Isotope effects in elimination reaction

For example, the rates of dehydrobromination of l-bromo-2-phenylethane and l-bromo-2,2-dideuterio-2-phenylethane differ by a factor of seven. 

The ratio of the rate constants for the undeuterated and deuterated compounds, expressed as is 

This result tells us that the C—H and C—D bonds are broken in the rate-deterrnining step of the reaction. And this in turn tells us that reaction occurs by an E2 mechanism. 

There is no deuterium isotope effect in an El process. There is still a difference in the rates at which the C—H and C—D bonds are broken, but we can t direcdy measure it because the C—H and C—D bonds break after the rate-determining step. 

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Saunders, W.H. (1985). Calculations of isotope effects in elimination reactions. New experimental criteria for tunneling in slow proton transfers. J. Am. Chem. Soc. 107, 164-169... 

The secondary Hke/T H KIE in the eliminations of 373, 374 and 375 presented above which are higher than this maximum possible secondary IE value, are taken as strongly implicating tunnelling. This conclusion has been supported also by intercomparison of secondary H/T and D/T isotope effects in E2 reactions of RNM3 1 Br at 50 °C. The secondary IE is depressed markedly when deuterium rather than proton is transferred, which also implicates tunnelling ... 

Chiao, W.-B., Saunders, W. H., Jr. Mechanisms of elimination reactions. 29. Deuterium kinetic isotope effects in eliminations from amine oxides. The consequences of nonlinear proton transfer. J. Am. Chem. Soc. 1978,100, 2802-2805. 

The diversity in isotope effects for elimination reactions (Table 3) is indicative of widely varying transition states, but these effects alone provide insufficient evidence of the extent of proton transfer and have to be considered in conjunction with alternative kinetic evidence. 

The above conclusion must be remembered when considering carbon isotope effects for elimination reactions. Certainly the alpha- and beta-carbon isotope effects for elimination from propyl trimethylammonium ion indicate a different extent of rehybridisation at the two carbon atoms in the transition state, but a more definite conclusion requires additional kinetic evidence. 

The increased strength and electron-withdrawing power of the C -N bond relative to the C - bond should cause reactions of the former to proceed via a more carbanionic transition state. Under the same reaction conditions, the sulphur isotope effect for 2-phenylethyldimethylsulphonium ion (I I, Table 6) indicates more C -X bond breaking than the nitrogen isotope effect in elimination from the corresponding ammonium salt. 

A distinction between these four possibilities can be made on the basis of the kinetic isotope effect. There is no isotope effect in the arylation of deuterated or tritiated benzenoid compounds with dibenzoyl peroxide, thereby ruling out mechanisms in which a C5— bond is broken in the rate-determining step of the substitution. Paths (ii) and (iii,b) are therefore eliminated. In path (i) the first reaction, Eq. (6), is almost certain to be rate-determining, for the union of tw o radicals, Eq. (7), is a process of very low activation energy, while the abstraction in which a C—H bond is broken would require activation. More significant evidence against this path is that dimers, Arz, should result from it, yet they are never isolated. For instance, no 4,4 -dinitrobiphenyl is formed during the phenylation of... 

Good selectivity for the oxidation of primary alcohols in the presence of secondary ones can be achieved. By appropriate choice of the reaction conditions, overoxidation of the aldehyde from a primary alcohol to carboxylic acid can be minimized. Kinetic isotope effects in the range of 2 to 3 testify about the relevance of the H+-elimination step upon the overall reactivity . In general, the efficiency of oxidation of alkanols is slightly lower... 

Various mechanisms for the aerobic oxidation of alcohols catalysed by (NHC)Pd (carboxylate)2(H20) complexes [NHC = l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] were investigated using DFT combined with a solvent model. Of these, reductive j3-hydride elimination, in which the -hydrogen of a palladium-bound alkoxide is transferred directly to the free oxygen of the bound carboxylate, provided the lowest-energy route and explained the published kinetic isotope effect, activation enthalpy, reaction orders, and dependence of rate on carboxylate pKa.26S... 

The existence of a free carbonium ion such as VII in a strongly solvating medium is highly improbable. Only if VII could exist in association with the palladium could decomposition to vinyl acetate be expected to occur with a reasonable degree of frequency, in competition with the reaction with acetate to form ethylidene diacetate. Similar results have been reported in the Wacker acetaldehyde synthesis when D2O is used as the solvent (25). Stern (54) has reported results in which 2-deuteropropylene was used as substrate in the reaction. Based on assumed /J-acetoxyalkylpalladium intermediates, on the absence of an appreciable isotope effect in the proton-loss step, and on the product distribution observed, excellent agreement between calculated (71%) and observed (75%) deuterium retention was obtained. Several problems inherent in this study (54) have been discussed in a recent review (I). Hence, considerable additional effort must be expended before a clear-cut decision can be made between a simple / -hydrogen elimination and a palladium-assisted hydride shift in this reaction. 

Bach, R. D., Braden, M. L. Primary and secondary kinetic isotope effects in the Cope and Hofmann elimination reactions. J. Org. Chem. 1991,56, 7194-7195. 

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