Problem 32 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is most acidic? (a) o-cresol (b) phenol (c) p-nitrophenol (d) anisole

Step-by-Step Solution

Answer

p-Nitrophenol is the most acidic due to the electron-withdrawing nitro group.

Step 1: Identify the Compound Types

First, identify each compound. o-Cresol and p-nitrophenol are phenol derivatives with additional substituents. Phenol is a simple aromatic alcohol, and anisole is an aromatic ether.

Step 2: Understand Acidity in Phenols

Phenols are aromatic compounds that can donate a proton from the hydroxyl group. The acidity of phenols is influenced by substituents. Electron-withdrawing groups enhance acidity by stabilizing the phenoxide ion through resonance.

Step 3: Assess the Substituents’ Effects

Analyze the effects of the substituents: o-Cresol has a methyl group (electron-donating), phenol has no substituents, p-nitrophenol has a nitro group (strong electron-withdrawing), and anisole has a methoxy group (electron-donating).

Step 4: Determine the Strongest Acid

Electron-withdrawing groups like the nitro group increase the acidity of phenolic compounds. Hence, p-nitrophenol is the most acidic due to the presence of the nitro group that stabilizes the phenoxide ion through resonance.

Key Concepts

Electron-withdrawing groups

In organic chemistry, electron-withdrawing groups (EWGs) play a crucial role in acid-base chemistry by affecting the acidity of compounds. These groups effectively pull electron density away from other parts of a molecule. This is achieved through resonance or inductive effects.

In the context of phenols, the presence of an electron-withdrawing group helps enhance the acidity by stabilizing the negative charge on the phenoxide ion, which forms when the phenol loses a proton. Some typical examples of these groups include the nitro group (-NO₂), carbonyl groups (like -C=O), and halogens (such as -F, -Cl).

• By stabilizing the negative charge via delocalization, these groups make it energetically favorable for the acidic proton to be released from the phenol.
• In effect, this increases the molecule's overall acidity.

When comparing different compounds, the presence of a strong electron-withdrawing group on the phenol often points to increased acidity, as seen in the case of p-nitrophenol.

Phenoxide ion stability

The phenoxide ion is the conjugate base of phenol, formed when phenol loses a proton (H⁺). The stability of this ion is a significant factor in determining the acidity of phenolic compounds. A more stable phenoxide ion leads to increased acidity.

• Stability is greatly affected by the presence of electron-withdrawing groups that can spread out the negative charge over a larger volume.
• This is often achieved through resonance, where the negative charge is delocalized across the aromatic ring, reducing the energy of the molecule.

In addition to resonance, substituents that withdraw electrons through an inductive effect (i.e., polar bonds with little overlap of orbitals) can also stabilize the phenoxide ion.

For instance, in p-nitrophenol, the nitro group is a strong electron-withdrawing group that stabilizes the phenoxide ion effectively through resonance, resulting in high acidity compared to phenol or other derivatives without such groups.

Phenol derivatives

Phenol derivatives are compounds in which phenol's hydrogen or other components are replaced with different atoms or groups. These substitutions dramatically influence the chemical behaviors, especially acidity, of the phenol.

Different substituents on the phenolic ring alter the distribution of electron density. This could lead to either increased or decreased acidity. Phenol derivatives often contain various functional groups attached to the benzene ring, which can affect acidity in distinct ways:

• Electron-donating groups, like methyl (CH₃) or methoxy (-OCH₃), decrease acidity because they increase electron density on the oxygen atom, destabilizing the phenoxide ion.
• Conversely, electron-withdrawing groups such as nitro (-NO₂) increase acidity by stabilizing the phenoxide ion.

In practical scenarios, analyzing these substitutions helps chemists predict and control the behavior of these molecules. For example, p-nitrophenol is more acidic than phenol because the nitro group stabilizes the phenoxide ion.