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Magazine Chapter 27: Problem 8
Phenol is less acidic than (a) O-nitrophenol (b) p-methylphenol (c) methanol (d) ethanol
Short Answer
Expert verified
Phenol is less acidic than O-nitrophenol.
Step by step solution
01
Understand Phenol's Acidity
Phenol is weakly acidic due to its ability to release a hydrogen ion (H+) from its hydroxyl group, forming a phenoxide ion. The relative acidity of phenol can be influenced by substituents on the benzene ring.
02
Evaluate O-nitrophenol
The nitro group is an electron-withdrawing group. In O-nitrophenol, the presence of a nitro group at the ortho position increases the acidity of phenol by stabilizing the phenoxide ion. Thus, O-nitrophenol is more acidic than phenol.
03
Evaluate P-methylphenol
The methyl group is an electron-donating group. In p-methylphenol, the presence of a methyl group at the para position slightly decreases the acidity of phenol compared to pure phenol. Thus, p-methylphenol is less acidic than phenol.
04
Evaluate Methanol
Methanol is an alcohol and does not have the acidic hydrogen that phenol has. It is considerably less acidic than phenol because it doesn鈥檛 stabilize a negative charge through resonance as in phenol.
05
Evaluate Ethanol
Similar to methanol, ethanol is an alcohol and significantly less acidic than phenol. Ethanol, like methanol, does not have the acidic feature present in phenol and its ionization does not benefit from resonance stabilization.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Electron-Withdrawing Groups
Electron-withdrawing groups (EWGs) play a crucial role in enhancing the acidity of phenol. These groups pull electron density away from the phenol's benzene ring. This increases the positive character of the hydrogen atom in the hydroxyl group, facilitating its release as a hydrogen ion (\( H^+ \)). When an EWG is present, it often stabilizes the resulting phenoxide ion by delocalizing the negative charge over the ring and substituent. Here are some effects of EWGs:
- They increase the electron deficiency at the oxygen atom in the hydroxyl group.
- This promotes the release of the hydrogen ion, increasing acidity.
- Z groups, particularly nitro groups like in o-nitrophenol, significantly stabilize the negative phenoxide ion by resonance.
Electron-Donating Groups
In contrast to electron-withdrawing groups, electron-donating groups (EDGs) tend to decrease the acidity of phenol. They do this by donating electron density into the phenol's benzene ring, reducing the positive character of the hydrogen in the hydroxyl group. EDGs, such as methyl groups in p-methylphenol, decrease acidity through the following effects:
- They increase electron density on the oxygen, making it less likely to lose the hydrogen ion (\( H^+ \)).
- By providing more electron density, they destabilize the phenoxide ion, reducing its resonance stabilization.
- This results in a lower propensity for the hydroxyl group to release its hydrogen atom.
Phenoxide Ion Stabilization
Phenol's acidity is greatly influenced by the stability of the phenoxide ion, which is formed when phenol releases a hydrogen ion (\( H^+ \)). The stability of this ion directly correlates with the acidity of phenol; more stable phenoxide ions equate to higher acidity.The stabilization of the phenoxide ion involves:
- Resonance Stability: The negative charge on the phenoxide ion can be delocalized over the aromatic ring, providing resonance stability. This is a key factor in acidity.
- Substituent Effects: EWGs can enhance stabilization by further delocalizing the charge, increasing acidity. Conversely, EDGs reduce such stabilization, decreasing acidity.
- Positional Variations: The position of substituents (i.e., ortho, meta, para) affects the effective resonance and thus the stability of the phenoxide ion.
