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, p-methylphenol, 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

(b) p-methylphenol, phenol, m-nitrophenol, p-nitrophenol.

Step 1: Understand the Factors Affecting Acidity

The acidity of phenols depends on the ability of the substituent groups to stabilize the negative charge on the phenoxide ion. Electron-withdrawing groups, such as nitro (enitro), increase acidity by stabilizing the phenoxide ion through delocalization. Electron-donating groups, like methyl (methyl), decrease acidity by destabilizing the phenoxide ion.

Step 2: Analyze Substituents and Their Effects

Identify the substituents on each phenol: phenol has no substituents, p-methylphenol has a para methyl group, m-nitrophenol has a meta nitro group, and p-nitrophenol has a para nitro group. Methyl is an electron-donating group, while nitro is an electron-withdrawing group.

Step 3: Determine Acidity Based on Substituent Position

Para positions can exert stronger influence than meta positions because they allow better overlap for resonance effects. Therefore, p-nitrophenol is more acidic than m-nitrophenol, as the para position allows more effective resonance and stabilization of the phenoxide ion.

Step 4: Evaluate the Order of Acidity

Knowing that electron-donating groups decrease acidity and electron-withdrawing groups increase acidity, the decreasing acidity order is: p-nitrophenol > m-nitrophenol > phenol > p-methylphenol. Therefore, the least acidic (or most basic) substance is p-methylphenol.

Step 5: Match with Given Options

Based on the determined order of p-methylphenol < phenol < m-nitrophenol < p-nitrophenol, compare it with the given options. The correct order, according to option (b), matches this sequence.

Key Concepts

Electron-withdrawing groups

When discussing the acidic strength of phenols, electron-withdrawing groups play a significant role. These groups, such as the nitro group \((NO_2)\), are known for pulling electrons away from the phenol ring.
When these groups are attached to the phenol, they enhance the molecule's acidity. This happens because they help stabilize the negative charge found on the phenoxide ion \( (C_6H_5O^-) \). As electrons are pulled away, this stabilization occurs through a mechanism called resonance.

• The nitro group, by its strong electron-withdrawing nature, can spread this negative charge over a wider range.
• This delocalization makes it easier for the phenol to lose a proton \((H^+)\), thereby enhancing its acidic nature.

The position of these groups on the aromatic ring also influences their effectiveness, as you’ll see in the substituent effects section.

Electron-donating groups

On the other side of the spectrum, electron-donating groups tend to decrease the acidity of phenols. These groups, like the methyl group \((CH_3)\), push electrons towards the phenol ring, opposing the loss of a proton.
Essentially, by donating electrons, these groups destabilize the negative charge on the phenoxide ion, making it less likely for the molecule to lose \((H^+)\). This is because:

• The electron-donating groups add to the electron density of the ring, counteracting the needed resonance stabilization of the phenoxide ion.
• As a result, this makes the phenol's acidic behavior weaker.

Especially in cases like p-methylphenol, the presence of a para methyl group reduces its acidity compared to phenol without any substituent. Understanding this effect of electron-donating groups helps predict which phenols are less acidic.

Substituent effects on acidity

The position and type of substituents on a phenol ring greatly affect its acidic strength. Both electron-withdrawing and electron-donating groups have different influences depending on their orientation on the aromatic ring.
The para position can exert a greater resonance effect compared to the meta position. Let's break down why and how:

• **Para substituents**, such as in p-nitrophenol, allow for effective resonance overlap. This increases the acid strength by stabilizing the negative charge effectively.
• **Meta substituents**, like in m-nitrophenol, can't participate in resonance as effectively as para substituents. This results in a lesser increase in acidity compared to para-positioned counterparts.

These details clarify why p-nitrophenol is more acidic than m-nitrophenol, and both are more acidic than phenol itself.
On the flip side, we see that para positions of electron-donating groups also lead to significant changes, such as a decreased acidity observed in p-methylphenol. This comprehensive view of substituent effects provides a clearer understanding for determining acid strength in substituted phenols.