Always apply the lowest locant rule first to determine the numerical positions of all substituents on a ring. Once the position numbers are fixed, arrange the substituent prefixes alphabetically in the final name regardless of their numbers.
Step 1: Understanding the Concept:
For trisubstituted benzenes where none of the substituents impart a special name (like phenol or aniline), the ring is numbered such that the set of locants (the position numbers) is the lowest. If multiple numbering schemes give the same set of lowest locants, we number based on alphabetical priority. The substituents are then listed in alphabetical order in the final name. Step 2: Detailed Explanation:
The substituents on the ring are:
- Chloro (C)
- Methyl (M)
- Nitro (N)
Let's evaluate numbering schemes:
1. Starting at Cl (1) and going towards Nitro: Locants are (1, 2, 4). Set = $\{1, 2, 4\}$.
2. Starting at Nitro (1) and going towards Cl: Locants are (1, 2, 5). Set = $\{1, 2, 5\}$.
3. Starting at Methyl (1) and going towards Nitro: Locants are (1, 3, 4). Set = $\{1, 3, 4\}$.
Comparing $\{1, 2, 4\}$, $\{1, 2, 5\}$, and $\{1, 3, 4\}$, the lowest set is $\{1, 2, 4\}$.
Using this numbering:
- Chlorine is at position 1.
- Nitro is at position 2.
- Methyl is at position 4.
Now, arrange alphabetically for the final name: Chloro (C) < Methyl (M) < Nitro (N).
Name: 1-Chloro-4-methyl-2-nitrobenzene. Step 3: Final Answer:
The correct IUPAC name is 1-Chloro-4-methyl-2-nitrobenzene. The correct option is (B).
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