Isotope Effects In Reactions Research Articles

Kinetic isotope effects in reactions of isotopic exchange of the hydrogen of ferrocene with acid and base

1. The kinetics of the hydrogen exchange of ferrocene, labeled with deuterium and tritium, was measured in acid and basic media. Exchange with liquid ammonia in the case of catalysis by potassium amide proceeds 30 times as rapidly in ferrocene as in benzene. 2. The kinetic isotope effect in the exchange of hydrogen in ferrocene with acid (KD/KT=1.2) is substantially lower than in the case of the exchange of hydrogen in aromatic compounds (KD/KT ≈ 2). 3. The kinetic isotope effect in the exchange of hydrogen in ferrocene with liquid ammonia, catalyzed by potassium amide, possesses a value close to that which was found for benzene and naphthalene, and shows that the protophilic mechanism of exchange in basic medium, established for other organic compounds, also holds in the case of ferrocene.

Tritium-Labeled Compounds IX. Determination of Isotope Effects in Reactions Yielding Water-t from Nonvolatile Reactants. Oxidation of Aldoses-1-t With Iodine.

A convenient method is presented for estimating isotope effects in reactions which yield water-t. The water-t is separated by freeze-drying, and k*/k is calculated from the extent of the reaction and the ratio of the radioactivity of the water-t to that of the initial reactant. Application of the method to the oxidation of aldoses-1-t at 25 °C with iodine in alkaline solution gives values for k*/k ranging from 0.12 to 0.17, with an average of 0.14. There are no significant differences in the values of k*/k obtained for aldoses differing in configuration, or for pentoses and hexoses. The isotope effect corresponds to a ratio of one to seven for the respective rates of reaction of the labeled and unlabeled molecules.

Chlorine Isotope Effects in the Reactions of Benzyl and Substituted Benzyl Chlorides with Various Nucleophiles

The kinetics and chlorine isotope effects in the nucleophilic substitution reactions of benzyl and substituted benzyl chlorides with water, cyanide ion, and thiosulfate ion were investigated. For those reactions exhibiting first-order kinetics, the kinetic isotope effect (k35//k37 ) observed was close to 1.0078, while for reactions displaying second-order kinetics the value was 1.0058. Those reactions proceeding by borderline kinetics gave isotope effects intermediate between these two extremes. Detailed analysis of the kinetic and isotope effect data for the reactions with cyanide and thiosulfate ions led to the conclusion that there is a smooth transition in the reaction mechanism from near ideal SN 1 to near ideal SN 2 as the electrondonating power of the substituent decreases. It appeared that the magnitude of the chlorine isotope effect may be used to differentiate between SN 1 and SN 2 reactions.

Billiard-Ball Collisions in the Reactions of Hot Hydrogen Atoms. Isotope Effect on the Hydrogen Displacement in Methane

The nature of collisions leading to reactions of hydrogen atoms of high kinetic energy (in the form of recoil tritium) is investigated. Two extreme models, both originally due to Libby, are examined. One assumes very weak coupling by valence bonds in the molecule attacked, so that the reaction can be represented by a quasi-elastic atom-atom collision (billiard-ball model) and the other assumes strong valence bond coupling (epithermal model). Calculations on the billiard-ball (hard sphere) model of the reaction T*+CH4→H+CH3T leads to an estimate of its relative cross section as a function of energy. An isotope effect for reaction with CH4 and with CD4 is calculated and measured experimentally. Consideration of both the calculations and the experimental results indicate that the billiard-ball model is unimportant in the reactions of gas phase hot hydrogen (as recoil tritium), and that reactions occur nearly exclusively at less than 10–20 ev of kinetic energy, by mechanisms involving strong bond coupling. The possible importance of the billiard-ball model in the reactions of other hot atoms is briefly discussed.

Further Evidence for Benzyne as an Intermediate in Nucleophilic Aromatic Substitution Reactions1

Some alternatives to the “benzyne” (elimination-addition) mechanism for aminations of non-activated halobenzenes have been disproved by the finding that the amination of iodobenzene-1-^(14)C-2,4,6^(-2)H_3 gives the same extent of ^(14)C rearrangement in the resulting aniline as does iodobenzene-1-^(14)C. The intermediacy of substituted benzynes in non-rearranging aminations of 3-substituted halobenzenes has been confirmed by a study of the hydrogen-deuterium kinetic isotope effect in reactions of 3-bromoanisole-2,4,6-^2H_3.

The magnitude of electronic isotope effects

Secondary isotope effects, i.e. kinetic isotope effects in reactions where the bond to the labeled atom is not broken, are now being widely used for investigations of organic reaction mechanisms. In a recent paper, Halevi has discussed secondary hydrogen isotope effects, and has suggested that there is an inductive contribution to such isotope effects. In particular, he suggests that there is more facile electron release from a CD bond than from a CH bond. The purpose of this communication is to examine in more detail some of the evidence cited by Halevi, and to estimate the theoretical limitations on such an effect.

The Effect of Ring Substituents on the Isotope Effects in Reactions of Carbonyl-C14 Esters and Ketones

The variations of the isotope fractionation factor k14/k12 with changes in ring substituents, reaction temperature, solvent, concentration, and the alcohol group have been studied for the saponification of ethyl benzoates (α-carbon-14). The variations of the k14/k12 ratio with changes in temperature and ring substituents have been studied for the formation of the 2,4-dinitrophenylhydrazones of acetophenones (α-carbon-14).