How many structural isomeric alkenes on hydrogenation give `-n`-Pentane.

Answer

To determine how many structural isomeric alkenes can yield n-pentane upon hydrogenation, we can follow these steps:

### Step 1: Write the molecular formula of n-pentane
The molecular formula for n-pentane is C5H12.

### Step 2: Identify the structural isomers of alkenes that can yield n-pentane
We need to find alkenes (C5H10) that can be hydrogenated to form n-pentane (C5H12).

### Step 3: Draw the possible structural isomers of alkenes
1. **1-Pentene**: CH2=CH-CH2-CH2-CH3
2. **2-Pentene**: There are two geometric forms (cis and trans), but they are not structural isomers.
- **Cis-2-Pentene**: CH3-CH=CH-CH2-CH3
- **Trans-2-Pentene**: CH3-CH=CH-CH2-CH3
3. **3-Pentene**: This isomer is the same as 2-Pentene due to symmetry.
- **3-Pentene**: CH3-CH2-CH=CH-CH3
4. **Cyclopentane**: This is not an alkene, so we won't consider it.
5. **2-Methyl-1-butene**: CH2=C(CH3)-CH2-CH3
6. **3-Methyl-1-butene**: CH2=CH-CH(CH3)-CH2
7. **2-Methyl-2-butene**: This is a branched alkene but does not yield n-pentane.

### Step 4: Count the unique structural isomers
From the above analysis, we can conclude:
- 1-Pentene
- 2-Pentene (considering only one structural form)
- 3-Pentene (considering only one structural form)
- 2-Methyl-1-butene
- 3-Methyl-1-butene

### Conclusion
Thus, the total number of structural isomeric alkenes that can yield n-pentane upon hydrogenation is **5**.