In the following compounds, the decreasing order of basic strength will be :

Options

A. `C_(2)H_(5)NH_(2)gtNH_(3)gt(C_(2)H_(5))_(2)NH`
B. `(C_(2)H_(5))_(2)NHgtNH_(3)gtC_(2)H_(5)NH_(2)`
C. `(C_(2)H_(5))_(2)NHgtC_(2)H_(5)NH_(2)gtNH_(3)`
D. `NH_(3)gtC_(2)H_(5)NH_(2)gt(C_(2)H_(5))_(2)NH`

Answer

To determine the decreasing order of basic strength among the given compounds (a primary amine, a secondary amine, and ammonia), we will analyze the basicity of these compounds in both the gas phase and aqueous solution.

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Primary amine: C2H5NH2 (ethylamine)
- Secondary amine: C2H5NH (diethylamine)
- Ammonia: NH3

2. **Understand Basicity**:
Basicity refers to the ability of a compound to accept protons (H+ ions). The more readily a compound can donate its lone pair of electrons to bond with a proton, the stronger the base it is.

3. **Analyze Basicity in Gas Phase**:
- In the gas phase, the basic strength is influenced primarily by the +I (inductive) effect of alkyl groups. Alkyl groups are electron-donating, which increases the electron density on the nitrogen atom, making it more basic.
- **Order in Gas Phase**:
- Tertiary amines > Secondary amines > Primary amines > Ammonia
- For our specific case:
- Diethylamine (C2H5)2NH has the most +I effect due to two ethyl groups.
- Ethylamine (C2H5)NH2 has one ethyl group.
- Ammonia (NH3) has no alkyl groups.

4. **Analyze Basicity in Aqueous Solution**:
- In aqueous solution, other factors come into play, such as steric hindrance and hydration energy.
- Steric hindrance can inhibit the approach of protons to the nitrogen atom, thus affecting basicity.
- Hydration energy is the energy released when a solute is dissolved in water; more hydration generally means a stronger base.
- **Order in Aqueous Solution**:
- Primary amines are more basic than secondary amines due to less steric hindrance and greater hydration.
- Ammonia is less basic than both primary and secondary amines.

5. **Final Order of Basic Strength**:
- Considering both phases, the order of basic strength from strongest to weakest is:
- Diethylamine (C2H5)2NH > Ethylamine (C2H5)NH2 > Ammonia (NH3)

### Conclusion:
The decreasing order of basic strength is:
**(C2H5)2NH > C2H5NH2 > NH3**