Question

What is the correct increasing order of basic strength for the following amines when measured inside an aqueous medium?
\[ \text{(I) } \mathrm{CH_3NH_2} \] \[ \text{(II) } \mathrm{(CH_3)_2NH} \] \[ \text{(III) } \mathrm{(CH_3)_3N} \] \[ \text{(IV) } \mathrm{NH_3} \]

• A. IV < III  <  I  <  II
• B. IV & Lt; I & Lt; II & Lt; III
• C. III & Lt; I & Lt; II & Lt; IV
• D. I & Lt; II & Lt; III & Lt; IV

Hint:

Remember these structural trends for aqueous amines to save time: For Methyl groups, use sequence 213 (\(2^\circ > 1^\circ > 3^\circ\)). For Ethyl groups, use sequence 231 (\(2^\circ > 3^\circ > 1^\circ\)).

Answer 1

The Correct Option is A

Step 1 : Understanding the Question:
The topic of this question is the Basic Strength of Amines. In the gaseous phase, basicity simply follows the $+I$ (inductive) effect of alkyl groups. However, in an aqueous medium, the trend becomes much more complex due to the interplay of three different factors: the inductive effect, steric hindrance, and solvation (hydration) effects. The question specifically asks for the increasing order of basicity for ammonia and methyl-substituted amines in water.
Step 2 : Key Formulas and approach:
To solve this, we must balance three effects:
1. Inductive Effect ($+I$): Increases basicity as more alkyl groups donate electron density to the Nitrogen ($3^\circ>2^\circ>1^\circ$).
2. Solvation Effect: Water stabilizes the conjugate acid ($RNH_3^+$) via hydrogen bonding. Smaller ions with more hydrogens are better solvated ($1^\circ>2^\circ>3^\circ$).
3. Steric Effect: Bulky alkyl groups block the approach of water molecules or protons ($1^\circ>2^\circ>3^\circ$).
The combination of these factors results in a specific order for different alkyl groups.
Step 3 : Detailed Explanation:

Let's evaluate the methyl-substituted amines: Ammonia ($IV$), Methylamine ($I$ - $1^\circ$), Dimethylamine ($II$ - $2^\circ$), and Trimethylamine ($III$ - $3^\circ$).

Ammonia (IV) is always the weakest base because it lacks any electron-donating alkyl groups to stabilize the positive charge on Nitrogen.

In the case of methyl groups, the small size of the methyl group allows for a unique balance. Secondary amine (II) is the strongest because it has two donating groups and is still well-solvated.

For the primary (I) and tertiary (III) amines, the competition between solvation and inductive effects is intense. In the case of methyl groups, the solvation of the primary amine's cation is so significant that it outweighs the $+I$ effect of the tertiary amine.

Therefore, Trimethylamine ($3^\circ$) is actually less basic than Methylamine ($1^\circ$) in water.

Combining these results, the overall order of increasing basicity is: $\text{Ammonia (IV)}<\text{Trimethylamine (III)}<\text{Methylamine (I)}<\text{Dimethylamine (II)}$.

This corresponds to the sequence $IV<III<I<II$, which matches option (A).

Step 4 : Final Answer:
In an aqueous medium, methyl amines follow the 2-1-3 order of basicity, with ammonia being the weakest. The correct option is (A).