Pick the compound having only primary hydrogen

Options

A. Cyclohexene
B. Propyne
C. But-2-ene
D. Propene

Answer

To solve the question of identifying the compound that has only primary hydrogens, we need to understand the definitions of primary hydrogen and primary carbon.

### Step-by-Step Solution:

1. **Understand Primary Hydrogen and Primary Carbon**:
- Primary hydrogen atoms are those that are attached to primary carbon atoms.
- A primary carbon is defined as a carbon atom that is attached to only one other carbon atom.

2. **Analyze Each Compound**:
- We will evaluate the given compounds one by one to determine if they contain only primary hydrogens.

3. **Evaluate Cyclohexane**:
- Cyclohexane has the formula C6H12 and is a cyclic compound.
- In cyclohexane, each carbon is bonded to two other carbons.
- Since all carbons are secondary (attached to two other carbons), there are no primary carbons and hence no primary hydrogens.

4. **Evaluate Prop-1-yne**:
- Prop-1-yne has the formula C3H4 and contains a triple bond.
- The structure can be represented as H-C≡C-CH3.
- The terminal carbon (attached to one carbon) is a primary carbon, and it has three primary hydrogens.
- The other carbon in the triple bond is also a primary carbon, contributing one primary hydrogen.
- Thus, prop-1-yne contains only primary hydrogens.

5. **Evaluate But-2-ene**:
- But-2-ene has the formula C4H8 and contains a double bond.
- The structure can be represented as CH3-CH=CH-CH3.
- In this case, the first and last carbons are primary, but the middle carbons are secondary (attached to two other carbons).
- Therefore, but-2-ene does not have only primary hydrogens.

6. **Evaluate Propene**:
- Propene has the formula C3H6 and contains a double bond.
- The structure can be represented as CH2=CH-CH3.
- Here, the first carbon is primary, but the second carbon is secondary (attached to two other carbons).
- Thus, propene also does not have only primary hydrogens.

7. **Conclusion**:
- After evaluating all the compounds, we find that only **Prop-1-yne** contains only primary hydrogens, as all hydrogens are attached to primary carbons.

### Final Answer:
The compound having only primary hydrogen is **Prop-1-yne**.