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 reactants, as the transition state is closer in energy to the reactants than to the products.

Meanwhile, in a single-step endothermic process, the structure of the transition state closely resembles that of the products, as the transition state is closer in energy to the products than to the reactants.

A reactive intermediate corresponds to an energy minimum between two transition states. Highly reactive intermediates are difficult to isolate and have a short lifespan, while those with lower energies have longer lifetimes.

The most common reactive intermediates in organic chemistry involve carbon radicals or carbon centers without four bonds. For example, carbocations act as electron acceptors and carbanions act as electron donors.