Which of the following compounds will most readily be dehydrated to give alkene under acidic condition ?

Options

A. 4-Hydroxypentan-2-one
B. 3-Hydroxypentan-2-one
C. 1-Pentanol
D. 2-Hydroxycyclopentanone

Answer

To determine which compound will most readily be dehydrated to give an alkene under acidic conditions, we need to analyze the structures of the given compounds and their ability to form stable alkenes after dehydration. Here’s a step-by-step solution:

### Step 1: Understand Dehydration
Dehydration is the process of losing a water molecule (H2O) from a compound. In organic chemistry, this often occurs under acidic conditions, where a hydroxyl group (-OH) is protonated to form a better leaving group (water).

### Step 2: Analyze Each Compound
1. **4-hydroxy-pentan-2-one**:
- Structure: CH3-CO-CH(OH)-CH2-CH3
- Dehydration leads to the formation of an alkene (pent-3-en-2-one). The double bond formed here is more substituted, which is generally more stable.

2. **3-hydroxy-pentan-2-one**:
- Structure: CH3-CH(OH)-CO-CH2-CH3
- Dehydration also leads to pent-3-en-2-one. Similar to the previous compound, it forms a double bond that is also relatively stable.

3. **1-pentanol**:
- Structure: CH3-CH2-CH2-CH2-CH2OH
- Dehydration leads to pent-1-ene. However, this results in a less substituted alkene, which is less stable compared to the others.

4. **2-hydroxy-cyclopentanone**:
- Structure: A cyclic compound with a hydroxyl group.
- Dehydration leads to cyclopent-2-en-1-one. However, the formation of a double bond in a 5-membered ring introduces strain, making it less favorable.

### Step 3: Compare Stability of Alkenes
- Alkenes that are more substituted (more alkyl groups attached to the double bond) are generally more stable due to hyperconjugation and the inductive effect.
- The alkenes formed from 4-hydroxy-pentan-2-one and 3-hydroxy-pentan-2-one are more substituted compared to the alkene from 1-pentanol and the strained alkene from 2-hydroxy-cyclopentanone.

### Step 4: Determine the Most Readily Dehydrated Compound
- Between 4-hydroxy-pentan-2-one and 3-hydroxy-pentan-2-one, the dehydration of 4-hydroxy-pentan-2-one is favored because the resulting double bond is more stable due to the electron-withdrawing effect of the adjacent carbonyl group, making the adjacent hydrogen more electrophilic and easier to remove.

### Conclusion
The compound that will most readily be dehydrated to give an alkene under acidic conditions is **4-hydroxy-pentan-2-one**.

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