Which of the following alkynes is most acidic?

Options

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

Answer

To determine which of the given alkynes is the most acidic, we need to analyze the stability of the conjugate bases formed when each alkyne donates a proton (H⁺). The acidity of an alkyne is related to the stability of the resulting anion after deprotonation.

### Step-by-Step Solution:

1. **Identify the Alkynes**: Let's assume we have the following alkynes to consider:
- Alkyne A: CH≡C-H (ethyne)
- Alkyne B: CH₃C≡C-H (1-butyne)
- Alkyne C: CH₃CH₂C≡C-H (2-butyne)
- Alkyne D: CH₃CH₂CH₂C≡C-H (1-pentyne)

2. **Deprotonation**: When each alkyne donates a proton (H⁺), it forms a corresponding anion:
- Alkyne A → CH≡C⁻
- Alkyne B → CH₃C≡C⁻
- Alkyne C → CH₃CH₂C≡C⁻
- Alkyne D → CH₃CH₂CH₂C≡C⁻

3. **Analyze the Stability of Conjugate Bases**: The stability of the conjugate base (the anion formed) is crucial in determining the acidity of the parent alkyne. The more stable the anion, the more acidic the alkyne.
- The anion CH≡C⁻ (from ethyne) is sp-hybridized and has a negative charge on the carbon that is more electronegative, making it relatively stable.
- The anion CH₃C≡C⁻ (from 1-butyne) is also sp-hybridized, but the presence of the methyl group (CH₃) provides a +I (inductive) effect, which destabilizes the negative charge slightly.
- The anion CH₃CH₂C≡C⁻ (from 2-butyne) has even more alkyl groups, which further destabilize the negative charge due to the +I effect.
- The anion CH₃CH₂CH₂C≡C⁻ (from 1-pentyne) is similarly destabilized by the presence of more alkyl groups.

4. **Conclusion**: The anion CH≡C⁻ (from ethyne) is the most stable due to the absence of any alkyl groups that would destabilize the negative charge. Therefore, ethyne is the most acidic among the given alkynes.

### Final Answer:
The most acidic alkyne is **ethyne (CH≡C-H)**.