scholarly journals KINETIC HYDROGEN ISOTOPE EFFECTS IN AROMATIC BROMODEPROTONATION

1966 ◽  
Vol 44 (3) ◽  
pp. 379-386 ◽  
Author(s):  
B. T. Baliga ◽  
A. N. Bourns
Keyword(s):  
Isotope Effect ◽  
Hydrogen Isotope ◽  
Salt Effect ◽  
Isotope Effects ◽  
Effect Study ◽  
Ion Concentration ◽  
Bromide Ion ◽  
Halogenation Reaction ◽  
Hydrogen Isotope Effect ◽  
Step Mechanism

The bromodeprotonation of sodium p-methoxybenzenesulfonate is an ordinary halogenation reaction in which a hydrogen ortho to the methoxy group is replaced by bromine. The kinetic data for this reaction do not distinguish between a one- and a two-step mechanism with Br2 as the brominating species. The two-step mechanism was confirmed by the observation of a variation in the kinetic hydrogen isotope effect, kH/kD, with changing bromide ion concentration. The observed isotope effects are 1.01, 1.07, 1.18, and 1.31 at bromide ion concentrations 0, 0.5, 1, and 2 M, respectively.The isotope effect study strongly establishes that in this reaction the small depression in rate produced by bromide ion, beyond that due to Br3− formation, arises mainly from a salt effect, and only a small amount of this depression is caused by return of the intermediate to reactants.

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.

KINETIC SULFUR ISOTOPE EFFECTS IN AROMATIC BROMODESULFONATION

1966 ◽  
Vol 44 (3) ◽  
pp. 363-378 ◽  
Author(s):  
B. T. Baliga ◽  
A. N. Bourns
Keyword(s):  
Sulfur Isotope ◽  
Isotope Effects ◽  
Ion Concentration ◽  
Step Process ◽  
Bromide Ion ◽  
Alternate Mechanism ◽  
One Step ◽  
The Individual ◽  
Step Mechanism

Both bromodeprotonation and bromodesulfonation occur during aqueous bromination of sodium p-methoxybenzenesulfonate, A, and potassium 1-methylnaphthalene-4-sulfonate, B. Whereas for A the bromodeprotonation increases appreciably in amount as the bromide ion concentration is raised, this reaction takes place to a relatively small extent for the less reactive B. A reinvestigation of the kinetics of bromination and a determination of sulfate yields for both of these compounds over a range of bromide ion concentration of 0 to 0.5 M have been made to evaluate the individual rates of these two competing reactions. The kinetic data for the bromodesulfonation of A favor a two-step mechanism with Br2 as the brominating species, but are not considered to eliminate completely a one- or two-step process involving Br+ (or H2OBr+). For B, the kinetics allow either a one- or two-step mechanism with molecular bromine as the brominating agent.Kinetic sulfur isotope effects (k32/k34) have been measured for the bromodesulfonation of A at different bromide ion concentrations. The observed effects are: at [Br−] = 0, 0.3%; at [Br−] = 0.03 M, 1.3%; at [Br−] = 0.5 M, 1.7%. These results confirm the two-step mechanism favored by kinetics and allow the rejection of the alternate mechanism with Br+ (or H2OBr+) as the brominating species. A similar study of B gave the following results: at [Br−] = 0, 0.2%; at [Br−] = 0.25 M, 0.3%; at [Br−] = 0.5 M, 0.4%; at [Br−] = 2.0 M, 0.8%. These results establish the two-step mechanism for this compound and completely eliminate the alternate one-step process.

Kinetic hydrogen isotope effects in the ionization of some ketonic substances

1965 ◽  
Vol 286 (1406) ◽  
pp. 285-299 ◽  
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Hydrogen Isotope ◽  
Organic Molecule ◽  
Isotope Effects ◽  
Methyl Malonate ◽  
Ethyl Malonate ◽  
Hydrogen Isotope Effect ◽  
Catalytic Power ◽  
Basic Strength

The kinetic hydrogen isotope effect has been measured for the water-catalysed bromination of acetylacetone, monobromoacetylacetone, ethyl acetoacetate, ethyl α-bromoacetoacetate, ethyl α-methylacetoacetate, ethyl malonate, ethyl monobromomalonate, and methyl methyl - malonate and for the bromination of ethyl a-methylacetoacetate catalysed by six basic anions. All these reactions are of zero order with respect to bromine, so that the process studied is the rate of transfer of a proton or deuteron from the organic molecule to the catalyst. For ethyl a-methylacetoacetate there is a correlation between the basic strength of the catalyst, the rate of proton transfer, and the magnitude of the isotope effect. Various possible explanations are considered for the variation in isotope effect, and it is concluded that current interpretations in terms of a three-centre model of the transition state are inadequate. The isotope effect also varies considerably with the nature of the organic molecule, but is not in general related to its reactivity. There is, however, a correlation between the magnitude of the isotope effect and the exponent β of the Brönsted relation between basic strength and catalytic power, suggesting that both quantities depend on the position of the proton in the transition state.

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.

Hydrogen Isotope Fractionation in Aqueous Alkaline Earth Chloride Solutions

2007 ◽  
Vol 62 (12) ◽  
pp. 721-728 ◽  
Author(s):  
Masahisa Kakiuchi
Keyword(s):  
Isotope Effect ◽  
Hydrogen Isotope ◽  
Alkaline Earth ◽  
Platinum Catalyst ◽  
Pure Water ◽  
Pressure Ratio ◽  
Hydrogen Gas ◽  
Chloride Solutions ◽  
Aqueous Chloride ◽  
Hydrogen Isotope Effect

The D/H ratio of hydrogen gas in equilibrium with aqueous alkaline earth (Mg, Ca, Sr or Ba) chloride solutions measured at 25◦C using a hydrophobic platinum catalyst, was found to be higher than the D/H ratio equilibrated with the applied pure water. The hydrogen isotope effect between such solutions and pure water changes with the molality of the solutions. The order of the D/H ratios in alkaline earth chlorides is found to be BaCl2 > SrCl2 ≥ CaCl2 ≥ MgCl2. The hydrogen isotope effect in the aqueous chloride solutions of Mg, Ca, Sr or Ba ions is significantly larger than that in the aqueous chloride solutions of Li, Na, K or Cs ions. For MgCl2 and CaCl2 solutions, the hydrogen isotope effect is opposite to the oxygen isotope effect. The results are compared with the free energy change of transfer from H2O to D2O, and are discussed for the vapour pressure ratio of H2O and D2O of CaCl2 solutions.

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