Which of the following is not correct decreasing `k_(a)` order .

Options

A. `CH_(4) gt NH_(3) gt H_(2)O gt HF`
B. `CH_(3) -OH gt CH_(3)-NH_(2) gt CH_(3)-Fgt CH_(3)-CH_(3)`
C. `Hl gt HBr gt HCl gt HF`
D. `PhOH gt H_(2)O gt C_(2)H_(5)OH gt CH_(3)-C-=CH`

Answer

To determine which of the given options is not in the correct decreasing order of acidity (represented by the acidity constant \( K_a \)), we need to analyze the acidity of the compounds based on their structures and the electronegativity of the atoms involved.

### Step-by-Step Solution:

1. **Understanding Acidity and \( K_a \)**:
- The acidity of a compound is often measured by its acidity constant \( K_a \). A higher \( K_a \) value indicates a stronger acid, while a lower \( K_a \) value indicates a weaker acid.

2. **Analyzing the First Option**:
- The order given is: **Methane (CH₄) > Ammonia (NH₃) > Water (H₂O) > Hydrofluoric Acid (HF)**.
- Methane is a very weak acid (almost non-acidic), ammonia is weakly acidic, water is more acidic than ammonia, and HF is a weak acid but stronger than water due to the electronegativity of fluorine.
- **Conclusion**: This order is incorrect because HF should be stronger than water, making this option not correct.

3. **Analyzing the Second Option**:
- The order given is: **Methyl Alcohol (CH₃OH) > Methyl Amine (CH₃NH₂) > Methyl Phenate (C₆H₅O⁻) > Ethane (C₂H₆)**.
- Methyl alcohol is more acidic than methyl amine because oxygen is more electronegative than nitrogen. Methyl phenate is more acidic than both because of resonance stabilization of the phenoxide ion. Ethane is non-acidic.
- **Conclusion**: This order is correct.

4. **Analyzing the Third Option**:
- The order given is: **HI > HBr > HCl > HF**.
- This order is based on bond length and size of the halogens. HI has the longest bond and is the strongest acid, followed by HBr, HCl, and HF.
- **Conclusion**: This order is correct.

5. **Analyzing the Fourth Option**:
- The order given is: **Phenol > Water > Ethyl Alcohol > Propylene**.
- Phenol is more acidic than water due to resonance stabilization of the phenoxide ion. Water is more acidic than ethyl alcohol, and propylene is the least acidic.
- **Conclusion**: This order is correct.

### Final Conclusion:
The first option (Methane > Ammonia > Water > HF) is not the correct decreasing order of acidity.