11/6 - Seminar (organic): Daniel Singleton, Texas A&M University
Daniel Singleton - Singleton Group website
Department of Chemistry
Texas A&M University
Host: Abby Doyle
Dynamic Effects in Ordinary Organic Reactions in Solution
Chemists understand the rates, selectivities, and mechanisms of organic reactions based on statistical theories such as transition state theory or RRKM theory. We have been interested in reactions where statistical theories are either inapplicable or make incorrect predictions about the experimental outcome. We describe reactions for which statistical theories are inadequate as being subject to "dynamic effects." Dynamic effects can arise in a variety of ways, and this talk will focus on simple organic reactions illustrating three of these ways.
Because the time scale of a reaction coordinate is faster than loss of energy to solution, mechanistic intermediates are formed with excess energy. In one category of dynamic effects intermediates simply react faster than their excess energy can be equilibrated within the molecule. This lecture will discuss the role of this dynamic effect in an ozonolysis reaction and the lessons provided about the nature of energy flow in molecules.
Conventional statistical rate theories assume that molecular systems passing through a transition state in the direction of products do not turn back and reform reactants. It was recognized from the beginning that this "no-recrossing assumption" is an approximation, and this led to the incorporation of transmission coefficients into transition state theory. The inaccuracy in transition state theory engendered by recrossing can be minimized by a sagacious choice of the transition state, but it is understood that some recrossing occurs that is not yet predictable by statistical methods. This recrossing remains a hidden phenomenon; little is known about its importance in ordinary reactions and there are no general experimental probes for its occurrence. The observation of isotope effects on recrossing and how this recrossing can change the understanding of the mechanism and selectivities of cycloadditions will be described.
The idea of an entropic bifurcation is that it can be downhill enthalpically from a transition state to form a particular product but entropy can favor the formation of some alternative product. Standard theoretical studies employing minimum-energy paths will not find the entropically favored branch of the reaction. The lecture will conclude by discussing the role of entropic bifurcations on the competition between [2,3]- and [1,2]-sigmatropic rearrangements.