Problem 150 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of relative basic strength of the following is (a) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{O}^{-}>\mathrm{CH} \equiv \mathrm{C}^{-}>-\mathrm{OH}\) (b) \(\mathrm{CH} \equiv \mathrm{C}^{-}>-\mathrm{OH}>\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{O}^{-}\) (c) \(\mathrm{CH} \equiv \mathrm{C}^{-}>\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{O}^{-}>-\mathrm{OH}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{O}^{-}>\mathrm{OH}^{-}>\mathrm{CH} \equiv \mathrm{C}^{-}\)

Step-by-Step Solution

Answer

Option (c): \( \mathrm{CH} \equiv \mathrm{C}^{-} > \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{O}^{-} > -\mathrm{OH} \).

Step 1: Identify Basicity Trend

The basicity of anions is inversely related to the stability of their conjugate acids. Strong bases have weak conjugate acids. We need to compare the conjugate acids of \( \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{O}^{-} \), \( \mathrm{CH} \equiv \mathrm{C}^{-} \), and \( \mathrm{OH}^{-} \).

Step 2: Analyze Conjugate Acids

- The conjugate acid of \( \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{O}^{-} \) is ethanol (\( \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{OH} \)), which is a weak acid.- The conjugate acid of \( \mathrm{CH} \equiv \mathrm{C}^{-} \) is acetylene (\( \mathrm{CH} \equiv \mathrm{CH} \)), which is even weaker than ethanol, making \( \mathrm{CH} \equiv \mathrm{C}^{-} \) a stronger base.- The conjugate acid of \( \mathrm{OH}^{-} \) is water (\( \mathrm{H}_{2}\mathrm{O} \)), which is a relatively stronger acid than the other two, making \( \mathrm{OH}^{-} \) the weakest base among the three.

Step 3: Order the Relative Basic Strength

Based on the relative acidity of their conjugate acids, the order of basicity from strongest to weakest is: 1. \( \mathrm{CH} \equiv \mathrm{C}^{-} \) 2. \( \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{O}^{-} \) 3. \( \mathrm{OH}^{-} \). This corresponds to option (c): \( \mathrm{CH} \equiv \mathrm{C}^{-} > \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{O}^{-} > -\mathrm{OH} \).

Key Concepts

Conjugate Acid Stability

Understanding conjugate acid stability is crucial when determining the relative strength of bases. A conjugate acid is what an acid becomes after it donates a proton. The stability of a conjugate acid is inversely related to the strength of the base from which it formed. This means that if a conjugate acid is weak (and thus stable), the original base will be strong. Consider the conjugate acids we are dealing with:

• For \( \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{O}^{-} \), the conjugate acid is ethanol (\( \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{OH} \)).
• For \( \mathrm{CH} \equiv \mathrm{C}^{-} \), the conjugate acid is acetylene (\( \mathrm{CH} \equiv \mathrm{CH} \)).
• For \( \mathrm{OH}^{-} \), the conjugate acid is water (\( \mathrm{H}_{2}\mathrm{O} \)).

Ethanol and acetylene are weaker acids compared to water, meaning they are more stable as conjugate acids. As a result, their conjugate bases, \( \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{O}^{-} \) and \( \mathrm{CH} \equiv \mathrm{C}^{-} \) respectively, are stronger bases. This concept is key in understanding why certain substances act as strong bases.

Anion Basicity

Anions, which are negatively charged ions, can function as bases by accepting protons. The basicity of an anion is influenced by the stability of its conjugate acid. The more stable the conjugate acid, the weaker the original anion base, and vice versa. In this exercise, the anions we're focusing on are:

• \( \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{O}^{-} \) - ethoxide ion
• \( \mathrm{CH} \equiv \mathrm{C}^{-} \) - acetylide ion
• \( \mathrm{OH}^{-} \) - hydroxide ion

Let’s dive into what makes these anions good or bad bases. The ethoxide ion and the acetylide ion are associated with weak conjugate acids, making them strong bases. However, the hydroxide ion is linked to water, a stronger acid, which results in \( \mathrm{OH}^{-} \) being a weaker base. Understanding these relationships is crucial for comparing and contrasting the basic strength of each anion.

Acidity Comparison

When we want to compare acidity, it's about understanding the ease with which a substance can donate a proton. The stronger the acid, the more readily it gives up a proton. This comparison forms the basis for evaluating the basicity of anions. Consider the polar opposite roles of acids and bases; as we evaluate the acidity of the conjugate acids, we effectively determine the order of base strength.In our exercise:

• Water (\( \mathrm{H}_{2}\mathrm{O} \)) is a relatively stronger acid, being able to donate a proton more easily compared to ethanol (\( \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{OH} \)) and acetylene (\( \mathrm{CH} \equiv \mathrm{CH} \)).
• Ethanol is weaker than water, and acetylene is the weakest among them.

From this perspective, since acetylene is the weakest acid, its conjugate base \( \mathrm{CH} \equiv \mathrm{C}^{-} \) is the strongest base. By applying these insights, the order of relative basic strength aligns perfectly with the stability of their conjugate acids: \( \mathrm{CH} \equiv \mathrm{C}^{-} > \mathrm{C}_{2} \mathrm{H}_{5}\mathrm{O}^{-} > \mathrm{OH}^{-} \). This provides a clear comparison of acidity and predicts the behavior of these anions in chemical reactions.