Question

Which one of the following is least acidic? (a) Phenol (b) O-fluorophenol (c) M-fluorophenol (d) P-fluorophenol

Short answer

M-fluorophenol is the least acidic.

Step-by-step solution

Understanding Acidity in Phenols

Acidity in phenols is influenced by the ability of the phenolic proton to dissociate, which depends on the stability of the resulting phenoxide ion. Electron-withdrawing groups (EWG) such as fluorine increase acidity by stabilizing the negative charge on the oxygen through resonance or inductive effects.

Effect of Fluorine Substituent Position

Fluorine is an electron-withdrawing group, and its effect on phenol's acidity depends on its position relative to the hydroxyl group. The ortho (o), meta (m), and para (p) positions will affect the phenol's acidity differently due to differing resonance and inductive effects.

Analyzing Substituent Effects

For o-fluorophenol, the fluorine is at the ortho position, which allows both resonance and inductive effects. For m-fluorophenol, the fluorine is at the meta position, predominantly allowing inductive effects, but little to no resonance effects as it doesn't directly influence the oxygen's lone pair. For p-fluorophenol, the fluorine is at the para position, allowing resonance but minimal inductive effects impacting the hydroxyl group.

Comparing Acidic Strengths

Phenol's acidity increases with the presence of electron-withdrawing groups near the hydroxyl group, particularly through resonance stabilization. Ortho and para positions allow better resonance interaction with the phenoxide ion. Since both o-fluorophenol and p-fluorophenol benefit from resonance stabilization, m-fluorophenol lacks the resonance interaction, making it the least acidic among the fluorinated phenols.

Conclusion on Acidic Strength

Comparing phenol, o-fluorophenol, m-fluorophenol, and p-fluorophenol, m-fluorophenol has the least ability to stabilize the phenoxide ion through resonance stabilization. As a result, m-fluorophenol is the least acidic due to limited electron-withdrawing influence in comparison to the others.

Key concepts

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) have a significant influence on the acidity of phenols. These groups pull electron density away from other parts of the molecule, essentially stabilizing any negative charges that might occur. This is particularly important in phenols, where the main concern is the stability of the phenoxide ion, which forms when the phenolic proton is lost.
In the context of the problem, fluorine acts as an EWG. It increases the acidity of phenol by stabilizing the phenoxide ion. The fluorine atom achieves this through its high electronegativity, which pulls electron density away from the oxygen atom in the phenoxide ion.
This means that the stronger the electron-withdrawing effect, the more likely the phenol is to donate its proton, resulting in higher acidity. This principle is key when analyzing how different substituent positions for fluorine affect phenol's overall acidity.

Resonance and Inductive Effects

When discussing why EWGs affect the acidity of phenols, it's important to understand the specific mechanisms at play, namely resonance and inductive effects. These two effects describe how electron density is redistributed within a molecule. They are pivotal in determining how electron-withdrawing groups stabilize the phenoxide ion.
The resonance effect involves the delocalization of electrons across the molecule. In the case of o-fluorophenol and p-fluorophenol, the electron-withdrawing fluorine can participate in resonance structures that spread the negative charge more evenly throughout the molecule, stabilizing the phenoxide ion.

• **Resonance Effect:** Allows electrons to be shared over different atoms, spreading out negative charge.
• **Inductive Effect:** Involves electron pull through sigma bonds, decreasing electron density at the oxygen.

The inductive effect comes into play via the sigma bonds in the molecule. Here, fluorine's electronegativity pulls electron density through bonds, creating a more stable ion by attracting electrons away from the oxygen atom in the phenoxide ion. Both these effects combined can lead to significant changes in acidity depending on whether the substituent is at the ortho, meta, or para positions.

Substituent Position Effects

The position of a substituent like fluorine on a phenol ring is crucial in determining the molecule's acidity. Each position—ortho, meta, and para—offers unique scenarios for both resonance and inductive effects.

• **Ortho Position (o-fluorophenol):** Proximity allows both inductive and resonance effects. The inductive effect is quite strong due to the closeness of fluorine to the hydroxyl group.
• **Meta Position (m-fluorophenol):** Primarily shows the inductive effect. Lacks significant resonance interaction because it cannot directly affect the oxygen's lone pair.
• **Para Position (p-fluorophenol):** Mainly favors resonance stabilization with limited inductive effects due to the longer distance, yet still provides more stabilization than meta.

To summarize, the meta position offers the least stabilization through resonance, making m-fluorophenol the least acidic of the derivatives examined. Therefore, when evaluating acidity changes, recognizing how substituent positions influence both inductive and resonance effects is essential.