Deuterium isotope effect primary kinetic

A second reason for the larger isotope effect observed by Jones and Maness (140) might be that in the less polar acetic acid solvent, there might be a small degree of E2 elimination (with solvent acting as base) superimposed on the dominant Sn 1 mechanism. Such an elimination would involve a primary kinetic deuterium isotope effect with a kn/ko s 2 to 6, and hence even a 1 to 5% contribution from such a pathway would have a significant effect on the experimentally observed kinetic isotope effect. 

The effect of solvent upon k2 has been reported , and it was concluded that the activated complex is not sufficiently polar to be called ionic . The oxidations of toluene and triphenylmethane exhibit primary kinetic deuterium isotope effects of 2.4 and ca. 4 respectively. No isotopic mixing occurred during formation of the Etard complex from a mixture of normal and deuterated o-nitrotoluene . The chromyl chloride oxidation of a series of substituted diphenylmethanes revealed that electron-withdrawing substituents slow reaction while electronreleasing groups have the opposite effect, the values ofp andp being —2.28 + 0.08 and —2.20 + 0.07 respectively . ... 

Other differences between singlet (concerted) insertion and triplet (abstraction-recombination) carbene insertion are seen in selectivity, stereochemistry, and the kinetic deuterium isotope effect. The triplet states are more selective in C—H insertion than the singlets. For example, the triplet shows higher tertiary to primary selectivity than the singlet in the insertion reaction with 2,3-dimethylbutane. Singlet carbene is shown to insert into C—H bond with retention of configuration, while racemization is expected for triplet insertion reaction from the abstraction-recombination mechanism. For example, the ratios of diastereomeric insertion product in the reaction of phenylcarbene with roc- and mcTO-2,3-dimethylbutanes are 98.5 1.5 and 3.5 96.5, respectively. ... 

The largest values of primary kinetic deuterium isotope effects are found for reactions where the bond to hydrogen is about one-half broken (kn/ D values are 6-8). Smaller values are found in reactions in which the bond to hydrogen is less than or more than one-half broken. Normally, kn/ D values less than maximum correspond to bond cleavage of <. Primary kinetic deuterium... 

Use of deuterated substrates gives hAd = 6.5. This is a primary kinetic deuterium isotope effect, indicating diat proton removal is an essential component of die rate-determining step. The lack of rate dependence on bromine requires diat bromine is added to die molecule after die rate-determining step. A mechanism consistent widi diese facts has proton removal and enolate formation rate determining. 

This means diat both the substrate and the ethoxide base are present in the transition state of the rate-determining step. The rate constants for die deuterated and protio substrates were measured. The magnitudes ( h/ d = 3-4) of the kinetic deuterium isotope effects for both substrates are typical primary kinetic deuterium isotope effects, which means that C-H bond breaking is involved at the transition state of the rate-determining step. This suggests that proton removal... 

Since the lower pathway has proton removal as an integral part of the mechanism, you could use isotopic substitution in two ways. First make the deuterated compound D. The kinetic deuterium isotope effect should be primary for the lower path but very near 1 for the upper path. Moreover, if you looked at the product, there should be no deuterium left in the product if the lower path is followed, whereas deuterium should be retained in the product if the addition elimination path is followed. 

Solvolysis of (29-X, X = I, Br, OBs) in 25vol.% acetonitrile in water gives elimination product (32) and substitution products (33a) and (33b).17 The rate of elimination increases with increasing acidity of the substrate (Bronsted a > 0) as evidenced by results for ring-substituted substrates (30-X) and (31-X). However, for elimination reactions of the brosylates (29-OBs) and (31-OBs), the small kinetic deuterium isotope effect (kH/kD = 2.0 0.1 and 2.8 0.1, respectively) is believed to be a consequence of competing El reaction via a primary ion pair. 

The observations that the pH-independent reactions of deuterium-labeled 5-met-hoxyindene oxide and 6-methoxy-1,2,3,4-tetrahydronaphthalene-1,2-oxide show significant primary kinetic deuterium isotope effects for the ketone-forming reactions, whereas the pH-independent reactions of deuterium-labeled naphthalene oxide and benzene oxide do not, are quite puzzling. Clearly, more work needs to be done to fully understand why transition-state structures for rearrangement of arene oxides to phenols differ from those for rearrangement of benzylic epoxides to ketones. 

An alternative proposal for the transition state of the ene reaction involving carbonyl groups, specifically mesoxalic esters, has been advanced. On the basis of little variation in the primary kinetic deuterium isotope effect (2.55 at 120 nC and 2.58 at 200 °C) it was concluded that the transition state involved a C-H-O arrangement significantly distorted from linear and involving participation of a lone-pair of electrons on oxygen as well as secondary orbital overlap7,8. 

The primary motivation for these studies is the analysis of the reactivity patterns of organic compounds, when Ce(IV) is used as an oxidant. These patterns are determined for the most part by product analysis of selected series of organic compounds. The results obtained in two studies that bear more directly on the chemical behavior of Ce(IV) as an oxidant for hydrocarbons have been interpreted to indicate different mechanistic behavior of Ce(IV). In a product study of the oxidation of isodurene (1,2,3,5-tetramethyl benzene) by ceric ammonium nitrate compared to anodic oxidation, Eberson and Oberrauch (1979) concluded that the oxidation by Ce(IV) occurs via a H atom transfer from the alkylaromatic compound to Ce(IV). Badocchi et al. (1980) measured the variation of second-order rate constants for the oxidation of a series of alkylaromatic compounds with added Ce(III). These results along with those from the determination of kinetic deuterium isotope effect were dted to support a mechanism involving radical cations. The Ce(IV)/Ce(III) functions as an electron acceptor/donor in such a mechanism. 

Primary, secondary, and solvent kinetic deuterium isotope effects (KDlEs ku/ko) provide important mechanistic information about organic reactions in aqueous reaction media, particularly acid-base-catalyzed processes. Measurements are usually restricted to H2O and D2O because of the inconvenience of handling reactions in radioactive T2O (tritium has a half-life of 12.5 years). 

Important additional evidence for aryl cations as intermediates comes from primary nitrogen and secondary deuterium isotope effects, investigated by Loudon et al. (1973) and by Swain et al. (1975 b, 1975 c). The kinetic isotope effect kH/ki5 measured in the dediazoniation of C6H515N = N in 1% aqueous H2S04 at 25 °C is 1.038, close to the calculated value (1.040-1.045) expected for complete C-N bond cleavage in the transition state. It should be mentioned, however, that a partial or almost complete cleavage of the C — N bond, and therefore a nitrogen isotope effect, is also to be expected for an ANDN-like mechanism, but not for an AN + DN mechanism. 

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