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

(a) Strong mesomeric effect.

Step-by-step solution

Understanding Electrophilic Substitution

Electrophilic substitution in aromatic compounds involves the replacement of a hydrogen atom on the aromatic ring by an electrophile. The reactivity of an aromatic compound towards electrophilic substitution depends on the electron density of the aromatic ring.

Examining the Role of Hydroxyl Group in Phenol

Phenol contains an -OH group directly attached to the benzene ring. This hydroxyl group is an activating group for electrophilic substitution due to its electron-donating effects, which increase the electron density on the benzene ring.

Mesomeric Effect of Upliftment

The -OH group in phenol can donate electrons via resonance (mesomeric effect) into the pi system of the benzene ring. This mesomeric effect enhances the electron density primarily at the ortho and para positions, making the ring more attractive to electrophiles compared to benzene.

Comparing Provided Options

Evaluate each option: (a) Strong mesomeric effect refers to resonance stabilization via electron donation, which is correct as previously explained. (b) Hyperconjugative effect is less relevant here as it's more associated with alkyl groups. (c) Inductive effect refers to electron withdrawal through sigma bonds, which is lesser effective in this context compared to mesomeric effect. (d) Hydrogen bonding could affect solubility but not the reactivity towards electrophilic substitution.

Reaching a Conclusion

Given these considerations, the most accurate reason for enhanced reactivity of phenol compared to benzene in electrophilic substitution is option (a), the strong mesomeric effect offered by the -OH group.

Key concepts

Mesomeric Effect

In the realm of organic chemistry, the mesomeric effect plays a vital role. It is a type of resonance effect due to the delocalization of \( ext{pi (}c0 ext{)}\) electrons through a conjugated system. In the context of phenol, this effect is particularly poignant due to the presence of an -OH group.
The hydroxyl group in phenol can release electrons into the aromatic ring, increasing the electron density. This happens through resonance, where the lone pair of electrons on the oxygen can be shared into the \( ext{pi} ext{)}\) system of the benzene ring.
- **Enhanced Reactivity:** The increase in electron density, especially at the ortho and para positions, enhances phenol's reactivity towards electrophiles compared to benzene.- **Ortho and Para Positions:** Due to resonance, these positions become more negatively charged, which means they are more "inviting" to positively charged species, i.e., electrophiles.
This mesomeric effect makes phenol an exciting and highly reactive compound regarding electrophilic aromatic substitution.

Reactivity of Aromatic Compounds

Reactivity in aromatic compounds largely depends on their structure and the functional groups present. Aromatic rings, like benzene, typically exhibit stability due to their conjugated pi systems. However, when substituents like the hydroxy group in phenol come into play, the dynamics change.
- **Electron Donating Groups (EDG):** Groups like -OH can push electrons into the ring, increasing the overall electron density. - **Substituent Effects:** Activating groups increase reactivity by making the ring more susceptible to attack by electrophiles. - **Comparison with Benzene:** Pure benzene lacks such substituents. Therefore, it is less reactive when compared to phenol, as there's no additional electron density uplift.
Understanding these concepts aids in predicting and explaining the outcomes of reactions involving substituted aromatic compounds. Phenol, with its activating hydroxyl group, serves as a prime example of enhanced reactivity due to these effects.

Electron Density in Aromatic Rings

The electron density in aromatic rings is crucial to understanding their chemical behavior, particularly in electrophilic substitution reactions. In a stable benzene ring, electrons are symmetrically delocalized, forming a stable structure.
With substituents such as the -OH group in phenol, this balance shifts. - **Impact of Electron Donors:** Electron-donating groups increase electron density within the ring, primarily at specific positions. - **Localization in Phenol:** The increased electron density is most significant at the ortho and para positions, which are favorably attacked by electrophiles. - **Resulting Reactivity:** Phenol becomes more reactive than benzene due to this enhanced electron availability, leading to a more significant attraction of electrophiles to these positions.
In summary, electron density distribution significantly impacts the reactivity and mechanism of reaction of aromatic compounds, as evident in phenol's behavior in electrophilic substitution.