Question

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

Short answer

(a) \( \mathrm{CH}_{3}-\mathrm{CH}_{2}^{-} > \mathrm{NH}_{2}^{-} > \mathrm{H}-\mathrm{C} \equiv \mathrm{C}^{-} > \mathrm{OH}^{-} \).

Step-by-step solution

Identify the Bases

The bases provided in the problem are \( \mathrm{OH}^{-} \), \( \mathrm{NH}_{2}^{-} \), \( \mathrm{H}-\mathrm{C} \equiv \mathrm{C}^{-} \), and \( \mathrm{CH}_{3}-\mathrm{CH}_{2}^{-} \). We need to assess their relative strengths.

Consider the Basicity of Hydroxide Ion

The hydroxide ion \( \mathrm{OH}^{-} \) is a commonly known weak base compared to strong anions that are conjugated from less acidic species.

Assess the Basicity of Amide Ion

The amide ion \( \mathrm{NH}_{2}^{-} \) is a strong base, as it is the conjugate base of ammonia, which is a weak acid. This makes it a stronger base than \( \mathrm{OH}^{-} \).

Evaluate Basicity of Acetylide Ion

The ion \( \mathrm{H}-\mathrm{C} \equiv \mathrm{C}^{-} \) (acetylide ion) is a moderately strong base derived from a very weak acid, acetylene, which makes it stronger than \( \mathrm{OH}^{-} \) but generally weaker than \( \mathrm{NH}_{2}^{-} \).

Analyze Basicity of Ethylide Ion

The ethylide ion \( \mathrm{CH}_{3}-\mathrm{CH}_{2}^{-} \) is a very strong base due to its organometallic characteristics, as it comes from ethane. It is stronger than \( \mathrm{NH}_{2}^{-} \) and the other ions listed.

Arrange the Bases in Decreasing Order of Strength

Based on the above assessments, the order of strength from strongest to weakest base is: \( \mathrm{CH}_{3}-\mathrm{CH}_{2}^{-} > \mathrm{NH}_{2}^{-} > \mathrm{H}-\mathrm{C} \equiv \mathrm{C}^{-} > \mathrm{OH}^{-} \).

Key concepts

Hydroxide Ion

The hydroxide ion, commonly represented as \( \mathrm{OH}^{-} \), is one of the most well-recognized bases in chemistry. It is a result of water losing a hydrogen ion, making it the conjugate base of water. Hydroxide ions play critical roles in many chemical reactions, particularly those involving neutralization. For its basicity, \( \mathrm{OH}^{-} \) is considered a weak base when compared to stronger, more reactive ions.
\[\text{Key Characteristics of Hydroxide Ion:}\]

• Derived from water, a very weak acid.
• Commonly found in many cleaning agents and is essential for many biological processes.
• Its weakness as a base is due to the relatively low stability of \( \mathrm{H}_2\mathrm{O} \) as an acid.

When comparing to other ions, hydroxide is often not the most reactive base. This makes it weaker than ions like the amide, acetylide, and ethylide ions.

Amide Ion

The amide ion, \( \mathrm{NH}_{2}^{-} \), is known for its strong basicity. It arises from the deprotonation of ammonia, a weak acid. The removal of a hydrogen ion from ammonia leads to \( \mathrm{NH}_{2}^{-} \), establishing it as a much stronger base relative to hydroxide. The amide ion's strength as a base is highly significant in organic synthesis, particularly in reactions involving deprotonation.
\[\text{Key Characteristics of Amide Ion:}\]

• The conjugate base of ammonia, a weak acid.
• Strong base used frequently in organic reactions.
• Utilized in deprotonating other species due to its high reactivity.

This enhanced basicity stems from the weaker acidic nature of ammonia compared to water, placing \( \mathrm{NH}_{2}^{-} \) above \( \mathrm{OH}^{-} \) in base strength.

Acetylide Ion

The acetylide ion, \( \mathrm{H}-\mathrm{C} \equiv \mathrm{C}^{-} \), emerges from the deprotonation of acetylene, a compound known for its weak acidity. This ion is moderately strong, acting as an effective base in various reactions. It finds frequent use in organic chemistry for creating carbon-carbon bonds, highlighting its reactivity.
\[\text{Key Characteristics of Acetylide Ion:}\]

• Derived from acetylene, a very weak acid.
• Moderately strong base useful in coupling reactions.
• Serves as a nucleophile in synthetic organic chemistry.

Although the acetylide ion is a strong base, it is typically weaker than the amide ion but stronger than the hydroxide ion, striking a balance between the two in terms of reactivity.

Ethylide Ion

The ethylide ion, \( \mathrm{CH}_{3}-\mathrm{CH}_{2}^{-} \), is derived from the very weakly acidic ethane. As a result, it is an exceptionally strong base. Its strength surpasses the other ions in question, making it a prominent choice in reactions requiring a powerful deprotonating agent. The organometallic nature of ethylide ions also contributes to their potent reactivity.
\[\text{Key Characteristics of Ethylide Ion:}\]

• Formed from ethane, among the weakest organic acids.
• Exceptionally strong base, stronger than \( \mathrm{NH}_{2}^{-} \).
• Utilized in strong base reactions and organometallic chemistry.

The strong basicity of ethylide ions places them at the top of the hierarchy among the given bases, surpassing other ions such as hydroxide, amide, and acetylide.