Among the following pairs identify the one which gives higher heat of hydrogenation :
`CH_(3) - CH = CH - CH_(3)" and "CH_(3) - CH_(2) - CH = CH_(2)`

Answer

To determine which of the two alkenes, `CH3-CH=CH-CH3` (butene) or `CH3-CH2-CH=CH2` (1-butene), has a higher heat of hydrogenation, we can follow these steps:

### Step 1: Understand Heat of Hydrogenation
Heat of hydrogenation refers to the amount of heat released when an alkene is hydrogenated (converted to an alkane) in the presence of a catalyst (like nickel or platinum). The more unstable the alkene, the higher the heat of hydrogenation because more energy is released when converting to a more stable alkane.

**Hint:** Remember that heat of hydrogenation is inversely related to the stability of the alkene.

### Step 2: Analyze the Structure of the Alkenes
- **Butene (CH3-CH=CH-CH3)**: This alkene is a disubstituted alkene because the double bond is between two carbon atoms that each have two alkyl groups attached (methyl groups).
- **1-Butene (CH3-CH2-CH=CH2)**: This alkene is a monosubstituted alkene because the double bond is at the end of the carbon chain, with only one alkyl group (methyl group) attached to one of the carbons in the double bond.

**Hint:** Identify the degree of substitution of each alkene. More substituted alkenes are generally more stable.

### Step 3: Compare Stability
- Disubstituted alkenes (like butene) are more stable than monosubstituted alkenes (like 1-butene).
- Therefore, butene is more stable than 1-butene.

**Hint:** Recall that more substituted alkenes have lower energy and are more stable.

### Step 4: Relate Stability to Heat of Hydrogenation
Since heat of hydrogenation is inversely proportional to the stability of the alkene:
- The more stable the alkene (butene), the lower the heat of hydrogenation.
- The less stable the alkene (1-butene), the higher the heat of hydrogenation.

**Hint:** Use the stability of the alkenes to determine which one has a higher heat of hydrogenation.

### Step 5: Conclusion
From the analysis, we conclude that `CH3-CH2-CH=CH2` (1-butene) will have a higher heat of hydrogenation compared to `CH3-CH=CH-CH3` (butene) because it is less stable.

**Final Answer:** `CH3-CH2-CH=CH2` (1-butene) gives a higher heat of hydrogenation.