The correct sequence regarding base strength of aliphatic amines
in aqueous solution is:

Options

A. `R_3 N gt R_2 NH gt RNH_2 gt NH_3`
B. `R_2 NH gt RNH_2 gt R_3 N gt NH_3`
C. `R_2 NH gt R_2 N gt RNH_3 gt NH_3`
D. `RNH_2 gt R_2 NH gt R_3 N gt NH_3`

Explanation

To determine the correct sequence regarding the base strength of aliphatic amines in aqueous solution, we need to analyze the factors affecting their basicity. Here's a step-by-step solution:

### Step 1: Understand the basicity of amines
Amines are classified based on the number of alkyl groups attached to the nitrogen atom:
- Primary amines (1°) have one alkyl group (RNH2).
- Secondary amines (2°) have two alkyl groups (R2NH).
- Tertiary amines (3°) have three alkyl groups (R3N).
- Ammonia (NH3) is also included for comparison.

### Step 2: Factors affecting basicity in aqueous solution
The basicity of amines in aqueous solution is influenced by:
1. **Inductive Effect (+I effect)**: Alkyl groups donate electron density to the nitrogen, enhancing its ability to donate a lone pair of electrons.
2. **Hydrogen Bonding**: The ability of amines to form hydrogen bonds with water molecules affects their stability and, consequently, their basicity.
3. **Steric Hindrance**: The presence of bulky alkyl groups can hinder the approach of water molecules, affecting hydrogen bonding.

### Step 3: Analyze the stability of ammonium cations
When amines accept a proton (H+), they form ammonium cations (RNH3+). The stability of these cations is crucial in determining the basicity of the corresponding amines:
- **Primary amines**: Form ammonium cations with two hydrogen bonds (more stable).
- **Secondary amines**: Form ammonium cations with one hydrogen bond (less stable than primary).
- **Tertiary amines**: Form ammonium cations with no hydrogen bonds (least stable due to steric hindrance).

### Step 4: Establish the order of basicity
Considering the above factors:
- **2° amines**: Strongest base due to the balance of inductive effect and minimal steric hindrance.
- **1° amines**: Strong base, but less than 2° due to more hydrogen bonding stability.
- **3° amines**: Weaker base due to steric hindrance and lack of hydrogen bonding.
- **Ammonia (NH3)**: Weakest base among them.

### Conclusion
The correct sequence regarding the base strength of aliphatic amines in aqueous solution is:
**2° amines > 1° amines > 3° amines > NH3**