Problem 99 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following facts explains as to why p-nitrophenol is more acidic than phenol? 1\. \(-\) I effect of nitro group 2\. Greater resonance effect of p-nitrophenoxy group 3\. Steric effect of bulky nitro group Select the correct answer using the codes given below: (a) 2 and 3 (b) 1 and 3 (c) 1 and 2 (d) 2 alone

Step-by-Step Solution

Answer

(c) 1 and 2

Step 1: Identifying Nature of p-nitrophenol and phenol

p-Nitrophenol contains a nitro group at the para position, while phenol does not. This will influence its acidic behavior.

Step 2: Exploring the -I Effect of Nitro Group

The nitro group is strongly electron-withdrawing due to its -I (inductive) effect, which stabilizes the phenoxide ion formed when p-nitrophenol donates a proton. This makes p-nitrophenol more acidic than phenol.

Step 3: Understanding the Greater Resonance Effect

The p-nitrophenoxy group allows electrons in the p-nitrophenol anion to delocalize over a larger area, enhancing stability through greater resonance, which increases acidity.

Step 4: Assessing the Steric Effect of Nitro Group

The steric effect refers to the influence of the spatial arrangement of atoms. Though the nitro group is bulky, this primarily affects sterics rather than acidity.

Key Concepts

Acid-Base Chemistry

In organic chemistry, understanding acid-base interactions is crucial for predicting the behavior of compounds in reaction mechanisms. The acidity of a compound is determined by its ability to donate a proton ( H^+ ). Acid strength is often influenced by the stability of the conjugate base formed after the proton is donated.
For instance, p-nitrophenol is more acidic than phenol because its conjugate base, the p-nitrophenoxide ion, is more stabilized by the presence of a nitro group at the para position.

Why is stability important?

• Stabilization decreases the energy of the conjugate base, making the deprotonated species more favorable.
• The more stable the conjugate base, the stronger the original acid.

Therefore, factors that enhance conjugate base stability directly enable the acid to more easily release a proton, demonstrating increased acidity.

Inductive Effect

The inductive effect ( -I effect ) refers to the electron-withdrawing or electron-donating properties of substituents through sigma ( σ ) bonds. This effect significantly impacts acidity in organic compounds.

• Electron-withdrawing groups (EWGs), such as nitro groups, exhibit the -I effect and pull electron density away from the atom, enhancing the molecule's ability to stabilize a negative charge.
• This effect is transmitted along sigma bonds, so the closer the EWG is to the acidic proton, the greater the influence on acidity.

In p-nitrophenol, the nitro group exerts a strong -I effect, making the phenoxide ion more stable compared to that formed from phenol. This greater stabilization through electron withdrawal increases the acidity of p-nitrophenol.

Resonance Effect

The resonance effect is a stabilizing influence that arises when a molecular structure can be represented by two or more equivalent Lewis structures. This phenomenon allows for electron delocalization, which increases stability.
In the context of aromatic systems like p-nitrophenol, resonance plays a crucial role in stabilizing the conjugate base, the p-nitrophenoxide ion.

• The delocalization of electrons over the aromatic system, further facilitated by the nitro group, increases the stabilization of the negative charge across the structure.
• Resonance often has a greater impact on stability and therefore acidity, than the inductive effect alone.

By providing more resonance structures, the p-nitrophenoxy group offers enhanced stability, making it a stronger acid relative to phenol.

Aromatic Compounds

Aromatic compounds are a class of organic molecules characterized by the presence of at least one aromatic ring, such as benzene. These rings provide unique properties, mainly due to their electron-rich, planar, and cyclic nature.
Aromaticity significantly influences the behavior of compounds, as it affects both their physical properties and reactivity.

• The stability of aromatic rings arises from the delocalization of π electrons, leading to a lowering of overall energy compared to non-aromatic systems.
• This electron cloud is a factor in the reactivity patterns seen in aromatic compounds, including their acidity or potential for undergoing electrophilic substitution reactions.

In p-nitrophenol, the aromatic ring assists in further stabilizing the phenoxide ion through resonance, leveraging both the aromaticity and the attached nitro group's effects. This integration highlights why aromatic compounds with substituent groups like nitro exhibit notable differences in chemical behavior compared to non-aromatic systems.