Question

Which of following reacts slower than benzene in electrophilic substitution? (1) ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CH}}_3$ (2) ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{NO}}_2$ (3) ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{OH}$ (4) ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{NH}}_2$

Short answer

(2) ${\text{C}}_6{\text{H}}_5{\text{NO}}_2$

Step-by-step solution

Understanding electrophilic substitution

Electrophilic substitution involves an electrophile attacking a benzene ring. The rate of this reaction depends on the substituents attached to the benzene ring.

Identifying electron-donating and electron-withdrawing groups

Electron-donating groups (EDGs) increase the electron density on the benzene ring and enhance its reactivity towards electrophiles. Electron-withdrawing groups (EWGs) decrease the electron density on the benzene ring and decrease its reactivity towards electrophiles.

Analyzing the substituents

Consider the listed substituents of each compound: 1. ${\text{C}}_6{\text{H}}_5{\text{CH}}_3$ has a methyl group (EDG). 2. ${\text{C}}_6{\text{H}}_5{\text{NO}}_2$ has a nitro group (EWG). 3. ${\text{C}}_6{\text{H}}_5\text{OH}$ has a hydroxyl group (EDG). 4. ${\text{C}}_6{\text{H}}_5{\text{NH}}_2$ has an amino group (EDG).

Conclusion

The nitro group is an electron-withdrawing group that decreases the reactivity of benzene towards electrophilic substitution. Hence, ${\text{C}}_6{\text{H}}_5{\text{NO}}_2$ (option 2) reacts slower than benzene in electrophilic substitution.

Key concepts

Electron Donating Groups (EDGs)

Electron donating groups (EDGs) are substituents that increase the electron density on a benzene ring. They do this by either through electron-releasing inductive effects or by resonance effects, which stabilizes the benzene ring and enhances its reactivity towards electrophiles. For example, in compound ${\text{C}}_6{\text{H}}_5{\text{CH}}_3$, the methyl group is an EDG. This group donates electrons through an inductive effect, making the benzene more reactive. Similarly, the hydroxyl group (as in ${\text{C}}_6{\text{H}}_5\text{OH}$) and the amino group (as in ${\text{C}}_6{\text{H}}_5{\text{NH}}_2$) donate electrons by resonance, further increasing reactivity. Electrophilic substitution reactions with benzene derivatives containing EDGs are often faster than with benzene itself.

Electron Withdrawing Groups (EWGs)

Electron withdrawing groups (EWGs) are substituents that reduce the electron density on a benzene ring. They achieve this by either withdrawing electrons through electronegativity or through resonance effects. This makes the benzene ring less reactive towards electrophiles. For instance, in compound ${\text{C}}_6{\text{H}}_5{\text{NO}}_2$, the nitro group is a strong EWG. It withdraws electron density through both inductive effects and resonance, leading to a decrease in the reactivity of the benzene ring. Thus, electrophilic substitution reactions involving benzene derivatives with EWGs occur slower than with benzene itself.

Benzene Derivatives Reaction Rates

The rate at which benzene derivatives undergo electrophilic substitution is influenced by the type of substituent present on the benzene ring. Electron donating groups (EDGs) enhance the reaction rate, while electron withdrawing groups (EWGs) slow it down. For examples:

• Compounds like ${\text{C}}_6{\text{H}}_5{\text{CH}}_3$ (with a methyl group) react faster than benzene due to the electron-donating nature of the methyl group.
• Compounds like ${\text{C}}_6{\text{H}}_5{\text{NO}}_2$ (with a nitro group) react slower than benzene because the nitro group is a strong EWG.
• Hydroxyl and amino groups, as seen in ${\text{C}}_6{\text{H}}_5\text{OH}$ and ${\text{C}}_6{\text{H}}_5{\text{NH}}_2$ respectively, also increase the reactivity due to their electron-donating effects.

Understanding whether a substituent is an EDG or EWG helps predict the reactivity of benzene derivatives in electrophilic substitution reactions.