Question

Column-I $$ \left(\mathbf{K}_{\mathbf{C}_{6} \mathrm{H}_{5} \mathrm{Y}} / \mathbf{K}_{\mathrm{C}_{6} \mathrm{H}_{6}}\right) $$ For chlorination (A) \(1.6 \times 10^{-5}\) (B) \(3.4 \times 10^{2}\) (C) \(4.2 \times 10^{2}\) (D) \(9.7 \times 10^{6}\) (E) \(3 \times 10^{-2}\) Column-II (Y) (P) \(-\mathrm{Cl}\) (Q) \(-\mathrm{OMe}\) \((\mathrm{R})-\stackrel{\oplus}{\mathrm{NMe}}_{3}\) (S) (T) \(-\mathrm{CH}_{3}\)

Short answer

The correct matching between Column-I and Column-II is: (A) \(1.6 \times 10^{-5}\) <-> (R) -\(\stackrel{\oplus}{\mathrm{NMe}}_{3}\) (B) \(3.4 \times 10^{2}\) <-> (Q) -OMe (C) \(4.2 \times 10^{2}\) <-> (T) -CH3 (D) \(9.7 \times 10^{6}\) <-> (S) No substituent (Incorrect value) (E) \(3 \times 10^{-2}\) <-> (P) -Cl

Step-by-step solution

Identify the effect of the groups on chlorination rate

Group substitution on an aromatic ring can either activate or deactivate the ring towards an electrophilic aromatic substitution like chlorination. Electron-donating groups (EDGs) increase the rate (\(\mathbf{K}\)) compared to benzene, while electron-withdrawing groups (EWGs) decrease the rate. We expect the rate constant ratios to be greater than 1 for EDGs and less than 1 for EWGs.

Match rate constant ratios based on activation/deactivation

We can identify the nature of the substituents in Column-II: (P) -Cl is a weakly deactivating group (EWG) (Q) -OMe is an activating group (EDG) (R) -\(\stackrel{\oplus}{\mathrm{NMe}}_{3}\) is a strongly deactivating group (EWG) (S) H, i.e., no substituent, this ratio will be 1 (T) -CH3 is an activating group (EDG) Now, we match these groups with the rate constant ratios: (A) \(1.6 \times 10^{-5}\): Since it's less than 1, it corresponds to a (EWG) which is -\(\stackrel{\oplus}{\mathrm{NMe}}_{3}\) (R) (B) \(3.4 \times 10^{2}\): Greater than 1, corresponds to an (EDG). The highest among the EDGs is -OMe (Q) (C) \(4.2 \times 10^{2}\): Greater than 1, corresponds to an (EDG). The other EDG is -CH3 (T) (D) \(9.7 \times 10^{6}\): This value represents no substituent (S). For unsubstituted benzene, the ratio should be 1, but here we have given an incorrect value. (E) \(3 \times 10^{-2}\): Since it's less than 1, it should be matched to an (EWG) group which is -Cl (P) The correct matching between Column-I and Column-II is: (A) - \(1.6 \times 10^{-5}\) <-> (R) -\(\stackrel{\oplus}{\mathrm{NMe}}_{3}\) (B) - \(3.4 \times 10^{2}\) <-> (Q) -OMe (C) - \(4.2 \times 10^{2}\) <-> (T) -CH3 (D) - \(9.7 \times 10^{6}\) <-> (S) No substituent (Incorrect value) (E) - \(3 \times 10^{-2}\) <-> (P) -Cl

Key concepts

Rate Constant

In the context of electrophilic aromatic substitution, the rate constant is an important concept. It represents the speed at which a reaction progresses and is influenced by the chemical environment of the aromatic ring.

• A higher rate constant means the reaction proceeds quickly.
• Conversely, a lower rate constant indicates a slower reaction.

Substituents attached to the aromatic ring can greatly affect this rate constant. When comparing the rate constant of a substituted benzene to that of benzene itself, expressed as \( \frac{K_{C_6H_5Y}}{K_{C_6H_6}} \), it provides insight into whether the substituent is enhancing or inhibiting the reaction rate. If the result is greater than 1, it suggests the substituent makes the reaction occur more readily than with benzene alone. If it is less than 1, the opposite effect is taking place.

Electron-donating groups (EDG)

Electron-donating groups are substituents that can enhance the reactivity of an aromatic compound in electrophilic aromatic substitution reactions. They do this by donating electron density to the aromatic ring, making it more reactive to electrophiles.

• Common EDGs include the methoxy group (-OMe) and methyl group (-CH₃).
• These groups stabilize the positive charge that forms during the transition state of the reaction.

EDGs typically lead to a rate constant ratio greater than 1 when compared to benzene. This is because the increased electron density in the ring facilitates the reaction by making it more attractive to incoming electrophiles. For example, when -OMe or -CH₃ is present, reactions can proceed more quickly due to this increased electron density.

Electron-withdrawing groups (EWG)

Electron-withdrawing groups have the opposite effect of electron-donating groups. They decrease the reactivity of an aromatic ring by pulling electron density away from it, thus making it less susceptible to attack by electrophiles in electrophilic aromatic substitution reactions.

• Examples of EWGs include the chloride ion (-Cl) and the positively charged trimethylammonium group (-\(\stackrel{\oplus}{\text{NMe}}_3\)).
• These groups destabilize the positively charged intermediate formed during the reaction, hindering progress.

When an EWG is present, the rate constant ratio is typically less than 1 compared to benzene. This indicates that the reaction requires more energy to overcome the destabilization effects caused by these electron-withdrawing groups. Thus, the substitution reaction proceeds more slowly than it would in unsubstituted benzene.