The correct IUPAC name for methylisopropylacetylene is
(a) 2-methyl-4-pentyne
(b) 4-methyl-2-pentyne
(c) isopropylmethylacetylene
(d) 3-methyl-4-pentyne
(e) 2-methyl-3-pentyne.

Answer

To determine the correct IUPAC name for methylisopropylacetylene, we can follow these steps:

### Step 1: Identify the Structure
- The compound is named methylisopropylacetylene, which indicates that it contains an isopropyl group (C3H7) and a methyl group (CH3) attached to an acetylene (triple bond) structure.

### Step 2: Draw the Structure
- The structure can be drawn as follows:
- Start with the acetylene backbone, which is a carbon-carbon triple bond.
- Attach the isopropyl group (C3H7) to one end of the acetylene.
- Attach the methyl group (CH3) to the other end of the acetylene.

### Step 3: Determine the Longest Carbon Chain
- The longest continuous carbon chain that includes the triple bond should be identified. In this case, we have a total of 5 carbons in the chain.

### Step 4: Number the Carbon Chain
- Number the carbon chain starting from the end closest to the triple bond to give it the lowest possible number:
- If we number from the end with the triple bond, we get:
- C1 (triple bond), C2 (methyl), C3 (isopropyl), C4, C5.

### Step 5: Identify the Substituents and Their Positions
- The methyl group is at position 4 and the isopropyl group is at position 2.
- Therefore, we have:
- 4-methyl and 2-pentyne (since the longest chain is 5 carbons).

### Step 6: Combine the Information
- The correct IUPAC name is formed by combining the substituents and the main chain:
- The name is 4-methyl-2-pentyne.

### Step 7: Verify with the Options
- Now, we can check the options provided:
- (a) 2-methyl-4-pentyne
- (b) 4-methyl-2-pentyne
- (c) isopropylmethylacetylene
- (d) 3-methyl-4-pentyne
- (e) 2-methyl-3-pentyne
- The correct answer is (b) 4-methyl-2-pentyne.

### Final Answer:
The correct IUPAC name for methylisopropylacetylene is **(b) 4-methyl-2-pentyne**.