Energy Diagrams, Transition States, and Intermediates

Energy diagrams are used to represent the change in energy for the molecules involved in a chemical reaction.

The free energy is measured along the y-axis, and the reaction coordinate is plotted on the x-axis.

The reaction coordinate indicates the progress of the conversion of reactants to products. Peaks on the energy diagram represent the transition states, whereas the valleys represent the reactive intermediates.

As the reaction progresses, the reactants pass through an unstable state of maximum free energy, called the activated complex, or the transition state. They last for less than one picosecond and cannot be isolated. The reactant’s ability to achieve a given transition state depends on the value of the activation energy.

The double dagger symbol is used to describe transition states. If the transition state is easy to achieve, the associated delta-G double-dagger is small, and the reaction is fast. Contrarily, if the transition state is difficult to achieve, the associated delta-G double-dagger is large, and the reaction rate is slow.

According to Hammond's postulate, in a single-step exothermic process, the structure of the transition state closely resembles that of the