Question

Which one of the following is least acidic? (a) Phenol (b) o-fluorophenol (c) \(\mathrm{m}\)-fluorophenol (d) p-fluorophenol

Short answer

(a) Phenol is least acidic.

Step-by-step solution

Understand Acidity

Acidity in phenolic compounds is influenced by the phenol's ability to stabilize the negative charge on the oxygen after losing a hydrogen ion. This stability is affected by the electron-withdrawing or electron-donating nature of substituents attached to the benzene ring.

Electron-Withdrawing Effects

Fluorine is an electron-withdrawing group due to its high electronegativity. When attached to a benzene ring, it can influence the acidity of phenol depending on its position (ortho, meta, or para). This effect stabilizes the phenoxide ion, increasing acidity.

Assessing the Position of Substituents

The position of the fluorine on the aromatic ring affects acidity differently: - **Ortho (o-) and para (p-) positions:** These positions enable resonance stabilization, making phenol more acidic. - **Meta (m-) position:** The fluorine at the meta position does not allow for resonance stabilization and, thus, has a weaker electron-withdrawing effect compared to ortho and para positions.

Compare Acidities

Phenol is less acidic than o-fluorophenol and p-fluorophenol because the added fluorine allows for charge stabilization through resonance. However, m-fluorophenol does not benefit significantly from this resonance stabilization, making it less acidic than o-fluorophenol and p-fluorophenol, but more acidic than phenol itself.

Key concepts

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) play a crucial role in the acidity of phenolic compounds. These are atoms or groups that, due to their electronegativity, pull electron density away from the rest of the molecule. In the context of phenolic compounds, the presence of EWGs can increase acidity. This is achieved by stabilizing the negative charge formed on the oxygen atom when a hydrogen ion ( ) is lost. Fluorine, with its high electronegativity, is an excellent example of an electron-withdrawing group. When fluorine is attached to a benzene ring in phenol, it draws electrons towards itself. This action helps stabilize the phenoxide ion, the conjugate base formed after hydrogen ion loss. In terms of positioning:

• EWGs at the ortho or para positions to the hydroxyl group greatly enhance acidity.
• They work less effectively at the meta position, with less influence on acidity.

Resonance Stabilization

Resonance stabilization is another key concept in understanding the acidity of phenolic compounds. Resonance occurs when electrons are delocalized across the molecule, creating different structures that contribute to the overall stability. For phenolic compounds, when the phenol loses a hydrogen ion, a negative charge accumulates on the oxygen atom. With potential resonance, this negative charge isn't confined to just the oxygen. Instead, it can be shared or "delocalized" across the aromatic ring. Overall, this leads to a more stable structure. Thanks to this added stability, the phenoxide ion becomes a more stable conjugate base, increasing the acidity of the compound. The positioning of substituents such as fluorine can impact how effectively resonance stabilization occurs:

• Ortho and para positions allow for resonance pathways that include the withdrawing group's effects, hence enhancing overall stability.
• The meta position doesn’t contribute to the resonance stabilization of the phenoxide ion, making it less effective in enhancing acidity.

Effects of Substituents on Acidity

Substituents attached to the aromatic ring of phenolic compounds dramatically influence the acidity of the compound. Their effects depend on their ability to withdraw or donate electron density and on their position relative to the hydroxyl group. Through these influences, substituents can either enhance or reduce the acidity of phenols. Substituents like fluorine, which are electron-withdrawing, increase acidity by stabilizing the phenoxide ion. This stabilization occurs through a combination of inductive effects and resonance stabilization. This means the more effectively a substituent can withdraw electron density, and the more optimal its position (ortho and para) for resonance, the more it increases the acidity of phenolic compounds. Here's a quick look at positional effects:

• **Ortho and para positions**: Substituents here can not only withdraw electrons but also facilitate resonance, enhancing acidity.
• **Meta position**: Has reduced capabilities for resonance interaction, making it less effective in modifying acidity compared to ortho and para positions.

Understanding these concepts helps predict and rationalize the relative acidities of substituted phenols compared to plain phenol.