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:
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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.
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:
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.
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)
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.
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.
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.
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.
