Q.Amongst the following, the strongest base in aqueous medium is ____.

The key idea is that basicity in aqueous medium depends on both the inductive effect and solvation of the conjugate acid. For aliphatic amines, the order is usually 2 degree > 1 degree > 3 degree due to a balance of +I effect and hydration.

Reasoning:

1. Electron-releasing groups (alkyl groups) increase electron density on nitrogen, making the amine more basic. The cyano group (-CN) in (ii) is strongly electron-withdrawing, so (ii) is the weakest.
2. Aromatic amines like (iv) are much weaker because the lone pair is delocalised into the benzene ring.
3. Among (i) and (iii), both are aliphatic. (CH3)2NH (a secondary amine) has two +I methyl groups, and its conjugate acid is still well solvated. In water, secondary amines are generally more basic than primary amines.

The question asks for the strongest base in aqueous medium. In water, basicity is not just about gas-phase proton affinity - it is about how well the conjugate acid is stabilised by both the inductive effect and solvation (hydration).

1. What makes an amine basic in water? Two factors matter: the inductive effect (electron-donating alkyl groups raise electron density on N; electron-withdrawing groups like -CN or a phenyl ring lower it) and solvation (more N-H bonds on the conjugate acid mean better hydrogen-bonding with water; bulky groups hinder solvation).

2. Examine each option:

   • (i) CH3NH2 (methylamine): one methyl group donates electron density; its conjugate acid has three N-H bonds, so solvation is good.
   • (ii) NCCH2NH2 (aminoacetonitrile): the -CN group is strongly electron-withdrawing by induction - the weakest base of the four.
   • (iii) (CH3)2NH (dimethylamine): two methyl groups donate more electron density than one, and the conjugate acid still has two N-H bonds for solvation - the strongest among simple alkylamines in water.
   • (iv) C6H5NHCH3 (N-methylaniline): the phenyl ring withdraws by resonance (the N lone pair delocalises into the ring), and the bulkier conjugate acid solvates poorly.

3. The classic trend for alkylamines in water is Secondary > Primary > Tertiary (for small alkyl groups): secondary amines balance inductive donation against still-adequate solvation, while tertiary amines' conjugate acids have only one N-H bond and solvate poorly.

4. Compare directly: option (ii) is clearly the weakest (strong electron withdrawal); option (iv) is also weak (resonance withdrawal by phenyl); between (i) and (iii), (iii) has two methyl groups instead of one, so the inductive effect favours it, and its conjugate acid still solvates well.

Concept: Basicity of Amines in Aqueous Medium

The strength of a base in water depends on the availability of the lone pair on nitrogen for protonation. Electron-donating groups (like alkyl groups) increase basicity; electron-withdrawing groups (like cyano, phenyl) decrease it.

Method: Inductive Effect + Solvation Effect Analysis

Step 1 – Identify the functional groups

• (A) $CH_3NH_2$ — primary aliphatic amine (methyl group)
• (B) $NCCH_2NH_2$ — primary amine with a cyano ($-CN$) group on the carbon next to nitrogen
• (C) $(CH_3)_2NH$ — secondary aliphatic amine (two methyl groups)
• (D) $C_6H_5NHCH_3$ — secondary amine but with a phenyl ring attached to nitrogen

Step 2 – Apply inductive effect

• Alkyl groups ($-CH_3$) are electron-donating → increase electron density on N → stronger base.
• Cyano group ($-CN$) is strongly electron-withdrawing → reduces electron density on N → weaker base.
• Phenyl ring ($C_6H_5-$) is electron-withdrawing via resonance (lone pair delocalises into ring) → weaker base.

Step 3 – Compare aliphatic amines

In water, secondary amines are generally stronger bases than primary amines of similar size because:

• Two alkyl groups donate more electron density.
• Solvation of the conjugate acid is still favourable.

So: $(CH_3)_2NH$ > $CH_3NH_2$ in basic strength.

Step 4 – Eliminate weaker ones

• (B) $NCCH_2NH_2$ — very weak due to $-CN$ group.
• (D) $C_6H_5NHCH_3$ — weak due to resonance delocalisation of lone pair into the ring.

Step 5 – Final comparison

Between (A) and (C):

• $(CH_3)_2NH$ (secondary) > $CH_3NH_2$ (primary)

Final Answer

The strongest base is:

Let’s break this down step-by-step. The question tests basicity of amines in aqueous medium — a classic JEE/NEET concept.

🔍 The Core Concept

Basicity in water depends on:

• Electron density on nitrogen (more electron density → stronger base)
• Resonance effects (delocalisation of lone pair reduces basicity)
• Inductive effects (electron-donating groups increase basicity; electron-withdrawing groups decrease it)
• Solvation of the conjugate acid (more H-bonding with water → more stable cation → stronger base)

✗ Common Mistake #1: Ignoring the effect of substituents on the nitrogen

Mistake: Students think all alkyl groups increase basicity equally, so they pick $(CH_3)_2NH$ without checking the other options carefully.

Why it’s wrong:

• $NCCH_2NH_2$ has a cyano group ($-CN$) which is strongly electron-withdrawing (by both inductive and resonance effects). This drastically reduces electron density on nitrogen.
• $C_6H_5NHCH_3$ has a phenyl ring that delocalises the lone pair of nitrogen into the ring (resonance), making it a much weaker base.

How to avoid:

• Always identify electron-donating groups (EDG) vs electron-withdrawing groups (EWG) on the amine.
• Alkyl groups ($-CH_3$, $-C_2H_5$) are EDG; $-CN$, $-NO_2$, $-COOH$, $-C_6H_5$ are EWG.

✗ Common Mistake #2: Forgetting the order of basicity for aliphatic amines in water

Mistake: Students memorise “$3^\circ > 2^\circ > 1^\circ > NH_3$” and pick $(CH_3)_3N$ if it were present, but here they pick $(CH_3)_2NH$ as strongest without checking.

Why it’s wrong:

In aqueous medium, the actual order for simple alkyl amines is:

This is because:

• $2^\circ$ amines have two alkyl groups (good inductive effect) and two hydrogens for solvation.
• $3^\circ$ amines have three alkyl groups (strong inductive effect) but no H on N → poor solvation of the conjugate acid.

So $(CH_3)_2NH$ (a $2^\circ$ amine) is indeed stronger than $CH_3NH_2$ (a $1^\circ$ amine) in water.

How to avoid:

• Remember: In water, solvation of the ammonium ion matters. More H atoms on N → better solvation → stronger base.
• The order is: $2^\circ > 1^\circ > 3^\circ > NH_3$ for simple alkyl amines.

✗ Common Mistake #3: Confusing gas-phase basicity with aqueous basicity

Mistake: Students apply gas-phase basicity order ($3^\circ > 2^\circ > 1^\circ > NH_3$) directly to aqueous medium.

Why it’s wrong:

• In gas phase, only inductive effect matters — so $3^\circ$ is strongest.
• In water, solvation reverses the order for $3^\circ$ vs $2^\circ$.

How to avoid:

• Always check the medium: “in aqueous medium” means solvation counts.
• If the question says “in gas phase” or “in absence of solvent”, then $3^\circ > 2^\circ > 1^\circ$.

✓ Correct Answer & Reasoning

Answer: $(CH_3)_2NH$ (Option C)

Why:

• $(CH_3)_2NH$ is a $2^\circ$ aliphatic amine — two electron-donating methyl groups + two H atoms for solvation.
• $CH_3NH_2$ is $1^\circ$ — only one methyl group → weaker inductive effect.
• $NCCH_2NH_2$ has a strong EWG ($-CN$) → very weak base.
• $C_6H_5NHCH_3$ has resonance delocalisation of lone pair into the ring → weak base.

Final tip: For such questions, first eliminate any amine with an EWG or aromatic ring. Then compare the remaining aliphatic amines using the $2^\circ > 1^\circ > 3^\circ$ rule in water.