Problem 155 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following acids: (1) \(\mathrm{CH}_{3} \mathrm{COOH}\) (2) \(\mathrm{Cl}_{2} \mathrm{CHCOOH}\) (3) \(\mathrm{ClCH}_{2} \mathrm{COOH}\) (3) \(\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\) The correct sequence of acidity is: (a) \(3>1>2>4\) (b) \(2>3>4>1\) (c) \(1>3>4>2\) (d) \(2>4>3>1\)

Step-by-Step Solution

Answer

(b) 2>3>4>1

Step 1: Identify Inductive Effect

Understand how the presence of electronegative groups affects acidity. The more electronegative atoms in proximity to the carboxylic group, the stronger the acid due to electron-withdrawing effects, which stabilize the carboxylate ion.

Step 2: Analyze Each Compound

Compare each compound: Compound (1) has no chlorine substituents, Compound (2) has two chlorine atoms attached directly next to the carboxyl group, Compound (3) has one chlorine atom next to the carboxyl group, Compound (4) has one chlorine atom further away from the carboxyl group.

Step 3: Determine Acidity Order

Compound (2) with the highest number of electron-withdrawing chlorine atoms directly adjacent to the carboxyl group is the strongest acid. Compound (3) follows as it has one chlorine atom next to the carboxyl group. Compound (4) is next as the chlorine is farther from the carboxyl group. Compound (1), with no chlorine atoms, is the weakest.

Step 4: Match with Given Options

Option (b) matches the determined order: Compound (2) > Compound (3) > Compound (4) > Compound (1).

Key Concepts

Inductive Effect

The inductive effect is an important concept when discussing acid strength. It refers to the shifting of electrons through σ (sigma) bonds within a molecule, typically as a result of the presence of electronegative atoms. These groups can either pull (electron-withdrawing) or push (electron-donating) electron density away from or towards the core of a molecule.

In acids, especially carboxylic acids, the inductive effect can greatly influence acid strength. When electronegative atoms or groups such as chlorine are present near the acidic site, they pull electron density towards themselves. This stabilizes the resulting anion when the acid dissociates, thereby increasing the acid's strength.

• An electron-withdrawing group increases acidity.
• An electron-donating group decreases acidity.

Understanding the inductive effect helps in predicting the relative acidity of compounds.

Carboxylic Acids

Carboxylic acids are a class of organic compounds characterized by the presence of the carboxyl group (\(-\text{COOH}\)). They are a well-studied group due to their acidity and prevalence in both industrial and biological processes.

The acidity of carboxylic acids stems from the ability of the carboxyl group to donate a hydrogen ion (\( ext{H}^+\)). When a carboxylic acid loses its hydrogen ion, it forms a carboxylate ion. The stability of this ion is crucial in determining the acid strength. Factors that stabilize the carboxylate ion, like the presence of electron-withdrawing groups, increase acidity. Consequently, carboxylic acids with substituents that enhance the stability of the carboxylate ion are stronger acids.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are a key player in determining the strength of acids, such as carboxylic acids. These groups attract electron density towards themselves and away from the rest of the molecule. In the context of acid strength, EWGs stabilize the anion formed when an acid donates its proton, making the acid stronger.

An example of an electron-withdrawing group is chlorine. Chlorine, being highly electronegative, pulls electrons toward itself through the sigma bonds of the molecule. This leads to stabilization of the carboxylate ion after the acid has donated a proton.

• The closer the EWG is to the acidic proton, the greater the effect.
• Multiple EWGs lead to an even stronger effect.

Thus, the presence and position of electron-withdrawing groups in a molecule are crucial in determining the relative acidity among similar compounds.