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Difficulty: HardFunctional Groups, Homologous Series, and IUPAC Nomenclature
An organic compound has the following condensed structural formula:
CH3CH(CH3)CH(C2H5)CH2C≡CH\text{CH}_3\text{CH}(\text{CH}_3)\text{CH}(\text{C}_2\text{H}_5)\text{CH}_2\text{C}\equiv\text{CH}

What is the correct IUPAC name for this compound?

  1. 4-ethyl-5-methylhex-1-yneAnswer
  2. B
    4-isopropylhex-1-yne
  3. C
    3-ethyl-2-methylhex-5-yne
  4. D
    3-isopropylhex-5-yne

Answer

4-ethyl-5-methylhex-1-yne
The compound is numbered starting from the terminal alkyne carbon to give the triple bond the lowest locant (C1C1). At C4C4, there are two possible 6-carbon chains. Following IUPAC tie-breaking guidelines, the continuous chain that yields the greater number of substituents (ethyl at C4C4 and methyl at C5C5) is chosen as the parent chain, resulting in 4-ethyl-5-methylhex-1-yne.

Step-by-Step Solution

1
Identify the principal functional group and assign lowest locant numbering
The alkyne triple bond (−C≡C−-C\equiv C-) is at position 1, so numbering starts from the rightmost carbon: C1 is HC≡HC\equiv, C2 is −C−-C-, C3 is −CH2−-CH_2-, C4 is −CH(C2H5)−-CH(C_2H_5)-.
IUPAC rules state that principal functional groups receive the lowest possible locants.
2
Determine the longest continuous carbon chain containing the triple bond
From C4, continuing straight through −CH(CH3)CH3-CH(CH_3)CH_3 gives 6 carbons (hex-1-yne). Going down the ethyl group −CH2CH3-CH_2CH_3 also gives 6 carbons (hex-1-yne).
Both potential paths yield a parent chain length of 6 carbons.
3
Apply the tie-breaking rule for chains of equal length
The chain straight through −CH(CH3)CH3-CH(CH_3)CH_3 has 2 substituents (ethyl at C4, methyl at C5). The path through the ethyl group has only 1 substituent (isopropyl at C4). Thus, the chain with 2 substituents is selected.
When two chains of equal length compete for parent status, IUPAC rules dictate selecting the chain with the maximum number of substituents.
4
Assemble the IUPAC name in alphabetical order
Alphabetize 'ethyl' before 'methyl' to give 4-ethyl-5-methylhex-1-yne.
Substituents are listed alphabetically regardless of their locant numbers.

Key Concept

IUPAC Nomenclature of Alkynes with Branched Chains and Tie-Breaking Rules
Estimated Time:2m 0s
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Topics

Functional Groups, Homologous Series, and IUPAC Nomenclature

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