Consider the given compounds :
(a) `CH_(3)-CH_(2)-NH_(2)` (b) `CH_(3)-CH=NH`
(c ) `CH_(3)-C=N` (d) `C_(2)H_(5)-NH-C_(2)H_(5)`
Arrange basicity of these compounds in decreasing order :

Options

A. `d gt a gt b gt c`
B. `a gt b gt c gt d`
C. `a gt d gt b gt c`
D. `d gt a gt c gt b`

Answer

To determine the basicity of the given compounds in decreasing order, we need to analyze the factors that influence basicity, particularly the availability of the lone pair of electrons on the nitrogen atom, the hybridization of nitrogen, and the stability of the conjugate acid formed after protonation.

### Step-by-Step Solution:

1. **Identify the Compounds:**
- (a) `CH3-CH2-NH2` (Ethylamine)
- (b) `CH3-CH=NH` (Vinylamine)
- (c) `CH3-C≡N` (Acetonitrile)
- (d) `C2H5-NH-C2H5` (Diethylamine)

2. **Analyze the Hybridization of Nitrogen:**
- In compound (a) `CH3-CH2-NH2`, nitrogen is **sp³ hybridized**.
- In compound (b) `CH3-CH=NH`, nitrogen is **sp² hybridized**.
- In compound (c) `CH3-C≡N`, nitrogen is **sp hybridized**.
- In compound (d) `C2H5-NH-C2H5`, nitrogen is **sp³ hybridized**.

3. **Lone Pair Availability:**
- **sp³ hybridized nitrogen** (in (a) and (d)) has the highest lone pair availability.
- **sp² hybridized nitrogen** (in (b)) has moderate lone pair availability.
- **sp hybridized nitrogen** (in (c)) has the least lone pair availability due to higher electronegativity.

4. **Consider Electron Releasing Effects:**
- Compounds (a) and (d) both have sp³ hybridized nitrogen. However, compound (d) has two ethyl groups, which provide a stronger +I (inductive) effect compared to the single ethyl group in (a). Thus, compound (d) will be more basic than compound (a).
- Compound (b) has a vinyl group which is less electron-donating compared to the ethyl group in (a).
- Compound (c) has a triple bond (C≡N), which makes it less basic due to the high electronegativity of sp hybridized nitrogen.

5. **Stability of Conjugate Acids:**
- The stability of the conjugate acid also plays a role. The more stable the conjugate acid, the stronger the base. Compounds with sp³ nitrogen will form more stable conjugate acids than those with sp or sp² hybridized nitrogen.

6. **Rank the Compounds:**
- **Most Basic:** (d) `C2H5-NH-C2H5` (Diethylamine) - sp³, strong +I effect.
- **Second:** (a) `CH3-CH2-NH2` (Ethylamine) - sp³, weaker +I effect than (d).
- **Third:** (b) `CH3-CH=NH` (Vinylamine) - sp², moderate basicity.
- **Least Basic:** (c) `CH3-C≡N` (Acetonitrile) - sp, lowest basicity.

### Final Order of Basicity:
1. (d) `C2H5-NH-C2H5` (Diethylamine)
2. (a) `CH3-CH2-NH2` (Ethylamine)
3. (b) `CH3-CH=NH` (Vinylamine)
4. (c) `CH3-C≡N` (Acetonitrile)