Arrange basicity of the given compounds in decreasing order -
`(a)CH_(3) - CH_(2) - NH_(2)" " (b)CH_(2) = CH - NH_(2)`
`(c) CH -= C - NH_(2)`

Options

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

Answer

To arrange the basicity of the given compounds in decreasing order, we will analyze each compound based on the presence of lone pairs, resonance effects, and hybridization.

### Step-by-Step Solution:

1. **Identify the Compounds:**
- (a) CH₃-CH₂-NH₂ (Ethylamine)
- (b) CH₂=CH-NH₂ (Vinylamine)
- (c) CH≡C-NH₂ (Propyne amine)

2. **Analyze Compound (a) CH₃-CH₂-NH₂:**
- Ethylamine has an amine group (-NH₂) attached to a carbon chain.
- The lone pairs on the nitrogen atom are localized and not involved in resonance.
- This means that the basicity is high since the lone pair can readily accept a proton (H⁺).

3. **Analyze Compound (b) CH₂=CH-NH₂:**
- Vinylamine has a double bond adjacent to the amine group.
- The lone pair on the nitrogen can participate in resonance with the double bond.
- This delocalization reduces the availability of the lone pair to accept a proton, thus decreasing its basicity compared to ethylamine.

4. **Analyze Compound (c) CH≡C-NH₂:**
- Propyne amine has a triple bond adjacent to the amine group.
- Similar to vinylamine, the lone pair on the nitrogen can also participate in resonance with the triple bond.
- The hybridization of the nitrogen's adjacent carbon is sp, which means it has more s character (50%).
- Higher s character in the hybridization of the adjacent carbon leads to decreased basicity because the lone pair is held closer to the nucleus.

5. **Compare Basicity:**
- Since compound (a) has no resonance and the lone pair is localized, it is the most basic.
- Between (b) and (c), (b) has less s character than (c) due to the sp² hybridization of the carbon in vinylamine (33% s character) compared to the sp hybridization in propyne amine (50% s character).
- Therefore, the order of basicity is:
- (a) CH₃-CH₂-NH₂ > (b) CH₂=CH-NH₂ > (c) CH≡C-NH₂

### Final Arrangement:
- **Basicity Order:** (a) > (b) > (c)