Problem 63 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

m-Fluorophenol is the least acidic.

Step 1: Understand the Concept of Acidity

Acidity in phenols is influenced by the substituents attached to the aromatic ring. An electron-withdrawing group increases the acidity, while an electron-donating group decreases it. The position of the substituent (ortho, meta, or para) also affects the acidity level, with para and ortho positions generally enhancing the effect more strongly than the meta position.

Step 2: Evaluate the Effect of Fluoro Substitution on Acidity

In the case of fluorophenols, the fluorine atom is an electron-withdrawing group due to its electronegativity. This withdrawal stabilizes the phenoxide ion (the conjugate base of phenol), increasing acidity. The effect varies with the position: para (p-), ortho (o-), and meta (m-).

Step 3: Compare Acidity with Substituent Positions

For fluoro-substituted phenols, the acidity order generally follows: para > ortho > meta. This is because the electron-withdrawing effect is most effective at the para and ortho positions, as they allow better resonance stabilization of the phenoxide ion. In the meta position, the resonance effect is weak.

Step 4: Identify the Least Acidic Compound

Given the choices: phenol, o-fluorophenol, m-fluorophenol, and p-fluorophenol, m-fluorophenol is the least acidic. This is because the fluorine's electron-withdrawing effect is least effective at the meta position compared to ortho and para. Phenol itself is less acidic than o- and p-fluorophenols due to the absence of electron-withdrawing substituents, but more acidic compared to m-fluorophenol due to its higher intrinsic acidity.

Key Concepts

Electron-Withdrawing Groups

In the world of organic chemistry, electron-withdrawing groups (EWGs) play a pivotal role in influencing the acidity of a compound. When an EWG is attached to a phenol, it significantly impacts the acidity level.
An EWG is an atom or group of atoms that pulls electron density away from the rest of the molecule. This is especially true for atoms like fluorine, which have high electronegativity. When we see a fluorine attached to a phenol, it acts as an EWG.
Why does this matter? With the EWG pulling electrons away, the phenoxide ion (the conjugate base formed when phenol loses a proton) is better stabilized. This stabilization is due to the negative charge being spread out, making it easier and more favorable for the hydrogen ion (proton) to dissociate from the phenol. Consequently, EWGs like fluorine increase the acidity of phenols by making the loss of a proton more favorable.
To summarize, EWGs increase the acidity of phenols by stabilizing the phenoxide ion through electron density withdrawal.

Substituent Positions

In aromatic compounds such as phenols, the position of the substituent can dramatically influence the overall acidity. The terms ortho, meta, and para describe the positions around the benzene ring where substituents can be located.
For example, in fluoro-substituted phenols:

• Ortho (o-) describes a substituent positioned next to the hydroxyl group.
• Meta (m-) describes a substituent spaced by one carbon from the hydroxyl group.
• Para (p-) describes a substituent positioned opposite the hydroxyl group on the benzene ring.

Each of these positions affects how much the substituent can influence the properties of the phenol. Ortho and para positions generally allow for greater resonance and inductive effects, which are crucial for stabilizing the conjugate base, thus increasing acidity. However, the meta position offers less overlap for resonance, reducing this effect.
This makes ortho- and para-substituted phenols generally more acidic than their meta counterparts.

Resonance Stabilization

Resonance stabilization is a crucial concept in understanding the acidity of substituted phenols. When a substituent like fluorine is added to the benzene ring of phenol, it can influence the distribution of electron density through resonance.
Resonance refers to the delocalization of electrons across different structures that a molecule can adopt. In the case of phenoxide ions, the presence of an electron-withdrawing group (EWG) like fluorine in the ortho or para position enhances resonance stabilization by allowing more effective delocalization of negative charge across the aromatic ring.
To visualize this, imagine the phenoxide ion created after the loss of a proton from phenol. The EWG helps disperse the negative charge by having multiple contributing resonance structures. This delocalization makes the ion more stable.
In summary, substituents at the ortho and para positions provide better resonance stabilization of the phenoxide ion compared to the meta position, resulting in increased acidity.