Problem 68 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Arrange the following in the decreasing order of acidic strength (i) Phenol (ii) p-nitrophenol (iii) \(\mathrm{m}\) -cresol (iv) \(\mathrm{p}\) -cresol (a) \(\mathrm{ii}>\mathrm{iii}>\mathrm{iv}>\mathrm{i}\) (b) \(\mathrm{ii}>\mathrm{i}>\mathrm{iii}>\mathrm{iv}\) (c) \(\mathrm{ii}>\mathrm{i}>\mathrm{iv}>\) iii (d) \(\mathrm{iii}>\mathrm{iv}>\mathrm{ii}>\mathrm{i}\)

Step-by-Step Solution

Answer

(b) ii > i > iii > iv

Step 1: Understand the Concept of Acidic Strength

The acidic strength of phenols can be influenced by substituents on the aromatic ring. Electron-withdrawing groups (EWG) increase acidic strength by stabilizing the negative charge on the phenoxide ion. Electron-donating groups (EDG) decrease acidic strength.

Step 2: Analyze the Substituents

Identify the substituents in each compound: - Phenol (no substituent) - p-Nitrophenol (para-nitro group) - m-Cresol (meta-methyl group) - p-Cresol (para-methyl group)

Step 3: Determine the Effect of Substituents

- The nitro group is an electron-withdrawing group, which increases acidic strength due to the resonance and inductive effect. - The methyl group is an electron-donating group, which decreases acidic strength.

Step 4: Order Based on Acidic Strength

Arrange the compounds based on the effect of the substituents: - p-Nitrophenol is the strongest acid because the nitro group increases acidity. - Phenol follows because it has no additional group to influence its acidity. - m-Cresol is less acidic than phenol due to the presence of a methyl group at the meta position. - p-Cresol is the weakest because the para-methyl group strongly donates electron density.

Key Concepts

Understanding Electron-Withdrawing Groups (EWG)

Electron-withdrawing groups (EWG) play a crucial role in influencing the acidic strength of phenols. These groups are characterized by their ability to pull electron density away from the aromatic ring. By doing so, they stabilize the negative charge that resides on the phenoxide ion, which is the deprotonated form of phenol.
The stabilization of the phenoxide ion is key to enhancing the acidic nature of phenol. If the negative charge on the oxygen of the phenoxide ion is effectively stabilized, the compound is more likely to release a proton, thus behaving as a stronger acid.
- Common examples of electron-withdrawing groups include: - Nitro group (-NO2) - Carbonyl group (C=O) - Halogens (such as -Cl, -Br) serves as an excellent example where the para-nitro group, being an EWG, significantly increases the acidic nature of phenol by delocalizing the negative charge formed after deprotonation.

Role of Electron-Donating Groups (EDG)

Unlike electron-withdrawing groups, electron-donating groups (EDG) tend to decrease the acidic strength of phenols. These groups push electron density towards the aromatic ring, thereby increasing the electron density on the phenoxide ion.
When more electron density builds up on the ion, the stabilization decreases. As a result, it becomes harder for the molecule to release a proton, thus reducing its acidity.
- Common examples of electron-donating groups include: - Alkyl groups (e.g., methyl group -CH3) - Hydroxyl group (-OH) In the context of the original problem, both and have a methyl group, which is an electron-donating group. The presence of the methyl group reduces the acidic strength compared to phenol itself, with being even less acidic due to the stronger electron donating effect at the para position.

The Significance of the Aromatic Ring

The backbone of phenolic compounds is the aromatic ring, which forms the core framework including the hydroxyl group ( (-OH) ). This aromatic system allows for a special type of electron cloud called the pi-electron cloud.
This cloud contributes to the stability of the molecule through resonance, a process where electrons are distributed across multiple atoms or structure forms.
- Key features include: - Resonance stabilization - Delocalization of electrons The resonance in an aromatic system helps in the delocalization of charges, such as the negative charge on the phenoxide ion, but it can be further influenced by substituents that are either electron-withdrawing or electron-donating.
Thus, the aromatic ring acts as both an integral structure for basic phenolic properties and a platform for modifications through substituents that alter acid strength.

Phenoxide Ion Stabilization Explained

The phenoxide ion is the ion formed when phenol loses a proton. The stability of this ion is directly related to the acid strength of the phenol. The more stable the phenoxide ion, the stronger the corresponding acid will be. This stabilization is heavily influenced by the presence of substituents on the aromatic ring.
- Things Improving Stability: - Resonance effects - Inductive effects of substituents - Stabilization strategies include: - Delocalization of the negative charge by resonance with the aromatic ring. - Incorporating electron-withdrawing substituents that reduce electron density on the oxygen. In compounds like p-nitrophenol, the nitro group helps in stabilizing the phenoxide ion through both inductive and resonance effects, making it more likely to give up a proton. Conversely, in p-cresol and m-cresol, the electron-donating methyl groups decrease stabilization, leading to weaker acidity.