Arrange benzene, n-hexane and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour.

Explanation

To arrange benzene, n-hexane, and ethyne in decreasing order of acidic behavior, we need to analyze the acidic properties of each compound based on their hybridization and s-character.

### Step-by-Step Solution:

1. **Identify the Hybridization**:
- **Benzene**: The carbon atoms in benzene are sp² hybridized.
- **n-Hexane**: The carbon atoms in n-hexane are sp³ hybridized.
- **Ethyne**: The carbon atoms in ethyne are sp hybridized.

2. **Determine the s-Character**:
- **Benzene (sp²)**: Has 33% s-character (2 out of 6 orbitals).
- **n-Hexane (sp³)**: Has 25% s-character (1 out of 4 orbitals).
- **Ethyne (sp)**: Has 50% s-character (1 out of 2 orbitals).

3. **Relate s-Character to Acidity**:
- The greater the s-character, the stronger the acidity. This is because s orbitals are closer to the nucleus, which allows for a stronger attraction of electrons towards the nucleus. This results in a greater partial negative charge on the carbon atom, making it easier for the hydrogen atom to be released as a proton (H⁺).

4. **Order the Compounds**:
- **Ethyne** (most acidic due to highest s-character of 50%)
- **Benzene** (moderately acidic with 33% s-character)
- **n-Hexane** (least acidic with 25% s-character)

### Final Arrangement:
**Ethyne > Benzene > n-Hexane**

### Reasoning:
- Ethyne is the most acidic because it has the highest s-character (50%), allowing for a stronger attraction of the electrons to the nucleus, resulting in a greater tendency to donate a proton.
- Benzene follows with a moderate acidity due to its sp² hybridization and 33% s-character.
- n-Hexane is the least acidic because it has the lowest s-character (25%), making it less likely to donate a proton.