Ch_322a_6.07

Ch_322a_6.07 - Enthalpy and Entropy of Activation While the...

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Enthalpy and Entropy of Activation While the overall rate of a chemical reaction depends on the ! G , the components of the free energy of activation (the enthalpy and entropy of activations ) often provide mechanistic information. ! G = ! H ! S - T ! H ! S measures the enthalpy change as the reactants move towards the transition state. Bonding changes make important contributions to this parameter. Solvation effects are also important. measures the change in entropy as the reactants move towards the transition state. In most reactions, the transition state is more ordered than the reactant state leading to a negative value for the entropy of activation, and an unfavorable energy contribution to the free energy of activation.
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Pyridine reacts rapidly with methyl iodide in a nucleophilic substitution reaction yielding N-methylpyridinium iodide: N : + CH 3 I pyridine acetonitrile 25 o C N CH 3 + + I - N-methylpyridinium iodide ! G o = -12 kcal/mol Exergonic A kinetic study of this reaction provides the overall rate of this
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Ch_322a_6.07 - Enthalpy and Entropy of Activation While the...

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