Question

which of the following compounds undergoes electrophilic aromatic substitution at a slower rate than benzene under identical conditions?

Short answer

Nitrobenzene undergoes electrophilic aromatic substitution at a slower rate than benzene under identical conditions because it has an electron-withdrawing nitro (-NO2) group, which decreases the electron density on the benzene ring and makes it less reactive towards electrophilic aromatic substitution.

Step-by-step solution

Benzene

Benzene has no substituents, and we consider its rate of electrophilic aromatic substitution as the reference for comparing other compounds.

Toluene

Toluene has a methyl (-CH3) group as a substituent on the benzene ring.

Nitrobenzene

Nitrobenzene has a nitro (-NO2) group as a substituent on the benzene ring.

Anisole

Anisole has a methoxy (-OCH3) group as a substituent on the benzene ring. Step 2: Determine the electronic effect of the substituents

Methyl group (-CH3) in Toluene

The methyl group is an electron-donating group (EDG), which increases the electron density on the benzene ring, making it more reactive towards electrophilic aromatic substitution.

Nitro group (-NO2) in Nitrobenzene

The nitro group is an electron-withdrawing group (EWG), which decreases the electron density on the benzene ring, making it less reactive towards electrophilic aromatic substitution.

Methoxy group (-OCH3) in Anisole

The methoxy group is also an electron-donating group (EDG), which increases the electron density on the benzene ring, making it more reactive towards electrophilic aromatic substitution. Step 3: Compare the reactivity of compounds with benzene

Comparison of Reactivity

Comparing the rates of electrophilic aromatic substitution, we see that: 1. Toluene (with an EDG) will be more reactive than benzene. 2. Nitrobenzene (with an EWG) will be less reactive than benzene. 3. Anisole (with an EDG) will be more reactive than benzene. Based on the comparison, the compound that undergoes electrophilic aromatic substitution at a slower rate than benzene under identical conditions is nitrobenzene.

Key concepts

Electronic Effects of Substituents

In electrophilic aromatic substitution (EAS) reactions, the electronic nature of substituents on the benzene ring can significantly influence the reaction rate. A substituent can either donate electrons to the ring, thereby enhancing its reactivity, or withdraw electrons from the ring, reducing its reactivity. Substituents are classified into two main types based on their electronic effects:

• **Electron-Donating Groups (EDGs):** These groups, such as a methyl (-CH3) or methoxy (-OCH3) group, increase electron density on the ring. This happens because they push electrons toward the aromatic ring through inductive or resonance effects.
• **Electron-Withdrawing Groups (EWGs):** Substituents such as a nitro (-NO2) group pull electrons away from the ring, reducing its electron density. They generally contain highly electronegative elements or multiple bonds to electronegative atoms that can delocalize electrons away from the benzene ring.

Understanding these effects is crucial because the electron-donating groups make the ring more nucleophilic, enhancing its ability to attack electrophiles, while electron-withdrawing groups have the opposite effect, making the ring less nucleophilic and, hence, less reactive in EAS reactions.

Electrophilic Reaction Rate Comparison

When comparing the rate of electrophilic aromatic substitution among different benzene derivatives, it's essential to focus on the nature of the substituents present. Benzene itself can serve as a benchmark because it neither donates nor withdraws electrons effectively. As noted in the exercise:

• Toluene, which has a methyl group, will have an increased rate of reaction compared to benzene because the methyl group is an EDG, making the ring more reactive.
• In contrast, nitrobenzene, which contains a nitro group, will have a decreased rate of reaction because the nitro group is an EWG, making the ring less reactive than benzene.
• Anisole, with a methoxy group, similarly to toluene, reacts faster than benzene due to the electron-donating effect of the methoxy group.

This comparison hinges on the fact that increased electron density due to EDGs facilitates the attack of electrophiles, enhancing EAS. However, reduced electron density from EWGs slows down the reaction by making the benzene ring less nucleophilic.

Substituent Effects in Aromatic Compounds

Substituents on aromatic rings can lead to profound changes in chemical reactivity and selectivity during electrophilic aromatic substitution reactions. This effect is a combination of the electronic influences previously discussed and the positional preferences these groups instill, known as "ortho-, meta-, and para-directing effects." For example:

• EDGs typically direct incoming electrophiles to the ortho and para positions. This is because the increased electron density in these positions stabilizes the intermediate complex formed during the reaction.
• EWGs predominantly direct electrophiles to the meta position, as this often avoids the destabilizing influence these groups exert at the ortho and para positions.

Understanding these effects is essential for predicting which isomers will form preferentially during reactions and for designing compounds with desired chemical behaviors. Recognizing the interplay of both electronic and steric factors gives chemists the predictive power to manipulate reactivity for specific outcomes, showcasing the importance of substituent effects in aromatic chemistry.