When of the following can work as a dehydrating agent for alcohols

Options

A. `H_2SO_4`
B. `Al_2O_3`
C. `H_3PO_4`
D. All of these

Answer

To determine which of the following can work as a dehydrating agent for alcohols, we need to analyze the given options: H2SO4 (sulfuric acid), Al2O3 (aluminum oxide), and H3PO4 (phosphoric acid).

### Step-by-Step Solution:

1. **Understanding Dehydrating Agents**:
- Dehydrating agents are substances that remove water (H2O) from other compounds. In the case of alcohols, they facilitate the elimination of water to form alkenes.

2. **Analyzing H2SO4 (Sulfuric Acid)**:
- Concentrated sulfuric acid is a well-known dehydrating agent. It can dehydrate alcohols by removing a water molecule, leading to the formation of alkenes.
- Therefore, H2SO4 can work as a dehydrating agent for alcohols.

3. **Analyzing Al2O3 (Aluminum Oxide)**:
- Aluminum oxide can also act as a dehydrating agent when alcohols are passed over it at high temperatures. It helps in the elimination of water from alcohols.
- Thus, Al2O3 can also work as a dehydrating agent for alcohols.

4. **Analyzing H3PO4 (Phosphoric Acid)**:
- Phosphoric acid, especially in its concentrated form, can also serve as a dehydrating agent for alcohols. It facilitates the removal of water during the dehydration process.
- Therefore, H3PO4 can work as a dehydrating agent for alcohols.

5. **Conclusion**:
- Since all three substances (H2SO4, Al2O3, and H3PO4) can function as dehydrating agents for alcohols, the correct answer is "all of these".

### Final Answer:
- The correct option is **D) all of these**.