Transition State Theory - The Sequel The potential-energy surface for a reaction R-X+ Y  R-Y + X (• denotes start of reaction, T denotes transition state, and contours are constant potential-energy surfaces) The activated complex, or transition state, decomposes to products through a vibration (translation) whose energy is plotted as the reaction coordinate. DG DG CHEE 323 J.S. Parent

Forward and Reverse Reactions The principle of microscopic reversibility suggests that a reaction and its reverse proceed by the same mechanism. Forward and reverse reactions must have the same intermediates and rate-determining transition states. Example: Alcohol Dehydration CHEE 323 J.S. Parent

Thermodynamic Properties of an Activated Complex For the reaction: A + B X‡ C we can write: where, X‡ DG A + B DG C According to transition state theory, both the enthalpy and entropy of activation affect the rate of an elementary reaction Enthalpy of Activation: Entropy of Activation: CHEE 323 J.S. Parent

Properties of an Activated Complex: Enthalpy Enthalpy of Transition Enthalpy of formation for activation State enthalpy Reactants A and B Transition states of high enthalpy of activation represent slow reactions, an example of which is a bimolecular nucleophilic substitution reaction: CHEE 323 J.S. Parent

Properties of an Activated Complex: Entropy Entropy of Transition Entropy of formation for activation State entropy Reactants A and B Although we often focus on the “activation energy” or enthalpy of activation, entropy can dominate elementary reaction kinetics. large-scale ordering to form a transition state (low entropy of activation) reduces the rate of reaction Consider hydrogen atom abstraction from methane by a variety of radical species. CH4 + R•  CH3• + R-H Radical A (M-1s-1) EA (kcal mole-1) F• 1.26*1011 1.2 Cl• 2.51*1010 3.9 CH3• 6.31*108 14.3 CH3O• 6.31*108 11.0 CHEE 323 J.S. Parent

Early and Late Transition States Reactant Product ‡ In endothermic reactions, the transition state resembles the product in terms of energy and structure. This is called a “late” transition state or product-like t.s. DG‡ DGo Reactant ‡ Product Exothermic reactions have a transition state more closely resembling the reactants in terms of both energy and structure. This is called an “early” transition state or reactant-like t.s. DG‡ DGo CHEE 323 J.S. Parent

Hammond Postulate The use of transition state theory to describe chemical kinetics requires us to consider the structure of the transition state. By definition, the transition state cannot be isolated. How can we make meaningful inferences regarding its structure? While there is no universal relationship between the stability of a reaction product and its rate of formation, many reactions can be characterized by the Hammond Postulate. The position of the transition state along the reaction coordinate, its energy, and its geometry are related, and depend on the relative stabilities of the reactant and the product. McMurray states in his text: The structure of a transition state resembles the structure of the nearest stable species. CHEE 323 J.S. Parent

Hammond Postulate: Examples Classify these reaction profiles in terms of: A. Product stability B. Transition State energy C. Position of the transition state (early/late) What generalizations can be made regarding the position of the transition state and the rate of reaction? CHEE 323 J.S. Parent

Hammond Postulate: Inappropriate Use Consider the polymerization of methylmethacrylate to produce a transparent, glassy polymer (tradename plexiglass) the reaction proceeds with “head-to-tail” regioselectivity to give linear polymer chains as is the case for most polymerizations, it is strongly exothermic. What would the reaction profile look like for these reactions? Can product stability arguments (Hammond Postulate) be used to explain the head-to-tail preference? CHEE 323 J.S. Parent

Case Study: Ethylene Hydrogenation Cycle Consider the catalytic hydrogenation of ethylene by Wilkinson’s catalyst RhCl(PPh3)3. Assuming this sequence is comprised of elementary reactions, what would the reaction profile look like? We will examine the migratory insertion reaction in greatest detail. CHEE 323 J.S. Parent

Complex 6 4-centred transition state Complex 5 Dihydrido-olefin Complex 8A Hydrido-alkyl

Case Study: Potential Energy Diagram CHEE 323 J.S. Parent