Question

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

Hint:

IUPAC rules prioritize the triple bond over alkyl substituents when numbering. Always start from the end that gives the triple bond the smallest number.

Answer 1

The Correct Option is B

Concept: Common names like "acetylene" derivatives name the substituents attached to the $C \equiv C$ triple bond. IUPAC nomenclature requires finding the longest chain containing the triple bond and numbering to give the triple bond the lowest possible locant.

Step 1: {Translate the common name into a structure.} "Acetylene" refers to the triple bond: $C \equiv C$. "Methyl" and "isopropyl" substituents are attached to each carbon of the triple bond: $\text{CH}_3 - C \equiv C - \text{CH}(\text{CH}_3)_2$.

Step 2: {Find the longest carbon chain.} The longest continuous chain containing the triple bond has 5 carbons (pent-). Structure: $\text{CH}_3(1) - \text{C}(2) \equiv \text{C}(3) - \text{CH}(4)(\text{CH}_3) - \text{CH}_3(5)$.

Step 3: {Number the chain and identify substituents.} Numbering from the left gives the triple bond the position 2. At position 4, there is a methyl group. The IUPAC name is 4-methyl-2-pentyne.