THE HYDROGEN ISOTOPE EFFECTS IN THE PYROLYSIS OF ETHYL-1,1,2,2-d4 BROMIDE AND OF ETHYL-d5 BROMIDE

1962 ◽  
Vol 40 (8) ◽  
pp. 1533-1539 ◽  
Author(s):  
Arthur T. Blades ◽  
P. W. Gilderson ◽  
M. G. H. Wallbridge
Keyword(s):  
Isotope Effect ◽  
Rate Constant ◽  
Hydrogen Isotope ◽  
Relative Rate ◽  
Isotope Effects ◽  
Side Reaction ◽  
Relative Rate Constant ◽  
Temperature Range ◽  
Rate Controlling Step ◽  
Constant Expression

The relative rate constant expression has been obtained for the decomposition of ethyl-1,1,2,2-d4 bromide under inhibiting conditions in the temperature range 697.6 to 999.1 °K,[Formula: see text]The pressure dependence of the isotope effect has been investigated both with and without inhibitor, and in each case it has been shown that the isotope effect increases with decreasing pressure.The relative rate constant expression for the ethyl-h5, ethyl-d5 bromide comparison was also obtained in the temperature range 730.9 to 964.8 °K,[Formula: see text]The isotope effect is again pressure dependent, falling to lower values as the pressure is decreased.The data are used to demonstrate that the inhibited decomposition of ethyl bromide is primarily a molecular process, and that the rate-controlling step involves a carbon–hydrogen bond break.A side reaction that produces small amounts of ethane has been observed.

THE SECONDARY HYDROGEN ISOTOPE EFFECTS IN THE PYROLYSIS OF ETHYL-d5 ACETATE AND ETHYL ACETATE-d3

1960 ◽  
Vol 38 (9) ◽  
pp. 1407-1411 ◽  
Author(s):  
Arthur T. Blades ◽  
P. W. Gilderson
Keyword(s):  
Ethyl Acetate ◽  
Rate Constant ◽  
Hydrogen Isotope ◽  
Experimental Error ◽  
Relative Rate ◽  
Isotope Effects ◽  
Relative Rate Constant ◽  
Temperature Ranges ◽  
Relative Rate Constants ◽  
Constant Expression

Rate constant expressions have been obtained for ethyl acetate and ethyl-d5 acetate in the temperature ranges 500–603 °C and 501–614 °C.[Formula: see text]By measuring the relative rate of production of C2H4 and C2D4 from identical mixtures of the two esters at the temperatures 387 and 490 °C, it has been possible to determine the temperature coefficient of the relative rate constant more accurately. This, coupled with the relative rate constants at 500 °C derived from the above equations, gives the relative rate constant expression.[Formula: see text]These data are compared with the intramolecular isotope effect in the decomposition of ethyl-1,1,2,2-d4 acetate, and the differences attributed to secondary isotope effects.The rate of decomposition of ethyl acetate-d3 was found to be identical within experimental error with that of the normal acetate.

THE HYDROGEN ISOTOPE EFFECT IN THE PYROLYSIS OF ETHYL-1,1,2,2-d4 CHLORIDE

1962 ◽  
Vol 40 (8) ◽  
pp. 1526-1532 ◽  
Author(s):  
Arthur T. Blades ◽  
P. W. Gilderson ◽  
M. G. H. Wallbridge
Keyword(s):  
Isotope Effect ◽  
Hydrogen Isotope ◽  
Pressure Effect ◽  
Relative Rate ◽  
Isotope Effects ◽  
Deuterium Atom ◽  
Qualitative Explanation ◽  
Relative Rate Constant ◽  
Hydrogen Isotope Effect ◽  
Constant Expression

The hydrogen isotope effect in the pyrolysis of ethyl-1,1,2,2-d4 chloride has been investigated in the temperature range 758–989 °C, yielding the relative rate constant expression[Formula: see text]where kH and kD are the rate constants for the production of C2D4 (and HCl) and of C2D3H (and DCl) per β-deuterium atom respectively.The isotope effect is pressure dependent, its value increasing with decreasing pressure. The pressure effect in ethyl chloride has also been studied, and a qualitative explanation is given for the pressure dependence of intermolecular and intramolecular isotope effects.The data are compared with the isotope effects in ethyl-d4 bromide and ethyl-d4 acetate, and the conclusion is reached that the evidence supports a four-centered transition-state complex in the case of the halides.

Untersuchungen zur Reaktion der Alkoholdehydrogenase mit Tritium-markierten Substraten

1966 ◽  
Vol 21 (6) ◽  
pp. 540-546 ◽  
Author(s):  
Dieter Palm
Keyword(s):  
Alcohol Dehydrogenase ◽  
Temperature Dependence ◽  
Isotope Effect ◽  
Substrate Binding ◽  
Isotope Effects ◽  
Direct Interaction ◽  
Enzyme Complex ◽  
Temperature Range ◽  
Yeast Alcohol Dehydrogenase ◽  
Rate Controlling Step

Unexpectedly, the isotope effect of ethanol-1-Τ as a substrate of yeast alcohol dehydrogenase, increases with rising temperature from kH/kT = 3.2 at 5 —15°C to 3.8—4.7 at 20 —35 °C. This suggests a change of the rate controlling step as proposed by MÜLLER-HILL and WALLENFELS, who investigated the temperature dependence of the activation energies in this temperature range. A comparison of the affinities of propanol and butanol with the isotope effects of the corresponding tritium labelled compounds (propanol-1-Τ 6.7 at 25 °C, butanol-1-Τ 6.8 at 25 °C) supports the proposal, that during substrate binding, there must be a direct interaction between the enzyme complex and hydrogen which is removed in the reaction. These influences are less pronounced for the ethanol homologues which are bound less tightly to the enzyme. Therefore the H transfering step proper gives a greater contribution to the overall experimental isotope effect.

Kinetics of the thermal decomposition of cyclobutanone

1969 ◽  
Vol 47 (4) ◽  
pp. 615-617 ◽  
Author(s):  
Arthur T. Blades
Keyword(s):  
Activation Energy ◽  
Thermal Decomposition ◽  
Carbon Monoxide ◽  
Rate Constant ◽  
Relative Rate ◽  
Relative Rate Constant ◽  
Pressure Sensitive ◽  
Photochemical Decomposition ◽  
Kinetics Of ◽  
Constant Expression

The thermal decomposition of cyclobutanone into cyclopropane and carbon monoxide has been shown to occur simultaneously with the major decomposition to ethylene and ketene. The relative rate constant expression is given by [Formula: see text] Both reactions are pressure sensitive below 10 Torr and this quasi-unimolecular behavior is most pronounced in the cyclopropane forming reaction, consistent with the higher activation energy. The data are also discussed in relation to the photochemical decomposition and it is shown that cyclopropane formation from the ground singlet is an important feature of the photolysis at 3130 Å.

Solvent isotope effects as a probe of general catalysis and solvation in phosphoryl transfer

1996 ◽  
Vol 74 (6) ◽  
pp. 931-938 ◽  
Author(s):  
Clinton D. Bryan ◽  
K. Barbara Schowen ◽  
Richard L. Schowen
Keyword(s):  
Ionic Strength ◽  
Transition State ◽  
Rate Constant ◽  
Relative Rate ◽  
Isotope Effects ◽  
Phosphoryl Transfer ◽  
Relative Rate Constant ◽  
Nitrophenyl Phosphate ◽  
Solvent Isotope ◽  
Solvent Isotope Effects

Phosphoryl transfer to methanol from tris(p-nitrophenyl) phosphate (PNNN), methyl bis(p-nitrophenyl) phosphate (PMNN), and dimethyl p-nitrophenyl phosphate (PMMN) exhibits general base catalysis by acetate ion but no detectable catalysis by acetic acid. For PNNN, acetate catalysis produces normal solvent isotope effects kROH/kROD of 1.68 ± 0.01 at high ionic strength (0.475) and 1.77 ± 0.04 at low ionic strength (0.048). A linear proton inventory indicates most simply that the isotope effect arises from a one-proton catalytic bridge in the transition state, although this model cannot strongly be distinguished from a generalized solvation effect. Reactions of methoxide ions produce slight inverse isotope effects kROD/kROH of 1.1–1.2, far smaller than the inverseeffect of about 2.5 expected for complete and uncompensated desolvation of the reactant-state methoxide ion. The transition state is thus stabilized by substantial interaction with the solvent. The proton inventory for the least reactive substrate PMMN (relative rate constant 1) is suggestive of transition-state stabilization by a combination of one-proton catalytic bridge(s) and distributed sites, while the proton inventory for the most reactive substrate PNNN (relative rate constant 1388) suggests only generalized transition-state solvation (many distributed sites); the proton inventory for PMNN, a substrate of intermediate reactivity (relative rate constant 60), suggests an intermediate situation. The data are consistent with a model in which transition states with exterior concentrations of charge favor stabilization of the charge by isotope-fractionating one-proton bridges, while transition states with distributed charge favor stabilization of the charge by many distributed sites. Key words: phosphoryl transfer, proton inventories, solvent isotope effects.

The hydrogen isotope effect in the bromination of 2-carbethoxy cyclo pentanone

1956 ◽  
Vol 235 (1203) ◽  
pp. 453-468 ◽  
Keyword(s):  
Isotope Effect ◽  
Hydrogen Isotope ◽  
Deuterium Oxide ◽  
Isotope Effects ◽  
Activation Energies ◽  
Tunnel Effect ◽  
Fluoride Ion ◽  
Energy Barriers ◽  
Hydrogen Isotope Effect ◽  
Kinetics Of

Measurements are reported on the kinetics of the base-catalyzed bromination of 2-car-bethoxycyclopentanone, with either hydrogen or deuterium in the active position. The solvent throughout was deuterium oxide, the catalysts employed were the solvent, monochloroacetate ion and fluoride ion, and measurements were made at 5° intervals over the range 10 to 70°C. The observed activation energies are all greater for the deutero- than for the proto-ester, but the differences are greater than would be expected on current theories of isotope effects. The observed collision factors are in every case greater for deuterium than for hydrogen, especially for catalysis by fluoride ion, where the ratio of these factors is A D / A H = 24 ± 4. These observations can only be accounted for by invoking the tunnel effect, i. e. by supposing that the motion of the proton is markedly non-classical in nature. It is shown that this hypothesis leads to reasonable dimensions for the energy barriers involved, and some if its general consequences are discussed.

Kinetics of the reaction H + C2H6 → H2 + C2H5 in the temperature region of 1000 K

1984 ◽  
Vol 62 (1) ◽  
pp. 86-91 ◽  
Author(s):  
J.-R. Cao ◽  
M. H. Back
Keyword(s):  
Equilibrium Constant ◽  
Rate Constant ◽  
Temperature Region ◽  
Recent Measurement ◽  
Hydrogen Atoms ◽  
Elementary Reactions ◽  
Temperature Range ◽  
Hydrogen Molecules ◽  
Kinetics Of ◽  
Rate Controlling Step

A system for the measurement of rate constants for elementary reactions of hydrogen atoms in the temperature region of 1000 K is described. The concentration of hydrogen atoms is controlled by the equilibrium constant for dissociation of hydrogen molecules. The kinetics of the rate of conversion of ethane to ethylene in this system has been studied over the temperature range 876–1016 K. The results show that the rate-controlling step is[Formula: see text]and the value obtained for the rate constant is[Formula: see text](R = 1.987 cal mol−1 deg−1). This value is compared with values obtained from other methods over the temperature range 300–1400 K. Combination with a recent measurement of the rate constant for the reverse reaction yields an experimental value for the equilibrium constant for the reaction.

The nature of the transition state in diarylmethyl cation – nucleophile combination reactions as probed by secondary α-deuterium isotope effects

2001 ◽  
Vol 79 (12) ◽  
pp. 1887-1897
Author(s):  
Thuy Van Pham ◽  
Robert A McClelland
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Rate Constant ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Isotope Effects ◽  
Equilibrium Constants ◽  
Bond Formation ◽  
Kinetic Isotope ◽  
Rate Limiting

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 correlations 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.Key words: carbocation, isotope effect, transition state, halide.

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