Electrophilic Substitution Reactivity of Phenol and Aniline

The reactivity of aromatic compounds, specifically phenol and aniline, towards electrophilic substitution reactions is greatly influenced by the type and strength of electron-donating or withdrawing substituents attached to their aromatic rings. This article delves into the detailed comparison between phenol and aniline, elucidating why aniline is slightly more reactive than phenol in electrophilic substitution reactions.

Structure of Phenol and Aniline

Phenol (C6H5OH) features a hydroxyl group (-OH) attached to the benzene ring, whereas aniline (C6H5NH2) has an amino group (-NH2) attached to the benzene ring. These functional groups play a pivotal role in determining their reactivity.

Electron-Donating Effects

The -OH group in phenol is an electron-donating group through resonance, which slightly activates the ring towards electrophilic substitution. However, the -OH group also has an -I (inductive) effect due to the electronegativity of oxygen, which withdraws electrons from the ring. In contrast, the -NH2 group in aniline is a stronger electron-donating group than -OH. The lone pair on nitrogen can delocalize into the ring, increasing the electron density and enhancing the reactivity of the ring.

Comparison of Reactivity

Both -OH and -NH2 groups increase the reactivity of their respective rings. However, the -NH2 group in aniline is more effective at stabilizing the positive charge that develops during the electrophilic substitution mechanism, making the aniline ring more reactive than the phenol ring. Additionally, aniline's resonance structures allow for greater stabilization of the intermediate formed during the electrophilic substitution, further increasing its reactivity.

Further Analysis

Both -OH and OCH3 groups exhibit Resonance Effect (R-effect). Structures II to IV (where ve and -ve charges are separated) are capable of self-stabilization by releasing a proton, a very stable chemical species. Structures VII to IX (also with ve and -ve charges) cannot stabilize themselves by releasing a methyl carbocation, which is a very unstable chemical species. This extra stability of structures II to IV, through the loss of a proton, is responsible for the greater reactivity of phenol over anisole. For example, phenol reacts with Br2 in water to give 2,4,6-tribromophenol, whereas anisole does not react with Br2 water at all.

Conclusion

In summary, aniline is slightly more reactive than phenol in electrophilic substitution reactions due to the stronger electron-donating resonance effect of the -NH2 group compared to the -OH group in phenol. The inductive effect of the hydroxyl group in phenol further diminishes the overall electron density, leading to its lower reactivity.