Problem 87 - A Complete Resource Book in Chemistry for JEE Main

Question

For phenol, p-methylphenol, m-nitrophenol and p-nitrophenol the order of increasing acidic strength is (a) Phenol, \(\quad\) p-methylphenol, \(\mathrm{p}\)-nitrophenol, m-nitrophenol (b) p-methylphenol, phenol, m-nitrophenol, p-nitrophenol (c) p-methylphenol, m-nitrophenol, phenol, p-nitrophenol (d) m-nitrophenol, p-nitrophenol, phenol, p-Methylphenol

Step-by-Step Solution

Answer

The correct order is (b): p-methylphenol, phenol, m-nitrophenol, p-nitrophenol.

Step 1: Understand Acidic Strength

Acidic strength depends on the stability of the anion formed after the donation of a proton. Resonance and inductive effects play a major role in this stability, with electron-withdrawing groups increasing acidic strength and electron-donating groups decreasing it.

Step 2: Analyze the Electron-Donating and Withdrawing Effects

Examine the substituents on the phenol ring: - Phenol itself has no substituents besides the -OH group. - p-Methylphenol has a methyl group, an electron-donating group, decreasing acidic strength. - m-Nitrophenol and p-Nitrophenol have nitro groups, which are electron-withdrawing, thus increasing acidic strength. - The position of the nitro group matters; para position (p-) is more effective in resonance stabilization compared to meta position (m-).

Step 3: Order Based on Acidic Strength

According to the substituents: 1. p-Nitrophenol has the highest acidic strength due to the strong electron-withdrawing effect of the nitro group in a para position. 2. m-Nitrophenol comes next, having a nitro group also, but in a less effective meta position. 3. Phenol comes after the nitro-substituted phenols as it has no extra stabilizing groups. 4. p-Methylphenol is the least acidic due to the electron-donating effect of the methyl group.

Key Concepts

Electron-Donating Groups

Electron-donating groups (EDGs) are substituents that can increase the electron density around a molecule. They often do this by releasing electrons through resonance or inductive effects. The presence of EDGs on an aromatic ring can decrease the acidity of the compound. This happens because the increased electron density destabilizes the anion formed when a proton is dissociated. For example, in p-methylphenol, the methyl group is an electron-donating group. It donates electrons to the phenol ring, making it less likely to give up its proton, and thus reducing its acidic strength. If you're looking at phenol derivatives and see an EDG, expect the acidity to decrease. Common characteristics of electron-donating groups include:

• They often contain carbon groups, like methyl groups.
• They can include lone pair donating atoms like oxygen or nitrogen.
• They tend to decrease the positive charge density on the target atom.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are substituents that can pull electron density away from a molecule, often making it more electrophilic and capable of stabilizing a negative charge. This stabilization increases the compound's acidity. In phenolic compounds, having an EWG like the nitro group (-NO₂) can greatly enhance acidic strength. The nitro group withdraws electron density via resonance and inductive effects. This makes the phenolate anion (the ion formed when phenol loses a hydrogen ion) more stable due to the dispersal of negative charge.
In m-nitrophenol and p-nitrophenol, the nitro group significantly increases their acidity compared to phenol. The position of the EWG on the ring matters: p-nitrophenol, with the nitro group at the para position, shows a greater increase in acidity than m-nitrophenol, where it is at the meta position. Characteristics of electron-withdrawing groups include:

• They often possess electronegative atoms like nitrogen or oxygen.
• They can contain multiple bonds to electronegative atoms, enhancing their withdrawing power.
• They tend to increase positive charge density on the targeted atom, making it more likely to release a proton.

Resonance Stabilization

Resonance stabilization is a key concept in understanding acidic strength in organic molecules. It refers to the ability of a molecule to delocalize electrons across different atoms in the structure, resulting in a more stable compound. When an acidic compound releases a proton, it forms an anion. If this anion can distribute its negative charge over multiple atoms through resonance, the entire species becomes more stable.
For example, in p-nitrophenol, the nitro group allows for extensive resonance stabilization of the phenolate ion. The nitro group at the para position engages in resonance by overlapping its p-orbitals with the aromatic system of phenol, allowing electrons to be shared across the entire system. This makes the loss of a proton more favorable, hence increasing acidic strength.
Resonance structures of phenolate ions can often be drawn to show how electrons are spread out over the molecule. Important points of resonance stabilization include:

• It typically increases with the number of resonance structures possible.
• Effective resonance requires proper alignment of p-orbitals across the molecule.
• The more delocalized the electrons, the more stable the ion tends to be.