Problem 33 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

What is the decreasing order of strength of the bases \(\mathrm{OH}^{-}, \mathrm{NH}_{2}^{-}, \mathrm{HC} \equiv \mathrm{C}^{-}\)and \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{-} ?\) (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}^{-}>\mathrm{NH}_{2}^{-}>\mathrm{H}-\mathrm{C} \equiv \mathrm{C}^{-}>\mathrm{OH}^{-}\) (b) \(\mathrm{H}-\mathrm{C} \equiv \mathrm{C}^{-}>\mathrm{CH}_{3}-\mathrm{CH}_{2}^{-}>\mathrm{NH}_{2}^{-}>\mathrm{OH}\) (c) \(\mathrm{OH}^{-}>\mathrm{NH}_{2}^{-}>\mathrm{H}-\mathrm{C} \equiv \mathrm{C}^{-}>\mathrm{CH}_{3}-\mathrm{CH}_{2}^{-}\) (d) \(\mathrm{NH}_{2}^{-}>\mathrm{H}-\mathrm{C} \equiv \mathrm{C}^{-}>\mathrm{OH}^{-}>\mathrm{CH}_{3}-\mathrm{CH}_{2}^{-}\)

Step-by-Step Solution

Answer

The correct order is (a): \(\mathrm{CH_3CH_2}^{-} > \mathrm{NH_2}^{-} > \mathrm{HC}\equiv\mathrm{C}^{-} > \mathrm{OH}^{-}\).

Step 1: Identify Key Properties

The strength of a base is influenced by its ability to donate electron pairs. Typically, bases containing atoms with higher electronegativity or greater resonance stability are weaker bases as they hold onto their electrons more tightly. In contrast, bases with less electronegative atoms and less ability to stabilize additional electrons are stronger bases.

Step 2: Evaluate Acid Conjugates

The strength of a base is inversely related to the acidity of its conjugate acid. The weaker the conjugate acid, the stronger the base. Identify the conjugate acids for each ion: - \(\text{Conjugate acid of } \mathrm{OH}^{-} \text{ is } \mathrm{H_2O}\)- \(\text{Conjugate acid of } \mathrm{NH_2}^{-} \text{ is } \mathrm{NH_3}\)- \(\text{Conjugate acid of } \mathrm{HC}\equiv\mathrm{C}^{-} \text{ is } \mathrm{HC}\equiv\mathrm{CH}\)- \(\text{Conjugate acid of } \mathrm{CH_3CH_2}^{-} \text{ is } \mathrm{CH_3CH_3}\)

Step 3: Rank Acidic Strengths of Conjugates

Arrange the conjugate acids by increasing acidity (hence, by decreasing corresponding base strength):1. \(\mathrm{CH_3CH_3}\) is least acidic, meaning \(\mathrm{CH_3CH_2}^{-}\) is the strongest base.2. \(\mathrm{NH_3}\), less acidic than water, makes \(\mathrm{NH_2}^{-}\) a strong base.3. \(\mathrm{HC}\equiv\mathrm{CH}\) is a relatively weak acid.4. \(\mathrm{H_2O}\) is quite acidic compared to the others, making \(\mathrm{OH}^{-}\) a weaker base.

Step 4: Determine Decreasing Order of Base Strength

Based on the acidity of their respective conjugate acids, we rank the bases from strongest to weakest: - \(\mathrm{CH_3CH_2}^{-} > \mathrm{NH_2}^{-} > \mathrm{HC}\equiv\mathrm{C}^{-} > \mathrm{OH}^{-}\).

Step 5: Check Against Options

Compare the determined order with the given options:- Option (a): \(\mathrm{CH_3CH_2}^{-} > \mathrm{NH_2}^{-} > \mathrm{HC}\equiv\mathrm{C}^{-} > \mathrm{OH}^{-}\) - This matches our determination.- Others do not match.

Key Concepts

Electronegativity

Electronegativity is a crucial concept in understanding the strength of bases. It measures an atom's ability to attract electrons towards itself. In the context of bases, an atom with higher electronegativity holds onto its electrons tighter, making it harder for the atom to donate those electrons. This generally results in a weaker base.

For example, in the base \( \mathrm{OH}^{-} \), oxygen has a high electronegativity. It strongly attracts electrons, which makes it less willing to donate them, resulting in a relatively weaker base compared to bases with less electronegative atoms.

When we look at bases such as \( \mathrm{CH_3CH_2}^{-} \), carbon is the key atom involved, which has lower electronegativity compared to oxygen. This means that \( \mathrm{CH_3CH_2}^{-} \) can donate its electrons more easily, contributing to its strength as a base. Consequently, in assessing base strength, those compounds featuring elements with lower electronegativity are typically stronger.

Conjugate Acid

In chemistry, the term conjugate acid plays a significant role when evaluating the basicity of a substance. When a base accepts a proton, it forms its conjugate acid. The strength of a base is inversely related to the strength of its conjugate acid. This means, the weaker the conjugate acid, the stronger the base.

Let's delve deeper with examples from the exercise:

• \( \mathrm{OH}^{-} \) has a conjugate acid of \( \mathrm{H_2O} \) (water). Since \( \mathrm{H_2O} \) is relatively strong as a conjugate acid, \( \mathrm{OH}^{-} \) is a weaker base.
• For \( \mathrm{NH_2}^{-} \), its conjugate acid is \( \mathrm{NH_3} \), which is not very strong, denoting that \( \mathrm{NH_2}^{-} \) itself is a strong base.
• The base \( \mathrm{CH_3CH_2}^{-} \) forms ethanes \( \mathrm{CH_3CH_3} \) as its conjugate acid. Since ethane is very weakly acidic, \( \mathrm{CH_3CH_2}^{-} \) is among the strongest bases in this list.

This evaluation is essential for predicting reactivity and interaction in chemical processes.

Acidity

Acidity is closely linked to the concept of base strength and provides a clear framework for comparing different bases. It basically refers to the ability of a compound to donate protons (\( \mathrm{H^+} \) ions) when in solution. The relative acidity of substances helps determine the corresponding basicity of ions or molecules related to them.

As seen in our original problem, we compare the acidity of conjugate acids to understand base strength:

• \( \mathrm{CH_3CH_3} \), being the conjugate acid of \( \mathrm{CH_3CH_2}^{-} \), is the least acidic. Thus, \( \mathrm{CH_3CH_2}^{-} \) is the strongest base.
• On the other hand, \( \mathrm{H_2O} \) is quite acidic, making \( \mathrm{OH}^{-} \) a weak base as it readily donates protons.

By understanding these relationships, we can predict and analyze chemical reactions more efficiently. The examination of conjugate acids’ acidity provides critical insights into the hierarchy of base strengths, revealing which bases are likely to participate more actively in chemical interactions.