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This work demonstrates that organic compounds can be synthesized up to 1240 times faster in sealed Teflon vessels in a microwave oven than by conventional (reflux) techniques. It is shown that all polar molecules absorb microwave energy rapidly and that the rate of energy absorption varies with the dielectric constant. The rates of reaction of polar molecules in nonpolar solvents are not increased appreciably by the microwave method. Also, the homogeneity of the reaction does not affect the rate enhancement. The rate enhancement arises predominantly because the oven superheats the solvent rapidly. Finally, pressure (temperature) measurements have shown that the maximum rate enhancement is achieved when the proper power level and volume of solvent are used. It appears that rate enhancements of approximately 200 are possible for many reactions if the reaction conditions are optimized.RICHARD N. GEDYE, FRANK E. SMITH et KENNETH CHARLES WESTAWAY. Can. J. Chem. 66, 17 (1988).Dans ce travail, on demontre que les composCs organiques peuvent &tre synthCtisCs 1240 fois plus rapidement dans des rkcipients de TCflon scelles, dans des fours a micro-ondes, que par les techniques conventionnelles de reflux. On dCmontre que toutes les molCcules polaires absorbent rapidement 1'Cnergie des micro-ondes et que les vitesses d'absorption de 1'Cnergie varient avec la constante diklectrique. La mCthode des micro-ondes ne provoque pas en une augmentation substantielle des vitesses des rCaetions impliquant des molCcules polaires dans des solvants qui ne sont pas polaires. De plus, I'homogCnCitC du milieu reactionnel n'influence pas le taux d'augmentation des vitesses des rkactions. L'augmentation des vitesses des reactions provient principalement du fait que le four provoque une surchauffe rapide du solvant. Enfin, des mesures de pression (tempkrature) ont permis de dtmontrer que le taux maximal d'augmentation des vitesses de rCaction est atteint lorsqu'on utilise un niveau de puissance et un volume de solvant appropriCs. I1 semble que l'on puisse atteindre des taux d'augmentation des vitesses de rCactions d'environ 200 si les conditions des rCactions sont optimisCes.[Traduit par la revue] Introduction Although several chemical applications of microwave heating have been reported (1-5), it is only recently that microwave ovens have been used in organic synthesis (6, 7). In a preliminary communication (6) we described the use of microwave ovens for synthesizing esters from carboxylic acids, carboxylic acids from alkyl benzenes and amides, and ethers from alkyl halides. In fact, these organic reactions occurred up to 1240 times more rapidIy in sealed Teflon containers in the microwave oven than by classical reflux methods. Since then, Giguere et al. (7) reported dramatic reductions in reaction time in microwave syntheses using the Diels-Alder, Claisen, and ene reactions.The present paper extends the scope of this new synthetic method and reports our investigations of several of the factors that affect the rate enhancements found in organi...

The reactant and transition-state structures for several s N 2 reactions between different nucleophiles and methyl and ethyl chloride and fluoride have been calculated at the H F / 6 -1 3 + G * level. The secondary a-deuterium kinetic isotope effects for these reactions were calculated with Sim's BEBOVIB-IV program. The results demonstrate that the magnitude of these isotope effects is determined by an inverse stretching vibration contribution and a normal bending vibration contribution to the isotope effect. The stretching vibration contribution to the isotope effect is essentially constant for each substrate while the bending vibration contribution varies with the nucleophile and the looseness of the s N 2 transition state. Thus, the out-of-plane bending vibration model for relating the magnitude of secondary a-deuterium kinetic isotope effects to transition-state structure is correct. The bending vibration contribution to the isotope effect is greater in the ethyl substrate reactions than in the methyl substrate reactions. As a result, larger isotope effects and looser transition states are found for the s N 2 reactions of larger substrates. Looser transition states and larger isotope effects are also observed for the s N 2 reactions with softer nucleophiles.

The secondary alpha-deuterium, the secondary beta-deuterium, the chlorine leaving-group, the nucleophile secondary nitrogen, the nucleophile (12)C/(13)C carbon, and the (11)C/(14)C alpha-carbon kinetic isotope effects (KIEs) and activation parameters have been measured for the S(N)2 reaction between tetrabutylammonium cyanide and ethyl chloride in DMSO at 30 degrees C. Then, thirty-nine readily available different theoretical methods, both including and excluding solvent, were used to calculate the structure of the transition state, the activation energy, and the kinetic isotope effects for the reaction. A comparison of the experimental and theoretical results by using semiempirical, ab initio, and density functional theory methods has shown that the density functional methods are most successful in calculating the experimental isotope effects. With two exceptions, including solvent in the calculation does not improve the fit with the experimental KIEs. Finally, none of the transition states and force constants obtained from the theoretical methods was able to predict all six of the KIEs found by experiment. Moreover, none of the calculated transition structures, which are all early and loose, agree with the late (product-like) transition-state structure suggested by interpreting the experimental KIEs.

Direct comparisons of the reactivity and mechanistic pathways for anionic systems in the gas phase and in solution are presented. Rate constants and kinetic isotope effects for the reactions of methyl, ethyl, isopropyl, and tert-butyl iodide with cyanide ion in the gas phase, as well as for the reactions of methyl and ethyl iodide with cyanide ion in several solvents, are reported. In addition to measuring the perdeutero kinetic isotope effect (KIE) for each reaction, the secondary alpha- and beta-deuterium KIEs were determined for the ethyl iodide reaction. Comparisons of experimental results with computational transition states, KIEs, and branching fractions are explored to determine how solvent affects these reactions. The KIEs show that the transition state does not change significantly when the solvent is changed from dimethyl sulfoxide/methanol (a protic solvent) to dimethyl sulfoxide (a strongly polar aprotic solvent) to tetrahydrofuran (a slightly polar aprotic solvent) in the ethyl iodide-cyanide ion S(N)2 reaction in solution, as the "Solvation Rule for S(N)2 Reactions" predicts. However, the Solvation Rule fails the ultimate test of predicting gas phase results, where significantly smaller (more inverse) KIEs indicate the existence of a tighter transition state. This result is primarily attributed to the greater electrostatic forces between the partial negative charges on the iodide and cyanide ions and the partial positive charge on the alpha carbon in the gas phase transition state. Nevertheless, in evaluating the competition between S(N)2 and E2 processes, the mechanistic results for the solution and gas phase reactions are strikingly similar. The reaction of cyanide ion with ethyl iodide occurs exclusively by an S(N)2 mechanism in solution and primarily by an S(N)2 mechanism in the gas phase; only approximately 1% of the gas phase reaction is ascribed to an elimination process.