Organic Mechanisms. Xiaoping Sun

Organic Mechanisms - Xiaoping Sun


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complete (~100%). Therefore, a reversible reaction has been essentially converted to an irreversible reaction.

      When the carboxylic acid (RCO2H) and the alcohol (R′OH) are used in 1:1 molar ratio, the conversion of the reactants to the products is 70–75%. If R′OH is used in 10‐folds of excess, the conversion of RCO2H (limiting reactant) to the ester product will be ~99%. In this case, the reversible reaction has been almost transformed into an irreversible reaction. For some esterification reactions, the essential quantitative conversion of the reactants to the ester product can also be obtained by removal of water from the reaction system once it is formed.

      The relationship between the equilibrium constant (K) and standard free energy (ΔG°) is formulated as

equation

      Equation 1.53 also shows that plot of lnK versus 1/T defines a straight line, from which both the standard enthalpy (Δ) and standard entropy (Δ) can be obtained from the slope and intercept, respectively, for an unknown reaction.

equation equation

      1.6.1 The Transition State and Activation Energy

Graphs depict the early transition state (a) and late transition state (b).

      

      1.6.2 The Hammond Postulate

      The exact structure of a transition state is in general not measurable due to its instability. It is in‐between the structures of reactants and products, which can be qualitatively predicted and described by the Hammond postulate [3]. It states that the structure of the transition state for a concerted reaction resembles (closer to) the species (reactant or product) to which it is most similar in energy. According to the Hammond postulate, if a concerted reaction is exergonic (or exothermic if entropy of the reaction ΔS is small and negligible) (Fig. 1.4a), the energy of the transition state is most similar to that of the reactant. Therefore, the structure of the transition state resembles


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