Problem 66 - A Complete Resource Book in Chemistry for JEE Main

Question

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

(a) \( \text{CH}_3-\text{CH}_2^- > \text{NH}_2^- > \text{H-C} \equiv \text{C}^- > \text{OH}^- \)

Step 1: Understand the concept of base strength

Base strength is generally determined by the ability of a species to donate an electron pair or accept a proton. Stronger bases are more eager to accept protons.

Step 2: Identify the base conjugate pairs

Consider each species as part of a conjugate pair: \( \text{OH}^- \) (with water), \( \text{NH}_2^- \) (with ammonia), \( \text{H-C} \equiv \text{C}^- \) (with acetylene), and \( \text{CH}_3-\text{CH}_2^- \) (with ethane).

Step 3: Compare the pKa values of conjugate acids

The strength of a base is inversely related to the pKa value of its conjugate acid. Lower pKa value of the conjugate acid means stronger base: hydroxide ion is derived from water (pKa ~15.7), amide ion from ammonia (pKa ~36), acetylide ion from acetylene (pKa ~25), and ethyl anion from ethane (pKa ~50).

Step 4: Rank the bases by strength

Using the pKa values, arrange the bases from strongest to weakest. Ethyl anion, with the highest pKa of the conjugate acid, is the strongest base, followed by amide ion, acetylide ion, and finally hydroxide ion as the weakest.

Step 5: Match with the given options

Compare your order from Step 4 with the options given: (a) \( \text{CH}_3-\text{CH}_2^- > \text{NH}_2^- > \text{H-C} \equiv \text{C}^- > \text{OH}^- \) corresponds to the order derived.

Key Concepts

Conjugate Acid pKa

The concept of conjugate acid pKa is fundamental to understanding base strength. pKa, or the acid dissociation constant, essentially measures the propensity of a conjugate acid to donate a proton.
The lower the pKa value, the stronger the acid; conversely, the weaker the conjugate base. Conversely, a higher pKa indicates a weaker acid and a stronger conjugate base.
In the context of our bases, consider the pKa values:

• Water (conjugate acid of \( ext{OH}^- \)): ~15.7
• Ammonia (conjugate acid of \( ext{NH}_2^- \)): ~36
• Acetylene (conjugate acid of \( ext{H-C} \equiv \text{C}^- \)): ~25
• Ethane (conjugate acid of \( ext{CH}_3-\text{CH}_2^- \)): ~50

The ethyl anion, derived from ethane, has the highest pKa value, making it the strongest base here. Understanding pKa values helps predict which base can best hold onto or accept a proton.

Electron Pair Donation

A crucial aspect of basicity is the ability of a compound to donate an electron pair. In chemistry, a base is typically a molecule that can donate an electron pair to form bonds.
The ready availability of an electron pair to be shared or donated distinguishes strong bases from weaker ones.
In our example, each base in question can donate an electron pair to form a new bond, thus behaving as a Lewis base.

• The ethyl anion \( \text{CH}_3-\text{CH}_2^- \) is highly effective in this regard because it is less involved in intrinsic atomic interactions that might otherwise reduce its electron availability.
• The ability of \( \text{NH}_2^- \) and \( \text{H-C} \equiv \text{C}^- \) to donate electron pairs comes next due to their molecular structure that aids in such donation.
• Lastly, \( \text{OH}^- \), while a capable donor, is weaker due to its higher electronegativity, which holds onto the electron pair more tightly.

Understanding electron pair donation aids in visualizing how reactions might proceed when bases interact with acids.

Proton Acceptance

In the realm of acids and bases, proton acceptance is a hallmark of base strength. Bases are characterized by their ability to accept protons (\( ext{H}^+ \)) from acids.
This acceptance leads to the formation of the conjugate acid of the base.
In the context of bases, proton acceptance relies significantly on the structure and chemical environment of the base:

• The ethyl anion, being extremely strong due to its high pKa conjugate acid, readily accepts protons.
• The amide ion \( \text{NH}_2^- \) and acetylide ion \( \text{H-C} \equiv \text{C}^- \) both comprise strong bases also willing to accept protons, but the ethyl anion typically does so with more vigor.
• Lastly, the hydroxide ion \( \text{OH}^- \) accepts protons to form water, demonstrating less eagerness compared to the others discussed.

By knowing the base’s ability to accept protons, students can better predict the direction of chemical reactions.

Conjugate Pairs

Understanding conjugate pairs is integral to acid-base chemistry. Each base has an associated conjugate acid, resulting from its interaction with a proton donor.
A conjugate pair consists of two substances that transform into one another by gaining or losing a proton, \( ext{H}^+ \).
In our exercise, consider:

• Hydroxide ion \( \text{OH}^- \) pairing with water \( \text{H}_2\text{O} \)
• Amide ion \( \text{NH}_2^- \) with ammonia \( \text{NH}_3 \)
• Acetylide ion \( \text{H-C} \equiv \text{C}^- \) with acetylene \( \text{HC} \equiv \text{CH} \)
• Ethyl anion \( \text{CH}_3-\text{CH}_2^- \) corresponding with ethane \( \text{CH}_3-\text{CH}_3 \)

These conjugate acid-base pairs help to visualize what happens during acid-base reactions, specifically how substances transform by the gain or loss of protons. Recognizing these pairs connects theoretical concepts to practical reactions, allowing students to predict the behavior of bases in various scenarios.