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

The correct sequence of acidity is (b) \(2>3>4>1\).

Step 1: Understand the Influence of Electronegativity

The electron-withdrawing nature of atoms within a compound greatly affects its acidity. The more electronegative atoms attached to the carboxylic group, the stronger the acid becomes. Halogens like chlorine increase acidity due to their electronegativity.

Step 2: Compare the Electron-Withdrawal in Each Acid

Analyze each compound:- (1) \(\text{CH}_{3}\text{COOH}\) has no electronegative substituents.- (2) \(\text{Cl}_{2}\text{CHCOOH}\) has two Cl, providing a strong electron-withdrawing effect.- (3) \(\text{ClCH}_{2}\text{COOH}\) has one Cl, lessening but still contributing strong electron withdrawal.- (4) \(\text{ClCH}_{2}\text{CH}_{2}\text{COOH}\) has one Cl farther from the carboxylic group, lessening its influence.

Step 3: Rank Acids Based on Chlorine Influence

Based on electron-withdrawing capability by chlorine atoms:- (2) \(\text{Cl}_{2}\text{CHCOOH}\) with two chlorines has the most withdrawal power, thus most acidic.- (3) \(\text{ClCH}_{2}\text{COOH}\) is next, with a single chlorine directly next to the carboxylic group.- (4) \(\text{ClCH}_{2}\text{CH}_{2}\text{COOH}\) is less acidic because its chlorine is further from the carboxyl group compared to (3).- (1) \(\text{CH}_{3}\text{COOH}\) is the least acidic with no chlorine substituent.

Step 4: Identify the Correct Sequence

Understanding the steps above, place the acids in order of decreasing acidity: (2) \(>\) (3) \(>\) (4) \(>\) (1).

Key Concepts

Electronegativity

Electronegativity refers to the ability of an atom to attract and hold onto electron pairs. In simple terms, it is like a measure of how strongly an atom can pull electrons towards itself in a chemical bond. This concept is crucial in understanding the strength of various acids, including carboxylic acids.
Electronegative atoms, such as chlorine, increase the acidity of a compound by pulling electron density away from specific areas.

• This electron withdrawal increases the stability of the acid's conjugate base.
• When the negative charge is spread out more, the acid more readily loses its hydrogen ion ( H^+ ), increasing its strength.

The presence of electronegative atoms near a carboxyl group ( COOH ) can significantly change the acidity of a carboxylic acid by altering these electron interactions.

Carboxylic Acids

Carboxylic acids are a group of organic compounds characterized by the presence of a carboxyl group ( COOH ). This functional group is crucial for their chemical behavior and reactions. The carboxyl group is made of a carbonyl ( C=O ) and a hydroxyl group ( OH ) bonded to the same carbon atom. Here’s why this is important:

• The carboxyl group is typically acidic, meaning it can donate a hydrogen ion ( H^+ ) to a solution.
• This donation of hydrogen ions is what defines the strength of the acid.
• The carboxylic acids with substituents like chlorine influence how easily they can release this hydrogen ion.

Modifications to the carboxylic acid structure, such as adding halogens (chlorine), affect the electron distribution in the molecule, impacting the acid's overall acidity.

Halogen Influence

Halogens, including chlorine, play a significant role in strengthening acids. They influence acidity through their electronegativity, which we discussed earlier. In carboxylic acids, when a halogen like chlorine is added, it exerts an electron-withdrawing effect through two main pathways:

• Inductive effect: This is a distance-dependent effect in which the electronegative halogen pulls electron density away from other atoms along the carbon chain.
• Localized influence: The closer the halogen is to the carboxyl group, the stronger its effect on increasing the acid’s strength.

For example, in the acids ClCH_2COOH and Cl_2CHCOOH , the presence of chlorine increases their acidity due to the strong electron-withdrawing capacity of the chlorine atoms. The more chlorine atoms and the closer they are to the carboxyl group, the more acidic the compound becomes.