Question

The correct order of basic nature on aqueous solution for the bases \( \text{NH}_3 \), \( \text{NH}_2 \), \( \text{CH}_3 \text{NH}_2 \), \( \text{CH}_3 \text{CH}_2 \text{NH}_2 \), \( (\text{CH}_3 \text{CH}_2)_2 \text{NH} \) is:

• A. \( \text{NH}_3>\text{NH}_2>\text{CH}_3 \text{NH}_2>\text{CH}_3 \text{CH}_2 \text{NH}_2>(\text{CH}_3 \text{CH}_2)_2 \text{NH} \)
• B. \( \text{NH}_2>\text{NH}_3>\text{CH}_3 \text{NH}_2>\text{CH}_3 \text{CH}_2 \text{NH}_2>(\text{CH}_3 \text{CH}_2)_2 \text{NH} \)
• C. \( \text{NH}_3>\text{CH}_3 \text{NH}_2>\text{NH}_2>\text{CH}_3 \text{CH}_2 \text{NH}_2>(\text{CH}_3 \text{CH}_2)_2 \text{NH} \)
• D. \( \text{H}_2\text{N}-\text{NH}_2 < \text{NH}_3 < \text{CH}_3\text{CH}_2\text{NH}_2 < (\text{CH}_3\text{CH}_2)_3\text{N} < (\text{CH}_3\text{CH}_2)_2\text{NH} \)

Hint:

The presence of alkyl groups enhances basic strength due to the electron-donating effect, but the bulkier groups like \( (\text{CH}_3 \text{CH}_2)_2 \text{NH} \) decrease the basicity due to steric hindrance and reduced availability of the lone pair.

Answer 1

The Correct Option is D

This problem asks for the correct increasing order of basic strength for a set of nitrogen-containing compounds in an aqueous solution. The basicity of these compounds depends on the availability of the lone pair of electrons on the nitrogen atom to accept a proton from water.

Concept Used:

The basicity of amines and related compounds in an aqueous solution is governed by a combination of several factors:

1. Inductive Effect (+I): Alkyl groups (like ethyl, \(-\text{CH}_2\text{CH}_3\)) are electron-donating. They increase the electron density on the nitrogen atom, making its lone pair more available for protonation. Based on this effect alone, the order of basicity would be: Tertiary amine > Secondary amine > Primary amine > Ammonia.
2. Solvation Effect (Hydration): The ammonium cation (conjugate acid) formed after accepting a proton is stabilized by hydrogen bonding with water molecules. Greater stabilization of the conjugate acid makes the parent amine a stronger base. The degree of solvation depends on the number of hydrogen atoms available for H-bonding on the nitrogen atom.
   • Primary amine conjugate acid (R-NH₃⁺): Best solvated (3 H-bonds)
   • Secondary amine conjugate acid (R₂-NH₂⁺): Moderately solvated (2 H-bonds)
   • Tertiary amine conjugate acid (R₃-NH⁺): Poorly solvated (1 H-bond)
3. Steric Hindrance: Bulky alkyl groups can physically block the lone pair on the nitrogen, hindering its ability to accept a proton. This effect is most pronounced in tertiary amines and reduces their basicity.
4. Inductive Effect (-I): Electronegative atoms or groups (like another -NH₂ group) can withdraw electron density, making the lone pair less available and decreasing basicity.

The final observed order is the net result of these competing effects.

Step-by-Step Solution:

Step 1: Analyze Hydrazine (\( \text{H}_2\text{N}-\text{NH}_2 \))

In hydrazine, two electronegative nitrogen atoms are bonded to each other. The lone pair on one nitrogen atom is pulled by the electron-withdrawing inductive effect (-I effect) of the adjacent -NH₂ group. This significantly reduces the availability of the lone pair for protonation, making hydrazine a very weak base, even weaker than ammonia. Therefore, hydrazine is the least basic compound in the list.

Step 2: Compare Hydrazine and Ammonia (\( \text{NH}_3 \))

Ammonia has no electron-donating or electron-withdrawing groups attached. As established in Step 1, due to the -I effect in hydrazine, ammonia is a stronger base than hydrazine.

\[ \text{H}_2\text{N}-\text{NH}_2 < \text{NH}_3 \]

Step 3: Compare Ammonia and Ethylamines

Ethylamine (\(\text{CH}_3\text{CH}_2\text{NH}_2\)), Diethylamine (\((\text{CH}_3\text{CH}_2)_2\text{NH}\)), and Triethylamine (\((\text{CH}_3\text{CH}_2)_3\text{N}\)) all have electron-donating ethyl groups. The +I effect of these alkyl groups increases the electron density on the nitrogen atom, making them all stronger bases than ammonia.

Step 4: Determine the Order of Basicity for Ethylamines (1°, 2°, 3°)

Here, the interplay between the inductive effect, solvation, and steric hindrance is crucial.

• Diethylamine (2°): Exhibits the strongest basicity. It benefits from a strong +I effect from two ethyl groups and its conjugate acid can still be reasonably well-solvated (forming two H-bonds). This represents the optimal balance of all factors.
• Triethylamine (3°): Has the strongest +I effect due to three ethyl groups. However, its conjugate acid is poorly solvated (only one H-bond), and there is significant steric hindrance from the three bulky ethyl groups, which impedes protonation.
• Ethylamine (1°): Has the weakest +I effect among the three amines, but its conjugate acid is the most effectively stabilized by solvation (three H-bonds).

For ethylamines in an aqueous solution, the experimentally determined order of basic strength is: Secondary > Tertiary > Primary. The powerful combined +I effect in triethylamine outweighs its poor solvation enough to make it more basic than ethylamine.

\[ \text{CH}_3\text{CH}_2\text{NH}_2 < (\text{CH}_3\text{CH}_2)_3\text{N} < (\text{CH}_3\text{CH}_2)_2\text{NH} \]

Step 5: Combine All Compounds into the Final Order

Assembling the comparisons from the previous steps gives the final increasing order of basic nature:

\[ \text{H}_2\text{N}-\text{NH}_2 < \text{NH}_3 < \text{CH}_3\text{CH}_2\text{NH}_2 < (\text{CH}_3\text{CH}_2)_3\text{N} < (\text{CH}_3\text{CH}_2)_2\text{NH} \]

This corresponds to option (4) in the list.