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STERIC, SOLVENT EFFECTS UNCOUPLED
Researchers deepen understanding of steric effects in ionic reactions
Steric effects reasonably explain the differing rates of certain reactions. SN2 reactions, for example, are slow when the carbon atom undergoing substitution is surrounded by large substituents, because the incoming nucleophile can't get close to the reaction center.
Chemists, of course, know that for reactions in solution, solvation must also affect reaction rates. But the rate changes that occur when one substituent is replaced with a bulkier one have all been attributed to steric effects, because separating those effects from solvation effects has been impossible.
||BUZZ OFF In the gas phase, the reaction when R = tert-butyl is slower than when R = methyl because of steric effects only. In solution, bulky substituents hinder the approach not only of the nucleophile but also of solvent molecules.
Until now. At Stanford University, chemistry professor John I. Brauman and coworkers have conducted studies that untangled the two factors by measuring intrinsic steric barriers in the gas phase, where solvation does not come into play. They conclude that the rise in potential energy barriers caused by changes in substituent size is about equally due to solvation effects as to steric effects [Science, 295, 2245 (2002)].
"The work advances the fundamental understanding of the role of steric effects in chemical reactions," James M. Farrar, a chemistry professor at the University of Rochester, tells C&EN. "I was a bit surprised at the results," he adds. "I had expected that the intrinsic steric barrier might be a larger fraction of the overall barrier increase."
With former graduate students Colleen K. Regan and Stephen L. Craig, Brauman used Fourier transform/ion cyclotron resonance mass spectrometry to measure steric effects. By following the gas-phase reactions of chloride ions (37Cl) with methylchloroacetonitrile and with tert-butylchloroacetonitrile, producing 35Cl, they find that the steric effect of replacing the methyl group with tert-butyl is 1.6 kcal/mol. That value is less than that of the barrier for comparable reactions in solution--Cl exchange with ethyl chloride and neopentyl chloride--which is about 5 to 7 kcal/mol.
?The discrepancy must be due to differences in solvation, the researchers believe. And Monte Carlo simulations show that the solvation energy difference between the transition state and the reactants is larger for substrates with bulkier groups. Furthermore, the calculated difference is of the same magnitude as the discrepancy in the measured steric effects for gas- and solution-phase reactions.
"The study calls attention to the idea that in ionic reactions of this kind, you have to be concerned with what the medium is doing, as well as the intrinsic behavior of the ions and molecules themselves," Brauman notes. "With big substituents around the central carbon, the transition state is not as well solvated. The substituents keep solvent molecules from getting close. Another way to think about it is that there's steric hindrance for solvation as well as for the reaction."
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