Problem 16 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The correct decreasing order for acid strength is: (a) \(\mathrm{NO}_{2} \mathrm{CH}_{2} \mathrm{COOH}>\mathrm{FCH}_{2} \mathrm{COOH}>\) \(\mathrm{CNCH}_{2} \mathrm{COOH}>\mathrm{ClCH}_{2} \mathrm{COOH}\) (b) \(\mathrm{FCH}_{2} \mathrm{COOH}>\mathrm{CNCH}_{2} \mathrm{COOH}>\) \(\mathrm{NO}_{2} \mathrm{CH}_{2} \mathrm{COOH}>\mathrm{ClCH}_{2} \mathrm{COOH}\) (c) \(\mathrm{CNCH}_{2} \mathrm{COOH}>\mathrm{NO}_{2} \mathrm{CH}_{2} \mathrm{COOH}>\) \(\mathrm{FCH}_{2} \mathrm{COOH}>\mathrm{ClCH}_{2} \mathrm{COOH}\) (d) \(\mathrm{NO}_{2} \mathrm{CH}_{2} \mathrm{COOH}>\mathrm{CNCH}_{2} \mathrm{COOH}>\) \(\mathrm{FCH}_{2} \mathrm{COOH}>\mathrm{ClCH}_{2} \mathrm{COOH}\)

Step-by-Step Solution

Answer

(d) \(\mathrm{NO}_{2} \mathrm{CH}_{2} \mathrm{COOH}>\mathrm{CNCH}_{2} \mathrm{COOH}>\mathrm{FCH}_{2} \mathrm{COOH}>\mathrm{ClCH}_{2}\mathrm{COOH}\)

Step 1: Review Acid Strength Concepts

The strength of a carboxylic acid is influenced by the substituents attached to it. Electron-withdrawing groups (EWGs), such as \(-NO_2\), \(-CN\), \(-F\), and \(-Cl\), increase acidity by stabilizing the negative charge on the carboxylate ion after deprotonation.

Step 2: Evaluate the Influence of Substituents

The power of electron-withdrawing ability generally follows the order: \(-NO_2 > -CN > -F > -Cl\). This order is based on the strength of these groups in stabilizing the carboxylate ion, which directly affects the acid strength.

Step 3: Apply the Substituent Influence to Rate Acidity

Using the order of electron-withdrawing ability, the acids can be ranked from strongest to weakest:1. \(-NO_2CH_2COOH\) with the \(-NO_2\) group is the strongest.2. \(-CNCH_2COOH\) with the \(-CN\) group.3. \(-FCH_2COOH\) with the \(-F\) group.4. \(-ClCH_2COOH\) with the \(-Cl\) group is the weakest.

Step 4: Match the Order with Given Options

Compare the established order \(-NO_2 > -CN > -F > -Cl\) with the options provided:- Option (d) correctly lists the acids in decreasing order of strength: \(-NO_2CH_2COOH > -CNCH_2COOH > -FCH_2COOH > -ClCH_2COOH\).

Key Concepts

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are functional groups that pull electron density away from the rest of a molecule. This electron-withdrawing effect stabilizes negative charges within molecules, particularly after deprotonation. In carboxylic acids, such stabilization of the carboxylate ion is crucial.
When a carboxylic acid loses a proton (H⁺), it forms a negatively charged ion, called the carboxylate ion. The stability of this carboxylate ion directly impacts the acid strength.
Common electron-withdrawing groups include:

• -NO₂: Nitro group, very strong EWG due to resonance and inductive effects.
• -CN: Cyano group, strong EWG due to its ability to delocalize charge.
• -F: Fluoro group, moderately strong due to high electronegativity.
• -Cl: Chloro group, weaker than -F but still effective.

The electron-withdrawing power helps in ranking acids according to their strength. A stronger EWG leads to a stronger acid because it makes the carboxylate ion more stable.

Carboxylic Acids

Carboxylic acids contain a functional group known as the carboxyl group (-COOH). This group consists of a carbonyl group (C=O) and a hydroxyl group (OH) attached to the same carbon.
Carboxylic acids are weak acids but can donate protons (H⁺) to solutions, influencing acidity. Their acidic nature is primarily due to the ability to form stable carboxylate ions after proton donation.
The strength of carboxylic acids can vary greatly depending on the types of substituents attached to them. This variability makes it interesting to study substituent effects.

• Stronger carboxylic acids fully dissociate in solution, having high Ka values and low pKa values.
  
• When electron-withdrawing groups are present near the carboxyl group, they increase the overall acid strength by stabilizing the carboxylate ion.

Understanding carboxylic acids and their derivatives is crucial for grasping the fundamental principles of acidity in organic chemistry.

Substituent Effects

Substituent effects describe how different groups attached to a molecule's core can influence its chemical properties. In the case of carboxylic acids, these effects are most evident when evaluating acid strength.
Substituents that are electron-withdrawing increase an acid's strength by stabilizing the negative charge in the carboxylate ion. This stabilization occurs through

• Inductive effect: Electronic pull through sigma bonds from an electronegative substituent, like a fluoro (-F) group.
• Resonance effect: Delocalization of electrons across π bonds, more common in groups like nitro (-NO₂) and cyano (-CN).

For instance, a nitro group attached to the carboxylic acid as a substituent considerably enhances acid strength, more so than a fluoro or chloro group.
By analyzing these effects, chemists can predict how changes to a molecule’s structure will alter its acidity, providing valuable insights for chemical synthesis and applications.