The compound having only primary hydrogen atom is :-

Options

A. Isobutene
B. 2,3-Dimethyl-1-butene
C. Cyclohexane
D. Pentene

Answer

To determine the compound that has only primary hydrogen atoms, we need to understand what primary hydrogen atoms are. Primary hydrogen atoms are those attached to primary carbon atoms. A primary carbon atom is one that is bonded to only one other carbon atom and has three hydrogen atoms attached to it.

Let's analyze the options step by step:

### Step 1: Understand Primary Carbon and Primary Hydrogen
- A primary carbon is attached to only one other carbon atom and has three hydrogen atoms.
- Therefore, a compound with only primary hydrogen atoms must consist entirely of primary carbon atoms.

### Step 2: Analyze the Given Compounds
1. **Isobutene (CH3-CH=CH-CH3)**:
- Structure: CH3-CH=CH-CH3
- The first and last carbon (CH3) are primary (attached to one other carbon and three hydrogens).
- The middle carbon (CH) is secondary (attached to two carbons).
- Conclusion: Not all hydrogens are primary.

2. **2,3-Dimethyl-1-butene**:
- Structure: CH3-CH(CH3)-CH=CH2
- The first carbon (CH3) is primary, but the second and third carbons are not primary (attached to more than one carbon).
- Conclusion: Not all hydrogens are primary.

3. **Cyclohexane (C6H12)**:
- Structure: A six-membered ring with each carbon bonded to two other carbons.
- Each carbon is attached to two other carbons and has two hydrogens.
- Conclusion: All hydrogens are secondary.

4. **Pentene (C5H10)**:
- Structure: CH2=CH-CH2-CH2-CH3
- The first carbon (CH2) is primary, but the second carbon is attached to two carbons.
- Conclusion: Not all hydrogens are primary.

### Step 3: Conclusion
After analyzing all options, we find that none of the compounds listed have only primary hydrogen atoms. However, if we were to consider a compound that is entirely made up of primary carbons, it would be a straight-chain alkane like butane (C4H10) or similar.

### Final Answer
The compound having only primary hydrogen atoms is **butane (C4H10)**, but it was not listed in the options provided.