The chlorination of benzene can be carried out by allowing...

The chlorination of benzene can be carried out by allowing benzene to react with $\mathrm{Cl}_{2}$ in the presence of $\mathrm{AlCl}_{3} .$ As discussed on page $1245, \mathrm{Cl}_{2}$ and $\mathrm{AlCl}_{3}$ react to form $\mathrm{Cl}^{+} \mathrm{AlCl}_{4}^{-}$. Sketch the reaction profile for the reaction of benzene and $\mathrm{Cl}^{+} \mathrm{AlCl}_{4}^{-}$. Use the same assumptions as in Exercise 35.

Key Concepts

Lewis Acid Catalysis

Lewis acids are used to activate electrophiles by forming complexes that generate a more reactive electrophilic species. This activation facilitates the attack on the aromatic ring during electrophilic aromatic substitution, effectively lowering the activation energy and improving the overall efficiency of the reaction.

Transition State and Activation Energy

The highest point on the reaction energy diagram corresponds to the transition state, which represents the peak energy barrier that must be overcome during the formation of the sigma complex. The difference in energy between the reactants and this transition state is known as the activation energy, a critical parameter that affects the reaction rate.

Sigma Complex (Arenium Ion) Formation

In these reactions, the electrophile adds to the aromatic system to form a non-aromatic, high-energy intermediate known as the sigma complex or arenium ion. This intermediate is central to the reaction profile, as its formation represents a significant energy barrier that must be overcome, and its stability influences the overall reaction rate.

Electrophilic Aromatic Substitution

This is a reaction mechanism in which an aromatic ring is attacked by an electrophile, temporarily losing its aromaticity by forming a sigma complex (arenium ion), and then regains its aromatic character after a deprotonation step. It underlies many substitution reactions on aromatic compounds and explains both the kinetic and thermodynamic aspects of the process.

Reaction Energy Profile Diagram

A reaction energy profile provides a graphical representation of the energy changes during the course of a reaction, including the initial reactants, activation energy required for transition states, energy of intermediates like the sigma complex, and the relatively lower energy of the final product once aromaticity is restored. It is a key tool for understanding the kinetics and thermodynamics of the process.

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