Problem 61 - A Complete Resource Book in Chemistry for JEE Main

Question

Which is most acidic (a) Oc1ccccc1 (b) Oc1cccc(Cl)c1 (c) Oc1ccccc1Cl (d) Oc1ccc(Cl)cc1

Step-by-Step Solution

Answer

c; o-Chlorophenol is the most acidic due to the chlorine's proximity to the hydroxyl group, enhancing ion stabilization.

Step 1: Understanding Acidity

Acidity in organic compounds is generally determined by the ease with which a hydrogen ion (H⁺) is released. In phenols or aromatic alcohols, substituents on the aromatic ring can influence acidity by their electron-withdrawing or electron-donating effects.

Step 2: Analyzing Each Compound

Evaluate each option: - (a) Phenol (Oc1ccccc1) has no substituents. - (b) p-Chlorophenol (Oc1cccc(Cl)c1) has a chlorine atom at the p-position, an electron-withdrawing group. - (c) o-Chlorophenol (Oc1ccccc1Cl) has a chlorine atom at the o-position, an electron-withdrawing group but closer to the hydroxyl group. - (d) m-Chlorophenol (Oc1ccc(Cl)cc1) has a chlorine atom at the m-position, which is less effective in resonance with the hydroxyl group.

Step 3: Evaluating the Effects of Substituents

Chlorine is an electron-withdrawing group due to its electronegativity, which stabilizes the phenoxide ion by delocalizing the negative charge. The closer the electron-withdrawing substituent to the hydroxyl group, the more effective it is in increasing acidity. Thus, o-chlorophenol's (c) chlorine is in the best position to stabilize the ion via resonance and inductive effects.

Step 4: Conclusion

Based on the position and effects of substituents: - (c) o-Chlorophenol is more acidic than the other options because the chlorine is optimally positioned to stabilize the phenoxide ion.

Key Concepts

Phenol Acidity

Phenol, a type of aromatic compound, has characteristic acidity due to the presence of a hydroxyl group (-OH) attached to a benzene ring. The acidity of phenol can be explained by how easily it can donate a hydrogen ion (H⁺). In phenol, this process is facilitated by the resonance stabilization of the resulting phenoxide ion (phenol minus H⁺). Resonance allows the negative charge on the phenoxide ion to be dispersed throughout the aromatic ring, making the ion more stable. This stabilization through resonance is key to understanding phenol’s acidic nature. Though phenol is more acidic than typical alcohols, it is still weaker than carboxylic acids.

Substituent Effects

The acidity of phenols can be significantly influenced by the presence of substituents on the aromatic ring. Substituents are groups of atoms that replace hydrogen atoms on the benzene ring. These groups can either increase or decrease acidity depending on their nature:

• Electron-Withdrawing Substituents: These groups increase acidity by stabilizing the phenoxide ion.
• Electron-Donating Substituents: These groups decrease acidity as they destabilize the phenoxide ion by adding electron density.

This influence of substituents is based on their ability to either augment or reduce electron density through resonance or inductive effects, which are key in determining the acidity of substituted phenols.

Electron-Withdrawing Groups

An electron-withdrawing group (EWG) is an atom or group of atoms that pulls electron density away from other parts of the molecule. This characteristic makes certain substituents, like chlorine, very effective in enhancing phenol acidity. In the case of chlorophenols, the chlorine atom, due to its high electronegativity, acts as an electron-withdrawing group. It increases acidity by stabilizing the phenoxide ion. This stabilization occurs because the electron-withdrawing effect extends across the aromatic system, delocalizing the negative charge that appears when phenol donates a hydrogen ion. The position of the chlorine atom in the structure is crucial, as it determines how effectively it stabilizes the phenoxide ion.

Resonance and Inductive Effects

The effectiveness of substituents, like chlorine, in influencing phenol acidity relies largely on resonance and inductive effects. Both of these effects help in stabilizing the phenoxide ion:

• Resonance Effect: This occurs when substituents allow the charge to be dispersed over the aromatic ring via delocalization. Chlorine can participate in resonance through its lone pairs, further stabilizing the ion.
• Inductive Effect: This effect involves the transmission of charge through the sigma bonds of a molecule. Chlorine induces an inductive effect by pulling electron density away from the oxygen of the hydroxyl group, making it more effective in releasing H⁺.

Both effects are influenced by the position of the substituent, with ortho and para positions usually being more effective than the meta position. The combined impact of these effects determines acidity changes in substituted phenols.