From the kinetic data below, calculate ΔH^* and ΔS^* for each nucleophilic substitution reaction

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From the kinetic data below, calculate ΔH^* and ΔS^* for...

From the kinetic data below, calculate $\Delta H^{*}$ and $\Delta S^{*}$ for each nucleophilic substitution reaction with $n$-butyl tosylate in methanol and DMSO. What trends do you note in $\Delta H^{*}$ and how would you explain them? What trends do you note in $\Delta S^{*}$ and how would you explain them?

Key Concepts

Activation Entropy (?S*)

Activation entropy reflects the change in disorder as reactants are transformed into the transition state. A negative ?S* often indicates a more ordered and organized transition state compared to the reactants, which is typical for bimolecular reactions where the alignment of molecules is necessary. A positive ?S* suggests increased disorder or less organizational requirement in the transition state. Changes in ?S* can provide insight into the degree of solvent reorganization and molecular ordering during the reaction.

Activation Enthalpy (?H*)

Activation enthalpy is the energy barrier that must be overcome for a reaction to proceed. It indicates the amount of heat required to form the transition state from the reactants. Trends in ?H* can reflect differences in the strength or nature of bonding interactions and can be influenced by factors such as the inherent reactivity of the substrate or the stabilizing effects of the solvent.

Solvent Effects on Reaction Kinetics

Solvent effects can profoundly influence both ?H* and ?S* by stabilizing or destabilizing the reactants, transition state, or intermediates. Polar protic and polar aprotic solvents, for instance, can differentially solvate nucleophiles and leaving groups, impacting the energy barrier and the molecular ordering required for the transition state. Observing trends in the activation parameters in different solvents helps elucidate how solvent interactions modify the reaction mechanism and energetics.

Transition State Theory

This theory relates the rate of a chemical reaction to the properties of a high?energy configuration called the transition state. It assumes that reactants pass through a short-lived intermediate state, and the reaction rate is determined by the energy barrier between the reactants and the transition state. The theory forms the basis for understanding how kinetic parameters, such as ?H* and ?S*, are related to the reaction mechanism.

Eyring Equation

The Eyring equation is a central expression in chemical kinetics that relates the rate constant of a reaction to temperature and the activation parameters—activation enthalpy (?H*) and activation entropy (?S*). By measuring rate constants at different temperatures and applying the Eyring equation, one can determine how much energy is required to reach the transition state and how the system's entropy changes during this process.

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