Problem 228 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of increasing acid strength of the compound is (a) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) (b) \(\mathrm{MeOCH}_{2} \mathrm{CO}_{2} \mathrm{H}\) (c) \(\mathrm{CF}_{3} \mathrm{CO}_{2} \mathrm{H}\) (d) \((\mathrm{Me})_{2} \mathrm{CH} \mathrm{CO}_{2} \mathrm{H}\) (a) \(\mathrm{b}<\mathrm{d}<\mathrm{a}<\mathrm{c}\) (b) \(\mathrm{d}<\mathrm{a}<\mathrm{c}<\mathrm{b}\) (c) \(\mathrm{d}<\mathrm{a}<\mathrm{b}<\mathrm{c}\) (d) \(\mathrm{a}<\mathrm{d}<\mathrm{c}<\mathrm{b}\)

Step-by-Step Solution

Answer

The correct order is (a) \(\mathrm{b}<\mathrm{d}<\mathrm{a}<\mathrm{c}\).

Step 1: Understanding Acid Strength

Acid strength depends on the stability of the conjugate base after losing the proton. More stable conjugate bases correlate with a stronger acid.

Step 2: Analyzing the Effect of Electron-Withdrawing Groups

Electron-withdrawing groups increase acid strength by stabilizing the negative charge on the conjugate base. For example, in \(\mathrm{CF}_3\mathrm{CO}_2\mathrm{H}\), the \(\mathrm{CF}_3\) group is a strong electron-withdrawing group that stabilizes the conjugate base, making it a strong acid.

Step 3: Evaluating Electron-Donating Group Impact

Electron-donating groups decrease acid strength as they reduce the stability of the conjugate base. For \(\mathrm{MeOCH}_2\mathrm{CO}_2\mathrm{H}\), the \(\mathrm{MeO}\) group is electron-donating, which makes the acid weaker.

Step 4: Comparing All Compounds

\(\mathrm{CF}_3\mathrm{CO}_2\mathrm{H}\) is the strongest due to the electron-withdrawing group. \(\mathrm{CH}_3\mathrm{CO}_2\mathrm{H}\) is stronger than \((\mathrm{Me})_2\mathrm{CHCO}_2\mathrm{H}\) because alkyl groups are weakly electron-donating, and \(\mathrm{MeOCH}_2\mathrm{CO}_2\mathrm{H}\) is the weakest due to its electron-donating \(\mathrm{MeO}\) group.

Step 5: Arranging in Order of Increasing Acid Strength

Based on the analysis, the order of increasing acid strength is \(\mathrm{MeOCH}_2\mathrm{CO}_2\mathrm{H} < (\mathrm{Me})_2\mathrm{CHCO}_2\mathrm{H} < \mathrm{CH}_3\mathrm{CO}_2\mathrm{H} < \mathrm{CF}_3\mathrm{CO}_2\mathrm{H}\).

Key Concepts

Electron-Withdrawing Groups

Acids are substances that can donate protons, and their strength is often linked to the stability of their conjugate base. One of the key factors influencing this stability is the presence of electron-withdrawing groups. These groups are like magnets for electrons and have a profound impact on acid strength.

Electron-withdrawing groups pull electron density towards themselves, which helps to stabilize the negative charge that forms on the conjugate base after the acid loses a proton. This increased stability makes the acid stronger because there's less energy needed to maintain the base's form post-hydrogen loss. For example, in the molecule \(\mathrm{CF}_3\mathrm{CO}_2\mathrm{H}\), the \(\mathrm{CF}_3\) group drastically pulls electron density away from the \(\mathrm{COO}^-\) group, stabilizing it and making \(\mathrm{CF}_3\mathrm{CO}_2\mathrm{H}\) a strong acid.

Some common electron-withdrawing groups include:

• Fluorine (\(\mathrm{F}\)) and other halogens
• Nitro groups (\(\mathrm{NO}_2\))
• Carbonyls (\(\mathrm{C=O}\))

These groups enhance the acid's ability to donate protons, thus increasing its acidity.

Conjugate Base Stability

Conjugate base stability is a central concept in understanding acid strength. When an acid donates a proton, it leaves behind a conjugate base that holds a negative charge. The more stable this conjugate base, the stronger the original acid.

Stability of the conjugate base depends on several factors:

• Resonance Stabilization: If the negative charge can be delocalized over multiple atoms through resonance, the conjugate base becomes more stable. This spreading out of charge reduces electron repulsion.
• Electronegative Atoms: Conjugate bases are more stable when the negative charge is on a more electronegative atom, as these atoms naturally "desire" electrons more.
• Inductive Effects: Nearby electron-withdrawing groups can also stabilize the base by pulling electrons away from areas of high electron density.

In essence, strong acids typically have conjugate bases that can effectively handle the additional electron density, through one or more of these stabilizing effects.

Electron-Donating Groups

While electron-withdrawing groups enhance acid strength, electron-donating groups have the opposite effect. These groups contribute electron density towards the molecule, destabilizing the conjugate base and leading to weaker acids.

Electron-donating groups increase the electron density on an already negatively charged conjugate base, making it less stable. This increased instability requires more energy to achieve, producing a weaker acid. For example, in \(\mathrm{MeOCH}_2\mathrm{CO}_2\mathrm{H}\), the \(\mathrm{MeO}\) group donates electrons, thus destabilizing the conjugate base and resulting in weaker acidity.

Common electron-donating groups include:

• Alkyl groups (such as \(\mathrm{CH}_3\)
• Alkoxies (such as \(\mathrm{OCH}_3\))
• Amino groups (such as \(\mathrm{NH}_2\))

These groups lessen the acid strength by making the conjugate base less stable.