Transition states and reaction rates

Abraham M H 1974 Solvent effects on transition states and reaction rates Prog. Rhys. Org. Ohem. 11 1-87... 

The QET is not the only theory in the field indeed, several apparently competitive statistical theories to describe the rate constant of a unimolecular reaction have been formulated. [10,14] Unfortunately, none of these theories has been able to quantitatively describe all reactions of a given ion. Nonetheless, QET is well established and even the simplified form allows sufficient insight into the behavior of isolated ions. Thus, we start out the chapter from the basic assumptions of QET. Following this trail will lead us from the neutral molecule to ions, and over transition states and reaction rates to fragmentation products and thus, through the basic concepts and definitions of gas phase ion chemistry. 

Solvent Effects on Transition States and Reaction Rates (Abraham). 

The concepts of energy surfaces for molecular motion, equilibrium geometries, transition structures and reaction paths depend on the Bora-Oppenheimer approximation to treat the motion of the nuclei separately from the motion of the electrons. Minima on the potential energy surface for the nuclei can then be identified with the classical picture of equilibrium structures of molecules saddle points can be related to transition states and reaction rates. If the Born-Oppenheimer approximation is not valid, for example in the vicinity of surface crossings, non-adiabatic effects are important and the meaning of classical chemical structures becomes less clear. Non-adiabatic effects are beyond the scope of this chapter and the discussion of energy surfaces and optimization will be restricted to situations where the Bom-Oppenheimer approximation is valid. 

Atkins, P. (1998). Physical Chemistry, 6th ed., W. H. Freeman and Company, New York. Abraham, M. H. "Solvent Effects on Transition States and Reaction Rates." Prog. 

This is a reaction in which neutral molecules react to give a dipolar or ionic transition state, and some rate acceleration from the added neutral salt is to be expected53, since the added salt will increase the polarity or effective dielectric constant of the medium. Some of the rate increases due to added neutral salts are attributable to this cause, but it is doubtful that they are all thus explained. The set of data for constant initial chloride and initial salt concentrations and variable initial amine concentrations affords some insight into this aspect of the problem. 

The effect of protic additives was rationalized by the formation of hydrogen-bonded adducts, such as that between benzoic acid and the carbonyl group of the coordinated benzoate, as observed by 1H NMR. Such an adduct would be expected to facilitate the formation of a polar transition state. The reaction rate also increases upon modification of the ligand with anionic groups and in polar solvents. Radical initiators have no effect. 

Equation (50) forms the basis upon which v can be evaluated (e.g. (1) by the radioactive tracer method to evaluate simultaneously and ), (2) by comparing i values at appropriate potentials for different reactant activities (3) coupling information from high and low overpotential regions of steady-state polarization curves " (extrapolated io and charge-transfer resistance, Rcr, respectively) (4) or by back-reaction correction analysis. 2 qqie first two methods involve determination of v at any single potential while the latter two procedures must assume that the same mechanism (and hence v) applies at different potentials (at which individual measurements are required) and that the reverse reaction occurs by the same path and has the same transition state and thus rate-determining step [for both forward (cathodic) and reverse reactions]. 

Later we shall consider the effect of resonance in the reactants and in the transition state on reaction rate. These correspond to the M and E effects. 

The essential feature of transition state theory is that there is a concentration of the species at the saddle point, the transition state or activated complex, that is in equihbrium with reactants and products. The Boltzmann Distribution Law governs the concentration of that transition state, and the rate of reaction is proportional to its concentration. Since the concentration... 

Other things being equal, when we compare the evaporation of two moles of reactants and one mole of transition states, the reaction rate in solution should be faster hy exp(A Vap/f ), which is of the order of 1(F. Another way to think about the difference is to compare the concentration of a reactant in the gas and the condensed phase. This equilibrium ratio equals exp(—AGvap//l7) = exp(—A//v /RT) exp(AS /if) and is ollen larger than 10 at room temperature. For more on this point see Section 11.1.3.1 on the cage effect. 

Green W H, Moore C B and Polik W F 1992 Transition states and rate constants for unimolecular reactions Ann. Rev. Phys. Chem. 43 591-626... 

Pairwise hydrophobic interactions can be used to alter the reactivity of organic molecules in water. For instance, the rate of hydrolysis reactions may be influenced significantly by the presence of hydrophobic cosolutes. The effect on reactivity has been analysed by comparirg the interactions between initial state and cosolute with those between transition state and cosolute. ... 

Another means of resolution depends on the difference in rates of reaction of two enantiomers with a chiral reagent. The transition-state energies for reaction of each enantiomer with one enantiomer of a chiral reagent will be different. This is because the transition states and intermediates (f -substrate... f -reactant) and (5-substrate... R-reactant) are diastereomeric. Kinetic resolution is the term used to describe the separation of enantiomers based on different reaction rates with an enantiomerically pure reagent. 

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