Which one is the correct order of acidity ?

Options

A. `CH_(2)=CH_(2) gt CH_(3)-CH=CH_(2) gt CH_(3)-C-=CH gt CH-=CH`
B. `CH-=CH gt CH_(3)-C-=CH gt CH_(2)=CH_(2) gt CH_(3)CH_(3)`
C. `CH-=CH gt CH_(2)=CH_(2) gt CH_(3)-C-=CH gt CH_(3)CH_(3)`
D. `CH_(3)CH_(3) gt CH_(2)=CH_(2) gt CH_(3)-C-=CH gt CH-=CH`

Answer

To determine the correct order of acidity among the given hydrocarbons, we will analyze the acidity based on the stability of their conjugate bases. The key principle here is that the more stable the conjugate base, the stronger the acid.

### Step-by-Step Solution:

1. **Understanding Acidity**:
- Acidity refers to the ability of a compound to donate a proton (H⁺). The more easily a compound can release H⁺, the stronger the acid it is.

2. **Identify the Compounds**:
- We are given a set of hydrocarbons, including ethyne (C₂H₂), ethene (C₂H₄), and ethane (C₂H₆).

3. **Analyze the Hybridization**:
- **Ethyne (C₂H₂)**: The carbon atoms in ethyne are sp hybridized, having 50% s character.
- **Ethene (C₂H₄)**: The carbon atoms in ethene are sp² hybridized, having 33.3% s character.
- **Ethane (C₂H₆)**: The carbon atoms in ethane are sp³ hybridized, having 25% s character.

4. **Stability of Conjugate Bases**:
- When a hydrogen ion is removed, the resulting conjugate base's stability is crucial in determining acidity.
- The more electronegative the atom holding the negative charge, the more stable the conjugate base.
- Since sp hybridized carbons (in ethyne) have the highest s character, they can stabilize the negative charge better than sp² (in ethene) and sp³ (in ethane).

5. **Comparing Acidity**:
- Ethyne (C₂H₂) will be the most acidic because its conjugate base is the most stable due to the high s character.
- Ethene (C₂H₄) will be less acidic than ethyne but more acidic than ethane.
- Ethane (C₂H₆) will be the least acidic since it has the least s character.

6. **Considering Substituents**:
- If we introduce a methyl group (CH₃) to ethyne, forming CH₃C≡CH, the +I (inductive) effect of the methyl group will decrease the stability of the conjugate base compared to ethyne alone. Thus, CH₃C≡CH will be less acidic than ethyne.

7. **Final Order of Acidity**:
- Based on the analysis, the correct order of acidity is:
- Ethyne (C₂H₂) > Methylacetylene (CH₃C≡CH) > Ethene (C₂H₄) > Ethane (C₂H₆).

### Conclusion:
The correct order of acidity is:
**Ethyne (C₂H₂) > Methylacetylene (CH₃C≡CH) > Ethene (C₂H₄) > Ethane (C₂H₆)**.