What is the IUPAC nomenclature of isoprene monomer present in natural rubber?

Options

A. 2 - methyl -1, 3 - butadiene
B. 1, 3- Hexadiene
C. 2, 3 - Dimethyl -1, 3 - butadiene
D. 2 - Methyl-1, 3 - pentadiene

Answer

To determine the IUPAC nomenclature of isoprene, the monomer present in natural rubber, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Structure of Isoprene**:
- Isoprene has the molecular formula C5H8 and its structure can be represented as:
\[ \text{CH}_2 = \text{C}(\text{CH}_3) - \text{CH} = \text{CH}_2 \]
- This shows that isoprene has two double bonds and a methyl group attached to the second carbon.

2. **Number the Carbon Atoms**:
- We need to number the carbon atoms in such a way that the double bonds and the substituents get the lowest possible numbers.
- Starting from the left, we number the carbon atoms:
- C1: CH2
- C2: CH (with the methyl group)
- C3: CH
- C4: CH2
- This gives us the numbering: 1, 2, 3, 4.

3. **Determine the Parent Chain**:
- The longest continuous chain of carbon atoms that includes the double bonds is 4 carbons long. Thus, the parent chain is butene.

4. **Identify the Substituents**:
- There is a methyl group (–CH3) attached to the second carbon.

5. **Identify the Type of Double Bonds**:
- Since there are double bonds between C1 and C2, and C3 and C4, it is a 1,3-diene.

6. **Construct the IUPAC Name**:
- Combining all the information:
- The parent chain is butene (4 carbons).
- The methyl group is on the second carbon.
- There are double bonds at positions 1 and 3.
- Therefore, the IUPAC name is **2-methylbut-1,3-diene**.

### Final Answer:
The IUPAC nomenclature of isoprene is **2-methylbut-1,3-diene**.

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