Problem 37 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following, the dissociation constant is highest for (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{OH}\) (c) \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CH}\) (d) \(\mathrm{CH}_{3} \mathrm{NH}_{3}{\underline{\phantom{xx}}}^{+} \mathrm{Cl}^{-}\)

Step-by-Step Solution

Answer

The dissociation constant is highest for \(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+} \mathrm{Cl}^{-}\).

Step 1: Understand the Concept of Dissociation Constant

The dissociation constant, often denoted as \( K_a \) for acids or \( K_b \) for bases, measures the extent of dissociation of a compound in solution. A higher dissociation constant indicates a greater degree of ionization, meaning the compound more readily releases ions into solution.

Step 2: Analyze the Given Compounds

We must identify the acidic or basic nature of each given compound to determine which would have the highest dissociation constant.1. **\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\)** (Phenol) is a weak acid and has a moderate dissociation constant.2. **\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{OH}\)** (Benzyl alcohol) is a very weak acid and is mainly neutral.3. **\(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CH}\)** (Ethyne) is not a typical acid or base with significant dissociation.4. **\(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+} \mathrm{Cl}^{-}\)** is an ammonium salt, which can dissociate to form \(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}\), an acid.

Step 3: Compare the Dissociation Tendencies

Phenol is a known weak acid with a relatively moderate \( K_a \). Benzyl alcohol is much less acidic than phenol. Ethyne is not typically considered acidic. The compound \(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+} \) is derived from a weak base \(\mathrm{CH}_{3} \mathrm{NH}_{2}\), and its conjugate acid can dissociate, potentially having a significant \( K_a \).

Step 4: Conclusion

Given that ammonium compounds like \(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}\) often have dissociation constants in the range of weak acids and are more ionized than phenol or benzyl alcohol, \(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}\) likely has the highest dissociation constant among the options.

Key Concepts

Acids and Bases

An acid is a substance that donates protons (H+) in a reaction, whereas a base accepts them. Understanding acids and bases is fundamental in chemistry as this concept explains how substances interact with water and affect pH levels.

• Acids like phenol (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\)) tend to release hydrogen ions, hence, they increase the concentration of hydrogen ions in a solution.
• Bases, on the other hand, like ammonia in water, can accept a hydrogen ion to form ammonium.

The strength of an acid or base is determined by its dissociation constant, where strong acids completely dissociate in water, while weak acids partially dissociate.
Thus, the concept of dissociation constant is crucial to understanding the behavior of acids and their ability to donate protons to a solution.

Ionization

Ionization is the process where a neutral molecule splits into charged ions when dissolved in a liquid such as water. This process is central to the concept of acids and bases as it explains their behavior in water.

• When an acid dissolves in water, it typically ionizes to produce H+ ions; in case of bases, they can accept H+ ions, forming hydroxide ions.
• For example, phenol (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\)) when dissolved in water, will release H+ ions, though not completely as it is a weak acid.

The extent to which a compound ionizes in solution is what is measured by the dissociation constant, with high ionization indicating a high constant for acids and bases.
This differentiates weak acids or bases from strong ones where ionization is nearly complete.

Chemical Compounds Behavior

Chemical compounds exhibit a wide variety of behaviors based on their structure and composition. This behavior influences how these compounds interact with each other and their surroundings.

• Phenols are typically weak acids because they only partially release hydrogen ions.
• Benzyl alcohol is even less acidic, hardly ionizing in water, making it largely neutral.
• Ethyne is considered neutral with respect to typical acid-base behavior.
• The ammonium ion (\(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}\)), derived from methylamine, shows acidic behavior, releasing protons when dissolved in water.

A compound like methylammonium chloride behaves as an acid because it donates protons to water, forming hydronium ions.
Each of these behaviors helps determine the chemical reactivity and applications of such compounds in different fields, including industry and pharmaceuticals.