Q.Arrange the following in decreasing order of their basic strength:
$C_6H_5NH_2$, $C_2H_5NH_2$, $(C_2H_5)_2NH$, $NH_3$

Concept: Basicity order of amines depends on the balance between the inductive effect (electron-donating alkyl groups increase electron density on nitrogen) and solvation/hindrance effects in aqueous medium.

Reasoning:

1. In water, aliphatic amines are more basic than ammonia because alkyl groups donate electron density via the +I effect, making the lone pair more available.
2. Among aliphatic amines, secondary amines ($(C_2H_5)_2NH$) are generally the most basic in water due to a favourable combination of inductive effect and solvation of the conjugate acid. …

The question asks us to compare basic strength across four nitrogen-containing compounds: aniline ($C_6H_5NH_2$), ethylamine ($C_2H_5NH_2$), diethylamine ($(C_2H_5)_2NH$), and ammonia ($NH_3$).

Basicity here means the tendency of the nitrogen atom to donate its lone pair to a proton (or to a Lewis acid). In aqueous solution — which is the standard context for such comparisons in Indian exams — two factors dominate: the electron-releasing or withdrawing effect of the groups attached to nitrogen, and the stabilisation of the conjugate acid (the protonated form) through solvation.

Let’s unpack each factor.

1. Inductive effect of alkyl groups

   Alkyl groups like ethyl ($-C_2H_5$) are electron-donating via the inductive effect (+I). They push electron density toward the nitrogen, making the lone pair more available for protonation. More alkyl groups generally mean greater electron density on nitrogen — so a secondary amine (two alkyl groups) should be more basic than a primary amine (one alkyl group), which in turn is more basic than ammonia (no alkyl groups).

   This suggests: $(C_2H_5)_2NH > C_2H_5NH_2 > NH_3$.

2. Solvation of the conjugate acid

   When an amine accepts a proton, it forms an ammonium ion ($R_3NH^+$). This cation is stabilised in water by hydrogen bonding between the N–H hydrogens and water molecules. The more N–H bonds the conjugate acid has, the better it is solvated, and the more stable it is — which shifts the equilibrium toward the protonated form, increasing basicity.

   Ammonia’s conjugate acid ($NH_4^+$) has four N–H bonds. Primary amine conjugate acids have three, secondary have two, and tertiary have only one. So solvation favours: $NH_3 > C_2H_5NH_2 > (C_2H_5)_2NH$.

   These two factors — inductive effect and solvation — pull in opposite directions. The observed order in aqueous solution for aliphatic amines is a compromise: secondary > primary > tertiary > ammonia is the usual pattern for simple alkyl amines, but here we have only up to secondary. For ethylamines, the order is indeed $(C_2H_5)_2NH > C_2H_5NH_2 > NH_3$.

3. The special case of aniline

   Aniline is dramatically less basic than aliphatic amines. Why? The lone pair on nitrogen is delocalised into the benzene ring through resonance. This conjugation makes the lone pair less available for protonation — it’s partly “tied up” in the $\pi$ system. …

Method: Inductive Effect + Solvation Effect Analysis (for Aliphatic vs Aromatic Amines)

This method compares electron availability on nitrogen by considering:

• +I effect of alkyl groups (pushes electrons toward N)
• Resonance delocalisation (in aniline, lone pair is shared with the ring)
• Solvation of the conjugate acid (more H atoms on N → better H-bonding with water)

Step 1 — Identify the type of each amine

Step 2 — Compare aliphatic amines with ammonia

• Alkyl groups are electron-donating (+I effect) → increase electron density on N.
• More alkyl groups → stronger +I effect → higher basicity in gas phase.
• In aqueous solution, solvation matters: The conjugate acid $RNH_3^+$ is stabilised by H-bonding with water. More H atoms on N → better solvation.

Result in water:

Secondary > Primary > Ammonia

So:

$(C_2H_5)_2NH > C_2H_5NH_2 > NH_3$

Step 3 — Compare aromatic amine with ammonia

• In $C_6H_5NH_2$, the lone pair on N is delocalised into the benzene ring (resonance).
• This makes the lone pair less available for protonation.
• Hence, aniline is weaker base than ammonia.

So:

$NH_3 > C_6H_5NH_2$

Step 4 — Combine all comparisons

From strongest to weakest base: …

Common Mistakes: Basicity Order of Amines

Students often lose marks on this exact question. Here are the most frequent errors and how to avoid them.

Mistake 1: Importing compounds that are not in the question

The error: While reciting the memorised ethyl-series order ($2^\circ > 1^\circ > 3^\circ > NH_3$ etc.), students write triethylamine, $(C_2H_5)_3N$, into their final answer — even though this question's list contains only four species: $C_6H_5NH_2$, $C_2H_5NH_2$, $(C_2H_5)_2NH$ and $NH_3$.

Why it's wrong: An ordering answer must contain exactly the species asked about — nothing added, nothing dropped. There is no tertiary amine in this list, so the whole $2^\circ$-vs-$3^\circ$ complication never arises here.

How to avoid: Before writing the final order, tick off each formula against the question's own list. For these four, both the inductive effect and simple solvation reasoning agree:

Mistake 2: Forgetting that aniline is much weaker than ammonia

The error: Students place $C_6H_5NH_2$ somewhere in the middle, thinking the phenyl ring is just a mild electron-withdrawing group.

Why it's wrong: The lone pair on N in aniline is delocalised into the benzene ring via resonance. This drastically reduces electron density on N, making it a very weak base — weaker than ammonia.

Correct placement: $C_6H_5NH_2$ is the weakest among these.

How to avoid: Draw the resonance structures of aniline. See how the lone pair is shared with the ring. Compare $pK_b$ values:

• $NH_3$: $pK_b \approx 4.75$
• $C_6H_5NH_2$: $pK_b \approx 9.38$

That difference of about 4.6 $pK_b$ units corresponds to roughly a 40,000-fold ($\approx 4 \times 10^4$) difference in $K_b$.

Mistake 3: Confusing the order of alkyl amines in water vs. gas phase

The error: Students memorise one order and apply it everywhere.

The correct orders (for the species in this question):