Propyne is less acidic than acetylene.

Answer

To understand why propyne is less acidic than acetylene, we can follow these steps:

### Step 1: Identify the Structures
- **Acetylene (C₂H₂)** has the structure H-C≡C-H.
- **Propyne (C₃H₄)** has the structure CH₃-C≡C-H.

### Step 2: Understand Acidity
- Acidity in hydrocarbons is determined by the ability of the compound to donate a proton (H⁺).
- The more stable the conjugate base formed after the donation of H⁺, the stronger the acid.

### Step 3: Form the Conjugate Bases
- When acetylene donates a proton, it forms the acetylide ion (C₂H⁻):
- H-C≡C-H → H⁺ + C≡C⁻
- When propyne donates a proton, it forms the propyne anion (C₃H⁻):
- CH₃-C≡C-H → H⁺ + CH₃-C≡C⁻

### Step 4: Analyze the Stability of Conjugate Bases
- The conjugate base of acetylene (C≡C⁻) has no alkyl groups attached, making it less electron-dense and more stable.
- The conjugate base of propyne (CH₃-C≡C⁻) has a methyl group (CH₃) attached, which exerts a +I (inductive) effect, increasing electron density around the negative charge.

### Step 5: Compare the Effects of Substituents
- The +I effect from the methyl group in propyne destabilizes the conjugate base because it increases the electron density on the negatively charged carbon.
- In contrast, the acetylide ion does not have such an electron-donating group, leading to a more stable conjugate base.

### Step 6: Conclusion
- Since the conjugate base of propyne is less stable than that of acetylene, propyne is less acidic than acetylene.

### Summary
- Acetylene is more acidic than propyne because its conjugate base is more stable due to the absence of electron-donating groups, while propyne's conjugate base is destabilized by the +I effect of the methyl group.

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