nitroethane to cesium n-propoxide in n-propanol have been measured under pseudo-first-order conditions with an excess of base for four temperatures between 5 and 35 °C. Using literature values of the fraction of cesium n-propoxide ion pairs that are dissociated into free ions, separate second-order rate constants for the proton and deuteron transfer to the ion pair and to the free ion have been calculated. The cesium n-propoxide ion pair is about 2.8 times more reactive than the free n-propoxide ion. The primary kinetic isotope effects for the two reactions are the same (kH/kD = 6.1–6.3 at 25 °C) within experimental error. The enthalpy of activation is smaller for the ion-pair reaction and the entropy of activation more negative than for the free-ion reaction. For proton transfer, ΔH±ion pair = 8.3 ± 0.2 kcal mol−1, ΔH±ion = 9.6 ± 1.0 kcal mol−1, ΔS±ion pair = −12.3 ± 0.6 cal mol−1 deg−1, ΔS±ion = −10.1 ± 3.4 cal mol−1 deg−1. The greater reactivity of the ion pair relative to the free ion is interpreted in terms of the weaker solvation shell of the ion pair in the initial state.

Identity-reaction proton transfers from oxygen acids yielding localized vs. delocalized conjugate bases. An ab initio study

1999 ◽  
Vol 77 (5-6) ◽  
pp. 810-816 ◽  
Author(s):  
James E Van Verth ◽  
William H Saunders, Jr.
Keyword(s):  
Proton Transfer ◽  
Ab Initio ◽  
Gas Phase ◽  
Heavy Atom ◽  
Proton Donor ◽  
Proton Transfers ◽  
Straight Line ◽  
Large Positive Deviation ◽  
Experimental Values ◽  
Conjugate Bases

Identity-reaction proton transfers from a series of oxygen acids to the corresponding conjugate bases have been studied by ab initio methods at the MP2/6-31+G*//MP2/6-31+G* level. The acids are H3O+, CH3OH2+, CH2 = OH+, HC(O)OH2+, CH2 = CHOH2+, H2O, CH3OH, HOOH, HOCH2OH, FOH, FCH2OH, HC(O)OH, and CH2 = CHOH. Gas-phase acidities were calculated at the G2(MP2) level in order to have benchmark values for all acidities regardless of whether experimental values were available. Barriers to proton transfer relative to the separated reactants, ΔHTS, show a straight-line relation to acidity for all but two of the neutral acids and for all but one of the cationic acids. Two neutral acids, HOOH and FOH, show negative deviations that can be attributed to polarizability of the atoms attached to the proton donor oxygens. The cationic acid HC(O)OH2+ shows a large positive deviation, which probably arises from substantial heavy-atom reorganization from reactant to TS. Charges provide evidence of a lag in delocalization in the reaction of CH2 = CHOH2+, though it does not show an elevated ΔHTS.Key words: ab initio, oxygen acids, proton transfer, acidity.

Kinetics of Proton Transfer Reactions of Carbon Acids. II. Reaction of Di-(4-nitrophenyl)methane with Alkoxide Bases

1972 ◽  
Vol 50 (1) ◽  
pp. 24-30 ◽  
Author(s):  
A. Jarczewski ◽  
K. T. Leffek
Keyword(s):  
Proton Transfer ◽  
Transition State ◽  
Rate Constants ◽  
Activation Parameters ◽  
Transfer Reactions ◽  
Activation Process ◽  
Proton Transfers ◽  
Proton Transfer Reactions ◽  
Carbon Acids ◽  
Kinetics Of

The second-order rate constants have been measured over a range of temperatures for the proton-transter reactions from di-(4-nitrophenyl)methane to ethoxide, isopropoxide, and t-butoxide ions in solvents consisting of the corresponding alcohols containing 10% toluene by volume. The activation parameters ΔH≠ and ΔS≠ have been calculated and an interpretation of them is given in terms of solvation effects during the activation process. A comparison between the activation parameters for proton transfers and E2 olefin-forming β-elimination reactions is made and discussed with respect to transition state character of the latter reactions.

Identity-reaction proton transfers from nitrogen acids yielding localized vs. delocalized conjugate bases. An ab initio study

1998 ◽  
Vol 76 (6) ◽  
pp. 821-827 ◽  
Author(s):  
James E. Van Verth ◽  
William H Saunders, Jr. ◽  
Thomas W Kermis
Keyword(s):  
Proton Transfer ◽  
Ab Initio ◽  
Negative Deviation ◽  
Proton Transfers ◽  
Straight Line ◽  
Transition Structures ◽  
Pi Systems ◽  
Experimental Values ◽  
Small Negative Deviation ◽  
Conjugate Bases

Identity-reaction proton transfers from a series of nitrogen acids to the corresponding conjugate bases have been studied by ab initio methods at the MP2/6-31+G*//MP2/6-31+G* level. The acids are NH4+, H2NNH3+, CH3NH3 +, CH2NH2, OCNH 2+, OCHNH3+, H2NCHNH2+ , HNCHNH 3+, NH3, CH3NH2, CH2NH, OCNH, OCHNH2,and HNCHNH2. Gas-phase acidities were calculated at the G2(MP2) level where experimental values are not available in order to have benchmark values for all acidities. Barriers to proton transfer relative to the separated reactants, ΔHTS, show a straight-line relation to acidity for all of the neutral acids and for all but four of the cationic acids. Three show ΔHTS values well above the line: HNCHNH3+, OCNH2+, and OCHNH3 +, in increasing order of positive deviation. One shows a small negative deviation: H2NNH3+ . The first three acids have localized pi systems but can yield delocalized transition structures and conjugate bases. The barriers result from a lag in delocalization relative to proton transfer in the transition structures. All of the other acids give transition structures that can only be localized, or if they can be delocalized they prefer to adopt conformations in which the unshared pair on nitrogen delocalizes rather than the electrons of the N---H bond. The negative deviation for H2NNH3 + is attributed to polarizability of the NH2 group.Key words: ab initio, nitrogen acids, proton transfer, acidity.

Study of the dissociation of the products of some proton transfer reactions in acetonitrile solvent

1992 ◽  
Vol 70 (3) ◽  
pp. 935-942 ◽  
Author(s):  
Wlodzimierz Galezowski ◽  
Arnold Jarczewski
Keyword(s):  
Proton Transfer ◽  
Rate Constants ◽  
Isotope Effects ◽  
Equilibrium Constants ◽  
Ion Pair ◽  
Transfer Reactions ◽  
Side Reactions ◽  
First Order ◽  
Kinetic Isotope ◽  
Proton Transfer Reactions

The conductometric study of the products of the proton transfer reactions of C-acids (nitriles, nitroalkanes, and 2,4,6-trinitrotoluene) with the strong amine bases (1,1,3,3-tetramethylguanidine (TMG), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,8-bis(dimethylamino)naphthalene (DMAN), and piperidine) in acetonitrile shows their large degree of dissociation into free ions. The dissociation constant values have been estimated at 25 °C to be larger than 1 × 10−4 M. This weakens the formalism commonly accepted in spectrophotometric kinetic studies of these systems of reactions, based on the assumption that the product is an ion pair. Spectrophotometric equilibrium and kinetic measurements provided evidence that reverse reaction is a second-order process (pseudo-first order because cation concentration is controlled by side reactions). The influence of the common cation (TMGH+) on the equilibria of the proton abstraction from 2-methyl-1-(4-nitrophenyl)-1-nitropropane and 4-nitrophenylcyanomethane with TMG base in acetonitrile at 25 °C was examined and was found to be compatible with the assumption of large dissociation of the reaction product for free ions. "Equilibrium constants" estimated by the Benesi and Hildebrand method (which assumes an ion-pair product) decreased with increasing concentration of added TMGH+ cation, but these "equilibrium constants" multiplied by [TMGH+] are constant. The observed pseudo-first-order rate constants of the proton transfer reaction, measured at large excess of the base over C-acid, grow with the cation concentration due to the increase of the backward reaction rate. The concentration of added common cation shows a negligible influence on the observed rate constants of deuteron transfer reaction. Thus, as a result of side reactions, in which extra amounts of cation are formed, some second-order rate constants [Formula: see text] and also kinetic isotope effects (KIEs) [Formula: see text] that have been measured in acetonitrile can be substantially overestimated. Keywords: ion-pair dissociation, proton transfer reactions, kinetic isotope effects.

Proton transfer reactions between ortho-methyl substituted derivatives of 4-nitrophenylphenylcyanomethane and nitrogen bases in acetonitrile solvent

1988 ◽  
Vol 66 (6) ◽  
pp. 1454-1458 ◽  
Author(s):  
Kenneth T. Leffek ◽  
Przemyslaw Pruszynski
Keyword(s):  
Proton Transfer ◽  
Isotope Effect ◽  
Rate Constants ◽  
Isotope Effects ◽  
Equilibrium Constants ◽  
Activation Parameters ◽  
Transfer Reactions ◽  
Methyl Groups ◽  
Methyl Group ◽  
Proton Transfer Reactions

Equilibrium constants, rate constants, primary deuterium isotope effects, and activation parameters have been measured for the proton transfer reactions in acetonitrile solvent of 4-nitrophenylphenylcyanomethane and 2-methyl-4-nitrophenylphenyl-cyanomethane with tetramethylguanidine base and for the reactions of 2-methyl-4-nitrophenylphenylcyanomethane and 2,6-di-methyl-4-nitrophenylphenylcyanomethane with 1,5-diazabicyclo[5.4.0]undec-7-ene base. Introduction of the ortho-methyl groups in the substrate molecule caused significant reductions in the equilibrium and rate constants. The expected rise in the kinetic primary deuterium isotope effect was not observed when the first ortho-methyl group was introduced, but a 20% increase did accompany the introduction of the second ortho-methyl group. Enthalpy of activation measurements indicated that there was no increase in the proton tunnelling contribution to the isotope effect when the amount of steric hindrance is increased with ortho-methyl groups.

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