Problem 84 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The correct order is (a): \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{-} > \mathrm{NH}_{2}^{-} > \mathrm{HC} \equiv \mathrm{C}^{-} > \mathrm{OH}^{-}\).

Step 1: Understand Basicity

The strength of a base is determined by its ability to donate electron pairs. Stronger bases have a higher tendency to donate electron pairs and usually have negative charges on less electronegative atoms.

Step 2: Identify the Conjugate Acids

Identify the conjugate acids of the given bases: for \(\mathrm{OH}^{-}\), it is water (\(\mathrm{H}_{2}\mathrm{O}\)); for \(\mathrm{NH}_{2}^{-}\), it is ammonia (\(\mathrm{NH}_{3}\)); for \(\mathrm{HC} \equiv \mathrm{C}^{-}\), it is acetylene (\(\mathrm{HC} \equiv \mathrm{CH}\)); and for \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{-}\), it is ethane (\(\mathrm{CH}_{3} \mathrm{CH}_{3}\)).

Step 3: Assess the Acidity of Conjugate Acids

The weaker the conjugate acid, the stronger the base. Water (\(\mathrm{H}_{2}\mathrm{O}\)) and ammonia (\(\mathrm{NH}_{3}\)) are relatively strong conjugate acids, whereas acetylene (\(\mathrm{HC} \equiv \mathrm{CH}\)) and ethane (\(\mathrm{CH}_{3} \mathrm{CH}_{3}\)) are weaker acids.

Step 4: Rank Base Strength Based on Conjugate Acids

Since ethane is the weakest acid (making \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{-}\) the strongest base), followed by acetylene, ammonia, and water, the order of bases from strongest to weakest is: \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{-}\), \(\mathrm{HC} \equiv \mathrm{C}^{-}\), \(\mathrm{NH}_{2}^{-}\), \(\mathrm{OH}^{-}\).

Step 5: Compare with Given Options

Match our ordered list of bases \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{-} > \mathrm{HC} \equiv \mathrm{C}^{-} > \mathrm{NH}_{2}^{-} > \mathrm{OH}^{-}\) with the options provided. This order corresponds to option (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{-} > \mathrm{NH}_{2}^{-} > \mathrm{HC} \equiv \mathrm{C}^{-} > \mathrm{OH}^{-}\).

Key Concepts

Conjugate Acids

When analyzing bases like \(\mathrm{OH}^{-},\ \mathrm{NH}_{2}^{-},\ \mathrm{HC} \equiv \mathrm{C}^{-}\), and \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{-}\), it is crucial to identify their conjugate acids. This helps us understand how strong or weak a base is, based on the properties of its conjugate acid. Every base has a counterpart acid, formed when it accepts a proton (\(\text{H}^{+}\)).

• For \(\mathrm{OH}^{-}\), the conjugate acid is water \(\mathrm{H}_{2} \mathrm{O}\).
• \(\mathrm{NH}_{2}^{-}\) becomes \(\mathrm{NH}_{3}\), known as ammonia.
• \(\mathrm{HC} \equiv \mathrm{C}^{-}\) transforms into acetylene \(\mathrm{HC} \equiv \mathrm{CH}\).
• Finally, \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{-}\) turns into ethane \(\mathrm{CH}_{3} \mathrm{CH}_{3}\).

Understanding these conjugate acids is a stepping stone in evaluating base strength. The weaker the conjugate acid, the stronger the base. This simple principle becomes very helpful in solving problems related to basicity.

Base Strength

Base strength is a measure of a base's ability to donate electron pairs. It's a key concept in understanding chemical reactivity. Strong bases are typically more willing to part with their electron pairs and often have a negative charge on an atom that isn't too electronegative.
Evaluating the base strength involves checking the stability and charge distribution of a molecule:

• A strong base like \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{-}\) is very capable of donating its electrons because of the less electronegative carbon atom.
• \(\mathrm{HC} \equiv \mathrm{C}^{-}\) is also a strong base, but slightly weaker compared to \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{-}\).
• \(\mathrm{NH}_{2}^{-}\) is next because nitrogen is more electronegative than carbon.
• Finally, \(\mathrm{OH}^{-}\) is the weakest among these bases due to the high electronegativity of oxygen holding onto its electron pairs tightly.

Thus, a clear ranking emerges when we examine how each base donates the electron pairs, determined by the nature of their atoms and the conjugate acid's strength.

Acidity of Conjugate Acids

The acidity of conjugate acids directly influences the strength of their corresponding bases. If a conjugate acid is potent, its base will be weaker, given that it forms easily and is more stable.
When we look at the acidity of the conjugate acids:

• Water \(\mathrm{H}_{2} \mathrm{O}\) and ammonia \(\mathrm{NH}_{3}\) are relatively strong acids among the group, making their conjugate bases \(\mathrm{OH}^{-}\) and \(\mathrm{NH}_{2}^{-}\) weaker.
• On the other hand, acetylene \(\mathrm{HC} \equiv \mathrm{CH}\) and ethane \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) are considered weak acids because they interact less readily with \(\text{H}^{+}\) ions.

In summary:- As \(\mathrm{CH}_{3} \mathrm{CH}_{3}\) is the weakest acid, its base \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{-}\) ranks as the strongest.- Acetylene comes next, followed by ammonia and water, aligning their conjugate bases as per their strength. Hence, understanding conjugate acid acidity is critical for determining which bases are strong or weak.