Identify the correct option in which the compound is not named as per IUPAC
(A)
(B)
(C)
(D)
Show Hint
For disubstituted cycloalkanes, a gem-disubstituted carbon (bearing two substituents) should be assigned locant 1 over a mono-substituted carbon to satisfy the lowest set of locants rule.
This is a common trap in IUPAC nomenclature questions.
Step 1: Understanding the Question:
The question asks us to identify the compound that is incorrectly named according to the official IUPAC nomenclature rules.
Step 2: Key Formula or Approach:
We must apply IUPAC nomenclature rules for substituted cycloalkanes:
1. Identify the principal functional group or substituents.
2. Number the ring carbons to give the lowest locant set at the first point of difference.
3. Alphabetize the substituents when writing the final IUPAC name.
Step 3: Detailed Explanation:
• Let us analyze the first option:
The structure given has a cyclohexane ring with a gem-dimethyl group and an ethyl group.
If we number starting from the ethyl-substituted carbon:
Locants are 1 (for ethyl) and 3,3 (for dimethyl). The set of locants is (1, 3, 3).
If we number starting from the dimethyl-substituted carbon:
Locants are 1,1 (for dimethyl) and 3 (for ethyl). The set of locants is (1, 1, 3).
Comparing the locant sets (1, 1, 3) and (1, 3, 3) at the first point of difference:
The set (1, 1, 3) has a lower locant at the second position ($1 \lt 3$).
Therefore, the correct numbering starts at the carbon with the two methyl groups.
The correct IUPAC name for this structure is 3-Ethyl-1,1-dimethylcyclohexane.
The name given in Option (A), "1-Ethyl-3, 3-dimethyl cyclohexane," violates the lowest locant rule.
• Let us verify the other options:
Option (B): Cyclohex-2-en-1-ol. The principal group is $-\text{OH}$ (assigned C1), and the double bond starts at C2. This name is correct.
Option (C): 2-Chloro-1-methyl-4-nitrobenzene. This is named as a derivative of toluene (methyl at C1). This name is correct.
Option (D): 4-Ethyl-1-fluoro-2-nitrobenzene. This follows correct alphabetical and numbering rules for trisubstituted benzenes. This name is correct.
Step 4: Final Answer:
The compound in Option (A) is not named correctly according to IUPAC guidelines.
