Question

The highest rate of electrophilic aromatic substitution is observed in (A) Chlorobenzene (B) Acetanilide (C) Benzene (D) Nitrobenzene

Short answer

The highest rate of electrophilic aromatic substitution is observed in Acetanilide (B), as its amide group is a weakly electron-donating group, increasing electron density in the aromatic ring and enhancing the rate of the reaction.

Step-by-step solution

Analyze the effect of the substituents on the aromatic ring

We have four compounds to analyze: (A) Chlorobenzene: The chloro group is an electron-withdrawing group, which reduces the electron density in the aromatic ring and therefore slows down the rate of electrophilic aromatic substitution. (B) Acetanilide: The amide group is a weakly electron-donating group, which increases the electron density in the aromatic ring and enhances the rate of electrophilic aromatic substitution. (C) Benzene: This is the parent compound with no substituents on the aromatic ring, which has a balanced electron density and moderate rate of electrophilic aromatic substitution. (D) Nitrobenzene: The nitro group is a strong electron-withdrawing group, which significantly reduces the electron density in the aromatic ring and results in the slowest rate of electrophilic aromatic substitution.

Determine the compound with the highest rate of electrophilic aromatic substitution

From the analysis in Step 1, we can rank the compounds according to how their substituents affect the rate of electrophilic aromatic substitution: - Acetanilide (with electron-donating group) > Benzene > Chlorobenzene (with weak electron-withdrawing group) > Nitrobenzene (with strong electron-withdrawing group) Thus, the compound with the highest rate of electrophilic aromatic substitution is Acetanilide.

Answer

The highest rate of electrophilic aromatic substitution is observed in Acetanilide (B).

Key concepts

Electron Withdrawing Groups

Electron withdrawing groups (EWGs) are substituents attached to an aromatic ring that decrease the electron density within the ring. They achieve this by pulling electrons away from the ring through inductive or resonance effects. As a result, the aromatic ring becomes less reactive towards electrophilic aromatic substitution (EAS) reactions. Understanding the behavior of EWGs is crucial because it affects the rate and position of electrophilic attacks on the aromatic compound. Common examples of typical EWGs include:

• Nitro group (-NO₂)
• Carbonyl group (-C=O)
• Halogens (though they have unique behavior)

In nitrobenzene, the strong electron-withdrawing nature of the nitro group leads to a significant decrease in the electron density of the benzene ring. This makes nitrobenzene less reactive and slower to undergo EAS. Similarly, chlorobenzene contains a chloro group, which is an electron-withdrawing group. However, due to its resonance donation capability, its withdrawing effect is somewhat moderated compared to other EWGs like the nitro group.

Electron Donating Groups

Electron donating groups (EDGs) are substituents that increase the electron density of an aromatic ring. They do this by donating electrons through resonance or inductive effects, which makes the ring more reactive towards EAS reactions. These groups are important because they can significantly enhance the rate of reaction and influence the position of attack by the electrophile. Typical examples of EDGs include:

• Alkyl groups (-R)
• Amine groups (-NH₂)
• Hydroxyl groups (-OH)

In acetanilide, the presence of an amide group acts as a mild electron-donating group. It increases the electron density in the benzene ring, thereby making it more susceptible and faster to undergo EAS compared to unsubstituted benzene. The ability of electron-donating groups to stabilize the transition state of the reaction is a key factor in their influence on reaction rates.

Aromatic Compounds

Aromatic compounds are a class of molecules that have a specific type of stability due to their conjugated pi electron clouds. The classic example of an aromatic compound is benzene, represented by the chemical formula C₆H₆. Benzene and its derivatives are characterized by:

• A ring structure with alternating single and double bonds
• Delocalized electrons, leading to enhanced stability
• Planarity, which contributes to the resonance of the pi system

In the context of electrophilic aromatic substitution, the reactivity of the aromatic compound is directly influenced by the presence and type of substituents on the benzene ring. Benzene itself can undergo EAS at a moderate rate due to its stable electron cloud, which is neither excessively electron-rich nor electron-poor. The introduction of substituents that are either electron withdrawing or donating can dramatically alter the rate of EAS by either stabilizing or destabilizing the transition state during the reaction.