Problem 35 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following, the weakest base is (a) \(\mathrm{CH}_{3} \mathrm{NHCHO}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (c) \(\mathrm{O}_{2} \mathrm{NCH}_{2} \mathrm{NH}_{2}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NHCH}_{3}\)

Step-by-Step Solution

Answer

The weakest base is (c) \(\mathrm{O}_2\mathrm{NCH}_2\mathrm{NH}_2\).

Step 1: Identify the Base Species

Examine each compound to identify which portion can potentially act as a base. Typically, it's the nitrogenous group that can accept protons in these compounds.

Step 2: Evaluate Electron Withdrawing and Donating Groups

For (a) \(\mathrm{CH}_3\mathrm{NHCHO}\), the formamide group \(\mathrm{NHCHO}\) withdraws electrons, making it a weaker base. (b) \(\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\mathrm{NH}_2\), the phenyl group is slightly electron-withdrawing but not very strongly. (c) \(\mathrm{O}_2\mathrm{NCH}_2\mathrm{NH}_2\), the \(\mathrm{NO}_2\) group strongly withdraws electrons, significantly reducing basicity. (d) \(\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\mathrm{NHCH}_3\), similar to (b) but with a secondary amine.

Step 3: Compare the Electronegativity Effects

Electronegative groups that withdraw electron density from nitrogen reduce the ability of nitrogen to donate electrons to protons (acting as a base). The \(\mathrm{NO}_2\) group in compound (c) is one of the strongest electron-withdrawing groups, making compound (c) the weakest base.

Step 4: Conclusion

Based on the analysis, compound (c) \(\mathrm{O}_2\mathrm{NCH}_2\mathrm{NH}_2\) has the strongest electron-withdrawing group, reducing its ability to act as a base. This makes it the weakest base among the options given.

Key Concepts

Electron-Withdrawing Groups

Electron-withdrawing groups play a crucial role in acid-base chemistry by affecting the basicity of compounds. These groups pull electron density away from other parts of the molecule. This results in a decrease in electron availability. When it comes to amines and other nitrogen-containing compounds, electron-withdrawing groups make the nitrogen less likely to donate its lone pair of electrons.

For instance, in the given exercise, the nitro group (NO\(_2\)) is an exceptionally strong electron-withdrawing group due to its high electronegativity and resonance-stabilized structure. It makes the amino group (NH\(_2\)) in the compound O\(_2\)NCH\(_2\)NH\(_2\) less basic. The nitro group effectively drains electron density from the nearby nitrogen atom, hindering its ability to act as a base.

• Strongly withdrawing groups: Nitro (NO\(_2\)), carbonyl (C=O\)), cyano (C\(_n\( \equiv \mathrm{N}\))), sulfonyl (SO\(_3\)H)
• Impact: Reduces basicity significantly by lowering electron availability

Basicity of Amines

Amines contain a nitrogen atom with a lone pair of electrons. This makes them good candidates to act as bases, as they can donate these electrons. However, the basicity of an amine is influenced by its surroundings.

In general, the presence of electron-donating groups can enhance the basicity of the amine by increasing the electron density on nitrogen. Conversely, electron-withdrawing groups reduce basicity.

In the given problem, different substituents attached to the nitrogen change each compound's ability to act as a base:

• CH\(_3\)NHCHO (a): Formamide is electron-withdrawing, diminishing basicity.
• C\(_6\)H\(_5\)CH\(_2\)NH\(_2\) (b): Slightly electron-withdrawing phenyl group; moderate impact on basicity.
• O\(_2\)NCH\(_2\)NH\(_2\) (c): Strong electron-withdrawing effect by the nitro group; significantly reduces basicity.
• C\(_6\)H\(_5\)CH\(_2\)NHCH\(_3\) (d): Secondary amine with phenyl, similar to (b).

The weakest base among them is the one with the strongest electron-withdrawing group, which is option (c).

Electronegativity Effects

Electronegativity refers to the ability of an atom to attract shared electrons. In molecules, this concept helps determine how electrons are distributed. In acid-base chemistry, it influences the ability of species like amines to donate electrons.

Atoms or groups with high electronegativity, when attached to a nitrogen atom, pull electrons towards themselves. This decreases the electron availability of the nitrogen atom, which lowers its basicity.

In the given exercise, the nitro group (NO\(_2\)) is highly electronegative. This group has a strong attraction for electrons, making it one of the most effective electron-withdrawing groups. Its presence in O\(_2\)NCH\(_2\)NH\(_2\) pulls electron density from nitrogen, effectively reducing the nitrogen's ability to act as a base.

• Electronegativity trend: higher electronegativity = stronger electron withdrawal
• Effect: lowers basicity by reducing electron availability at the nitrogen center.

The nitro group's strong electronegative nature explains why it has such a profound impact on reducing basicity in the given compound.