Catalyst, general isotopic molecules

In the formation of any of these products, the first step of the reaction involves activation of the alkane molecule. Experimental results point to the fact that in general, breaking of the first C—H bond is a rate-limiting step. For example, on a V-P-O catalyst, a sizable deuterium kinetic isotope effect was... 

Detailed studies of the temperature dependence of the kinetics of cation-pseudobase equilibration have been reported92 for three cations, and activation parameters have been evaluated for each of k0H, kH20, kt, and k2 in these cases. Whereas entropies of pseudobase formation from the cation are positive (Section III), the entropies of activation associated with k0H are quite negative (-11 to -17 cal mol-1 deg-1). For direct hydroxide ion attack on the cation via transition state A, one would predict an entropy of activation similar to the entropy of formation of the pseudobase from heterocyclic cation and hydroxide ion. Interpretation of k0H in terms of transition state B in which hydroxide ion acts as a general-base catalyst for the attack of a water molecule seems to be more consistent with the observed entropies of activation. The k2 step, which is the microscopic reverse of fcoH would then be interpreted as general-acid-catalyzed (by a water molecule) decomposition of the neutral pseudobase to the cation. These interpretations of /cqH and k2 are also consistent with the observed solvent isotope effects for the reactions in HzO and DzO and with the presence of general... 

The previous work on cyclopentane (10) resulted in isotopic distribution patterns like run 290 of Table X, which exhibits a maximum at perdeuterocyclopentane. However, only twice as many molecules exchanged by single-atom as by many-atom exchange in run 290, whereas on the previous catalyst (10) the ratio was 16. Our previous patterns (10) seem, however, generally characteristic of microcrystalline catalysts. Many-atom exchange with the amorphous catalysts, runs 285 and 296, leads to a distribution in which Di is constant between i equal 3 to 7 followed by a decline in Di with a minimum at the perdeutero. 

In the hydrolysis of diethylphenyl orthoformate no general base catalysis is observed. The steric effects of substituents on the imidazole catalyst are very small, and the activation parameters have been shown to have large negative entropies. These observations, in conjunction with a solvent isotope effect, point to participation of water molecules in the transition state, i.e., at least one water molecule intervenes between imidazolium ion and the ortho ester (see (66)). General acid catalysis is assumed <87JCS(P2)669>. 

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