Problem 142 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of decreasing acidity of the acids given below is 1\. \(\mathrm{Cl}_{3} \mathrm{CCH}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{COOH}\) 2\. \(\mathrm{H}_{3} \mathrm{CCH}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{COOH}\) 3\. \(\mathrm{Cl}_{3} \mathrm{CCH}=\mathrm{CH}-\mathrm{COOH}\) 4\. \(\mathrm{H}_{3} \mathrm{CCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\) (a) \(1>3>2>4\) (b) \(3>1>2>4\) (c) \(3>4>1>2\) (d) \(3>1>4>2\)

Step-by-Step Solution

Answer

The correct order is (b) 3 > 1 > 2 > 4.

Step 1: Analyze carboxylic acids structure

Examine how the structure of each acid affects acidity. Carboxylic acids are more acidic when electron-withdrawing groups are present because they stabilize the negative charge on the carboxylate anion.

Step 2: Assess electron-withdrawing effects

Compare the effects of substituents. Chlorine atoms are highly electronegative and withdraw electron density via inductive effect, increasing acidity. Fully substituted chlorinated compounds (like in (3)) are more acidic than partially substituted ones (like in (1)). Methyl groups (as in (2) and (4)) provide electron-donating effects, reducing acidity.

Step 3: Compare acidity within groups

Rank the acids. Acid (3), with three Cl atoms adjacent to the carboxylic group, is most acidic. Acid (1), with Cl atoms further away, follows. Acid (2), with a closer methyl group, is less acidic than Acid (1). Acid (4), with a methyl group further and saturated chain, is least acidic.

Step 4: Evaluate given options

Match the determined order with the provided options. Our order is 3 > 1 > 2 > 4, which corresponds with option (b).

Key Concepts

Carboxylic Acids

Carboxylic acids are a significant category of organic acids characterized by the presence of a carboxyl functional group, denoted as \(-COOH\). This group consists of a carbonyl (\(C=O\)) and a hydroxyl (\(-OH\)) group attached to a carbon atom. Carboxylic acids are known for their ability to donate a proton (\(H^+\)) from the hydroxyl, making them acidic. The stability of the carboxylate anion formed after losing a proton is crucial in determining the acidity of the carboxylic acid.
Understanding the acidity of carboxylic acids involves considering how well the carboxylate anion is stabilized. When electronic effects such as electronegativity and resonance are at play, they greatly influence how acidic the molecule is. For instance, substituents on the carbon structure that pull electron density away from the carboxyl group, like in trichloroacetic acid, significantly enhance acidity by stabilizing the negative charge on the anion. It's this delicate interplay of structural features and electronic factors that dictates the acidity level in carboxylic acids.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are substituents attached to a molecule that draw electron density away from other parts of the molecule. These groups can substantially increase the acidity of carboxylic acids. This happens due to their ability to stabilize the negative charge on the carboxylate ion after deprotonation.

• Halogens like chlorine are common EWGs. They achieve electron withdrawal through their high electronegativity.
• The closer these groups are to the carboxyl group, the stronger their effect on acidity.
• In compounds where the carboxylic acid is adjacent to EWGs, it's easier for the compound to release a proton, resulting in a stronger acid.

EWGs play an essential role in modifying the acidity of organic compounds. For example, in our exercise, acid (3) that contains three chlorine atoms near the carboxyl group is the most acidic when compared to other acids with fewer or no EWGs.

Inductive Effect

The inductive effect is a crucial concept in organic chemistry, involving the transmission of an electrical charge through a chain of atoms in a molecule. This effect is primarily determined by the electronegativity of the atoms involved and occurs due to the presence of a polar bond. It can either be electron-withdrawing or electron-donating, affecting the acidity or basicity of molecules.

• An electron-withdrawing inductive effect increases acidity. This is observed when an electronegative atom, like chlorine, is connected near an acidic group such as the carboxyl group.
• Conversely, an electron-donating inductive effect, often seen with alkyl groups like methyl, can decrease acidity.

In the context of carboxylic acids, the inductive effect of EWGs nearby enhances the acid’s ability to donate a proton. The negative charge on the resulting carboxylate ion is then better stabilized. This is why, in the given exercise, the compound with the highest number of chlorine atoms closely positioned to the carboxyl group showed the greatest acidity, following the principle of the inductive effect.