Rate Determining Step in Electrophilic Substitution Reaction

The rate determining step (RDS) in a reaction is the slowest step that determines the overall rate of the reaction. In electrophilic substitution reactions, the RDS is the step that involves the attack of an electrophilic reagent on the benzene ring. This is represented by option 'B' in the given choices.

Explanation:

1. Electrophilic Substitution Reactions

Electrophilic substitution reactions involve the substitution of an atom or a group in a molecule with an electrophile. These reactions are typically observed in aromatic compounds, such as benzene, and proceed through several steps.

2. Mechanism of Electrophilic Substitution Reactions

The mechanism of electrophilic substitution reactions can be summarized as follows:

a. Generation of Electrophile: The first step involves the generation of an electrophile, which is an electron-deficient species that can accept a pair of electrons from the benzene ring. This step is not the RDS because it is usually fast and reversible.

b. Attack by Electrophile: The second step is the attack of the electrophile on the benzene ring, resulting in the formation of a sigma complex. This step involves the breaking of one of the carbon-carbon bonds in the benzene ring and the formation of a new bond with the electrophile. This step is the RDS because it is relatively slow compared to the other steps.

c. Rearrangement: In some cases, a rearrangement of the sigma complex may occur to stabilize the intermediate. This step is usually fast and reversible.

d. Loss of Proton: The next step involves the loss of a proton from the sigma complex, resulting in the formation of an intermediate called a carbocation. This step is usually fast and reversible.

e. Regeneration of Aromaticity: The final step involves the removal of a proton from the solvent or the addition of a nucleophile to regenerate the aromaticity of the benzene ring. This step is usually fast and reversible.

3. Conclusion

In summary, the attack of an electrophilic reagent on the benzene ring is the rate determining step in electrophilic substitution reactions. This step is relatively slow compared to the other steps in the reaction mechanism. Understanding the RDS is important for predicting the rate and outcome of electrophilic substitution reactions.