Phenol is acidic primarily due to the presence of the hydroxyl (-OH) group attached to the aromatic benzene ring. The acidity of phenol can be understood through the following points:
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Resonance Stabilization of Phenoxide Ion: When phenol (C6H5OH) donates a proton (H+), it forms the phenoxide ion (C6H5O−). The negative charge on the oxygen atom can be delocalized into the aromatic ring through resonance. This resonance stabilization decreases the energy of the conjugate base (phenoxide ion), making it more favorable for phenol to lose a proton.
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Electronegativity: The oxygen atom in the hydroxyl group is highly electronegative, which helps to stabilize the negative charge after deprotonation. Hence, phenol is more acidic than aliphatic alcohols, as the resulting alkoxide ions are less stabilized.
Effect of Substituents on Acidity
The acidity of phenols can be significantly affected by the presence of other substituents on the aromatic ring. The two main types of substituents are electron-donating groups (EDGs) and electron-withdrawing groups (EWGs).
1. Electron Donating Groups (EDGs):
- Examples: Alkyl groups (-CH3, -C2H5), -OH, -O-, -NH2, etc.
- Effect: EDGs tend to raise the electron density of the aromatic ring and the hydroxyl oxygen because they donate electrons. This increased electron density reduces the stability of the phenoxide ion because the negative charge on the oxygen becomes less stabilized (due to increased repulsion). As a result, phenols substituted with electron-donating groups are generally less acidic compared to unsubstituted phenol.
2. Electron Withdrawing Groups (EWGs):
- Examples: Nitro (-NO2), carbonyl (-C=O), -CN, halogens (-F, -Cl, -Br, -I), etc.
- Effect: EWGs pull electron density away from the aromatic ring and the hydroxyl oxygen, thereby increasing the stability of the phenoxide ion by delocalizing the negative charge. This increased stabilization makes it easier for phenol to donate a proton, rendering phenols with electron-withdrawing substituents more acidic compared to unsubstituted phenol.
Summary
- Phenol is acidic due to resonance stabilization of the phenoxide ion formed upon deprotonation.
- Electron-donating groups decrease the acidity of phenols by destabilizing the phenoxide ion.
- Electron-withdrawing groups increase the acidity of phenols by stabilizing the phenoxide ion through better delocalization of the negative charge.
This relationship between substituents and acidity is a fundamental concept in organic chemistry, particularly in the context of aromatic compounds.
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