Acidity of Phenols: Phenols are organic compounds that contain a hydroxyl group (-OH) attached to an aromatic ring. The presence of the hydroxyl group makes phenols weak acids. Compared to alcohols, phenols exhibit greater acidity due to the resonance stabilization of the phenoxide ion formed after deprotonation.
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The acidity of phenols can be attributed to the following factors:
- Resonance Stabilization: The aromatic ring in phenols undergoes resonance, which stabilizes the phenoxide ion (PhO-) formed when the phenol loses a proton. The delocalization of the negative charge over the aromatic ring enhances the stability of the phenoxide ion and facilitates deprotonation.
- Electronegativity of the Substituents: Substituents attached to the aromatic ring can influence the acidity of phenols. Electron-withdrawing groups, such as nitro (-NO2) or carbonyl (-C=O) groups, increase the acidity of phenols by destabilizing the phenoxide ion through electron withdrawal from the aromatic ring. Conversely, electron-donating groups, like alkyl (-CH3, -C2H5) or hydroxy (-OH) groups, decrease the acidity by donating electron density to the aromatic ring and stabilizing the phenoxide ion.
- Degree of Substitution: Generally, phenols with more electron-withdrawing substituents are more acidic than those with electron-donating substituents. Additionally, ortho- and para-substituted phenols tend to be more acidic than meta-substituted phenols due to the proximity of the substituents to the hydroxyl group.
The acidity of phenols can be measured using pKa values, which represent the negative logarithm of the acid dissociation constant (Ka). Lower pKa values correspond to stronger acids. Phenols typically have pKa values in the range of 9-11, making them stronger acids than alcohols (pKa ~ 16-18).
The enhanced acidity of phenols compared to alcohols makes them useful in various chemical reactions. Phenols can undergo reactions such as nucleophilic substitution, oxidation, and condensation reactions due to the availability of the phenoxide ion.
It’s worth noting that the acidity of phenols is relatively modest compared to strong mineral acids, such as sulfuric acid or hydrochloric acid. However, in the context of organic chemistry, phenols are considered weak acids due to their ability to donate a proton to a base.
ACTUAL NOTES:
PATH: PHARMD/PHARMD NOTES/ PHARMD FIRST YEAR NOTES/ ORGANIC CHEMISTRY/ PHARMACEUTICAL ORGANIC CHEMISTRY/ ACIDITY OF PHENOLS.
