Question

The most reactive compound towards electrophilic aromatic substitution is (A) Anisole (B) Toluene (C) Aniline (D) Phenoxide ion

Short answer

The most reactive compound towards electrophilic aromatic substitution is the Phenoxide ion (D) because it has the most electron-donating substituent (-O^-) among the given compounds, increasing its reactivity.

Step-by-step solution

Identify the substituents in each compound.

In anisole (C6H5OCH3), the substituent is a methoxy group (-OCH3). In toluene (C6H5CH3), it's a methyl group (-CH3). In aniline (C6H5NH2), there's an amino group (-NH2). Finally, in the phenoxide ion (C6H5O^-), the substituent is an oxo group (-O^-).

Classify the groups according to their electron-donating power.

The electron-donating power of a group can be classified as follows: -O^- > -NH2 > -OCH3 > -CH3. The reasoning behind this order is based on how good the group is at donating electron density to the aromatic ring.

Combine information from Steps 1 and 2 to answer the question.

According to the electron-donating power of substituents, the phenoxide ion with the -O^- group is the most electron-donating, meaning it has the highest reactivity towards electrophilic aromatic substitution among these four compounds.

Final Answer: Phenoxide ion

Hence, the most reactive compound towards electrophilic aromatic substitution is the Phenoxide ion (D).

Key concepts

Phenoxide Ion Reactivity

The phenoxide ion, represented as C6H5O^-, displays a remarkable reactivity in electrophilic aromatic substitution (EAS) reactions. This reactivity can be attributed to the ion’s structure, where the negative charge on the oxygen atom is delocalized over the aromatic ring. Delocalization increases the electron density on the ring, making it more nucleophilic. Nucleophilic species are attractive to electrophiles, or electron-seeking species. In the context of EAS, this amplified nucleophilicity signifies that the phenoxide ion is highly reactive, as it can readily donate electron density to an incoming electrophile.

Furthermore, the oxygen atom in the phenoxide ion has a higher electronegativity compared to nitrogen or carbon atoms present in groups like -NH2 or -CH3. This means oxygen is better at stabilizing the negative charge through resonance, which further contributes to the high reactivity of the phenoxide ion in EAS processes.

Substituent Effects in EAS

In electrophilic aromatic substitution reactions, substituents already present on the aromatic ring have a significant influence on the reactivity and orientation of the incoming electrophile. These substituents can be either electron-donating or electron-withdrawing.

Electron-Donating Substituents

Groups like -OH, -OCH3, and -NH2 donate electron density into the ring via resonance or induction, making the ring more nucleophilic. Consequently, these substituents activate the ring towards EAS, facilitating the attack by electrophiles.

Electron-Withdrawing Substituents

On the other hand, groups such as -NO2, -SO3H, or -COOH pull electron density away from the aromatic ring, decreasing its nucleophilicity. These groups deactivate the ring and make it less reactive to electrophiles.

Substituents also influence the position where new groups are added during EAS. Electron-donating groups generally direct incoming electrophiles to the ortho and para positions, while electron-withdrawing groups typically direct to the meta position.

Electron-Donating Groups

Electron-donating groups (EDGs) are fundamental in determining the reactivity of aromatic compounds towards EAS. These groups, as the name suggests, donate electrons and increase the electron density of the aromatic ring. Examples include alkyl groups (-CH3, -C2H5), methoxy (-OCH3), and amino (-NH2) groups. They can push electrons towards the ring either through resonance - where electrons are shared through overlapping p-orbitals, or through inductive effect - the shifting of electron density through sigma bonds.

EDGs are characterized by their powerful ability to activate aromatic rings, making them more likely to react with electrophiles. Furthermore, the position of these substituents on the aromatic ring dictates the pattern of the substitution reaction, enhancing reactivity at specific sites on the ring (generally the ortho and para positions, relative to the location of the EDG). Recognizing the presence and impact of electron-donating groups is crucial for predicting the outcomes of electrophilic aromatic substitution reactions.