Question

Phenol is more reactive than benzene towards eletrophillic substitution due to (a) strong mesomeric effect (b) hyperconjugative effect (c) Inductive effect only (d) hydrogen bonding

Short answer

Phenol is more reactive due to the strong mesomeric effect.

Step-by-step solution

Understanding the Problem

The exercise asks why phenol is more reactive towards electrophilic substitution than benzene. We need to identify the factor that increases phenol's reactivity in comparison to benzene.

Comparing Structures

Benzene is a simple aromatic ring, whereas phenol has an -OH group attached to the benzene ring. This -OH group can affect the electronic distribution within the ring.

Analyzing Effects

We consider different effects: the mesomeric effect, hyperconjugative effect, inductive effect, and hydrogen bonding. Each effect influences how the electron density is distributed in phenol differently.

Mesomeric Effect Explanation

The -OH group on phenol exhibits a +M (positive mesomeric) effect by donating electrons into the aromatic ring through resonance, increasing the electron density on the ortho and para positions.

Evaluating Other Effects

The hyperconjugative effect primarily involves sigma bonds and is not significant in phenol. Inductive effects are present but weaker than resonance (mesomeric) effects. Hydrogen bonding affects physical properties more than reactivity towards electrophilic substitution.

Conclusion Based Analysis

The primary reason phenol is more reactive towards electrophilic substitution than benzene is due to the strong mesomeric effect of the -OH group, which activates the ring, making it more attractive to electrophiles.

Key concepts

Mesomeric Effect

The mesomeric effect is a key concept to understand why phenol is more reactive towards electrophilic substitution compared to benzene. In phenol, an -OH group is attached to the aromatic benzene ring. This -OH group donates electrons through resonance, a process called the positive mesomeric effect (+M effect). This donation increases the electron density, especially at the ortho and para positions of the benzene ring.
This enhanced electron density makes these positions more prone to attack by electrophiles, as they are positively charged species attracted to electrons. As a result, the benzene ring in phenol becomes more reactive due to this mesomeric effect, facilitating electrophilic substitution.

Phenol Reactivity

Phenol displays higher reactivity in electrophilic substitution reactions compared to benzene due to its structure and the presence of the -OH group. The key player here is the positive mesomeric effect, which we've already discussed. This effect significantly influences phenol's reactivity by

• increasing electron density in specific positions (ortho and para),
• making these positions more susceptible to electrophile attack,
• leading to faster and more favorable reactions compared to benzene.

For students, it's critical to grasp that the presence of the -OH group and its mesomeric effect are what makes phenol distinctively reactive in comparison to unsubstituted benzene, which lacks such activating groups.

Comparison with Benzene

Understanding the difference in the reactivity between phenol and benzene starts with examining the structure of these compounds.
Phenol is simply benzene with an additional -OH group. While benzene itself is known for its stability and relative lack of reactivity with electrophiles, phenol's attached -OH group changes this.
The -OH group movement of electrons and the introduction of positive mesomeric effect make phenol more active. This starkly contrasts benzene's reactivity, which, without such electron-donating substituents, remains much less active towards electrophilic substitution reactions.

Electronic Effects in Aromatic Compounds

In aromatic compounds, electronic effects play crucial roles in dictating reactivity. Besides the mesomeric effect, other effects like inductive effects can occur. However, in the case of phenol and its enhanced reactivity, the mesomeric effect overshadows the inductive effect and any potential hyperconjugation effects.

• The inductive effect involves the donation or withdrawal of electrons through sigma bonds but is weaker than the powerful resonance of the mesomeric effect.
• Hyperconjugation, more applicable in contexts with alkyl groups, is not significant for phenol.
• Hydrogen bonding, although important for physical characteristics like solubility, does not directly affect reactivity towards electrophilic substitution.

It's these electronic effects, with the strong focus on resonance, that really shape the behavior of aromatic compounds in reaction scenarios.