Question

The compound which is most reactive towards electrophilic aromatic substitution is (A) Benzophenone (B) Phenyl benzoate (C) Ethylbenzoate (D) Benzoic acid

Short answer

(B) Phenyl benzoate

Step-by-step solution

Identify the substituent groups

First, let's identify the substituent groups in each compound: (A) Benzophenone: C6H5COC6H5 - has phenyl (C6H5) group as substituent (B) Phenyl benzoate: C6H5COOC6H5 - has phenoxide (C6H5COO) group as substituent (C) Ethylbenzoate: C6H5COOCH2CH3 - has ethoxide (CH2CH3COO) group as substituent (D) Benzoic acid: C6H5COOH - has carboxyl (COOH) group as substituent

Determine the electron-donating/withdrawing nature of the substituent groups

Now that we have identified the substituent groups, let's determine if they are electron-donating or electron-withdrawing: Phenyl group (C6H5): Weakly electron-donating Phenoxide group (C6H5COO): Electron-donating (due to resonance effect) Ethoxide group (CH2CH3COO): Weakly electron-donating (due to resonance effect) Carboxyl group (COOH): Electron-withdrawing (due to resonance effect)

Compare the reactivity of the compounds based on their substituent groups

Compounds with electron-donating groups will be more reactive towards electrophilic aromatic substitution, as they increase the electron density of the ring and make it more attractive to electrophiles. Based on the electron-donating/withdrawing nature of the substituent groups of these compounds, we can rank their reactivity as follows: Phenyl benzoate > Ethylbenzoate > Benzophenone > Benzoic acid Therefore, the compound most reactive towards electrophilic aromatic substitution is:

Answer

(B) Phenyl benzoate

Key concepts

Electron-Donating Groups in EAS

Understanding the role of electron-donating groups (EDGs) is crucial when studying electrophilic aromatic substitution (EAS). These groups release electrons into an aromatic ring, enhancing its electron density. An increased electron density makes the aromatic compound more reactive to electrophiles, as these compounds seek out areas of high electron concentration in a reaction.

EDGs, such as alkyl and methoxy groups, can influence where electrophiles attach to the aromatic ring. This is referred to as the directing effect, where EDGs typically direct the incoming electrophile to positions ortho and para to themselves. The presence of an EDG can significantly increase the rate of an EAS reaction compared to a benzene ring without any substituents.

Substituent Effects in EAS

The ability of a substituent group attached to an aromatic ring to either donate or withdraw electrons can significantly affect the reactivity of the compound in EAS reactions. Substituent effects can be divided into two categories: electron-donating and electron-withdrawing effects.

Electron-withdrawing groups (EWGs) like nitro (NO2), carbonyl (C=O), and sulfonyl (SO2) groups decrease the electron density of the aromatic ring, thereby making the ring less reactive to electrophilic attack. In contrast, EDGs such as hydroxyl (-OH) and amino (-NH2) groups enhance the reactivity of the aromatic ring by increasing electron density through resonance or inductive effects. This interplay of electron availability is a guiding principle in predicting the outcome of EAS reactions.

Reactivity in Aromatic Compounds

Aromatic compounds vary in their reactivity toward EAS based on the substituents present. The electron density of the ring, influenced by these substituents, dictates how readily the ring will undergo reaction with an electrophile. Typically, compounds with EDGs will react faster and at milder conditions compared to those with EWGs or no substituents at all.

Comparing various substituents, the strongest EDGs will promote the highest reactivity, whereas strong EWGs can not only decrease reactivity but also redirect the electrophilic attack to less expected positions on the ring. For instance, a methyl group would augment reactivity, while a nitro group would decrease it. In the given exercise, Phenyl benzoate tops the reactivity scale among the listed options due to its strong electron-donating phenoxide group.