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

Step 1: Understanding Base Strength

The strength of a base is determined by its ability to donate electrons. A stronger base will more readily donate its electrons to a proton.

Step 2: Analyzing Molecular Structure

Evaluate each base. - \(\mathrm{OH}^-\): Hydroxide ion, often a weaker base compared to carbanions due to its proton affinity.- \(\mathrm{NH}_2^-\): Amide ion, strong base due to its higher electron density around the nitrogen.- \(\mathrm{H}-\mathrm{C} \equiv \mathrm{C}^-\): Acetylide ion, a base with high electron density around carbon but less than carbanions.- \(\mathrm{CH}_3-\mathrm{CH}_2^-\): Ethyl carbanion, generally a very strong base due to carbon's higher willingness to donate electrons.

Step 3: Compare Base Strength

Compare the ions: 1. Ethyl carbanion \((\mathrm{CH}_3-\mathrm{CH}_2^-)\) is typically the strongest base due to carbon's high willingness to hold electrons.2. Amide ion \((\mathrm{NH}_2^-)\) is usually strong, but slightly less so than carbanions.3. Acetylide ion \((\mathrm{H}-\mathrm{C} \equiv \mathrm{C}^-)\) is less strong due to increased s-character and electron-withdrawing triple bond, reducing basicity.4. Hydroxide ion \((\mathrm{OH}^-)\) is the weakest in this group.

Step 4: Determine Base Strength Order

Combining the observations, the decreasing order of strength is: \( \mathrm{CH}_3-\mathrm{CH}_2^- > \mathrm{NH}_2^- > \mathrm{H}-\mathrm{C} \equiv \mathrm{C}^- > \mathrm{OH}^- \).

Key Concepts

Electron Donation

Electron donation is a critical factor in determining the strength of a base. A base can donate its electrons to a proton (or H extsuperscript{+}), and the more readily it can do this, the stronger the base. Bases with higher electron density are more potent electron donors. For example, carbanions, such as the ethyl carbanion (\(\text{CH}_3\text{-CH}_2^-\)), have a significant negative charge concentrated on the carbon atom, making them excellent electron donors. This willingness to donate electrons enhances their reactivity as bases. From this, we conclude that the ability to donate electrons not only depends on the charge but also on the overall electronic environment of the molecule or ion.

Molecular Structure

The molecular structure of a species heavily influences its ability to act as a base. Key structural features include the type of atoms involved, hybridization, and the presence of functional groups. For instance, the acetate ion (\(\text{H}-\text{C}=\text{C}^-\)) has high electron density due to sp-hybridization, where electrons are more tightly held, resulting in a decreased ability to donate electrons compared to other configurations like sp\(^{3}\)-hybridized carbon atoms.

• Atoms with less s-character, like those in sp\(^{3}\) hybridization, can donate electrons more freely than sp hybridized atoms.
• The presence of electronegative elements can also pull electron density away from the active site, reducing basicity.
• Thus, the base strength order is impacted by how the structure of each ion facilitates or hinders electron donation.

Carbanions

Carbanions are negatively charged ions where the negative charge is localized on a carbon atom. Due to this localization, carbanions such as the ethyl carbanion (\(\text{CH}_3\text{-CH}_2^-\)) inherently possess high electron density, making them strong bases. One significant factor is the hybridization of the carbon atom bearing the charge:

• Carbanions with an sp\(^{3}\) hybridized carbon (more p-character) are generally stronger bases than those with an sp hybrid (more s-character) because electrons in p orbitals are less tightly held than those closer to the nucleus in s orbitals.
• Additionally, the surrounding alkyl groups can provide additional electron donation due to hyperconjugation, further increasing the basicity of carbanions.

Amide Ion

The amide ion (\(\text{NH}_2^-\)) is known for its substantial base strength due to the nitrogen atom's high tendency to donate electrons. This tendency arises from:

• The presence of a lone pair on the nitrogen makes it a potent electron donor.
• Nitrogen configuration also plays a role, as it is less electronegative than oxygen, allowing the electrons to be less tightly held and, therefore, more easily donated to protons.
• The base's ability to pull the proton towards itself highlights its reactive nature in chemical reactions like deprotonation.

Acetylide Ion

An acetylide ion (\(\text{H}-\text{C} \equiv \text{C}^-\)), features a carbon-carbon triple bond where one of the carbons bears a negative charge. This structure impacts its basicity due to:

• The high s-character in the sp hybridized carbon, attracting the electron pair closer to the nucleus and making it less available for donation.
• Despite this, the ion remains a strong base as compared to more electronegative elements that hold their electrons tightly, such as oxygen in hydroxide ions.
• The acetylide ion's unique structure allows it to act bases of relatively high strength, adept at protonation reactions, though weaker than some carbanions and amide ions.