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 Question:
We need to determine the correct IUPAC name of the given trisubstituted benzene derivative.
The substituents present on the benzene ring are:
- A chlorine atom ($-\text{Cl}$, chloro group)
- A nitro group ($-\text{NO}_2$, nitro group)
- A methyl group ($-\text{CH}_3$, methyl group)
Step 2: Key Formula or Approach:
According to IUPAC nomenclature rules for trisubstituted benzenes:
1. The substituents must be numbered according to the lowest locant rule.
2. The substituents are then listed alphabetically in the final IUPAC name.
Step 3: Detailed Explanation:
Let us evaluate different numbering schemes to find the lowest locant set:
- Scheme 1: Numbering starting from carbon with $-\text{Cl}$ as C1, going clockwise:
- Chlorine is at position 1.
- Nitro is at position 2.
- Methyl is at position 4.
- This gives the locant set: $(1, 2, 4)$.
- Scheme 2: Numbering starting from carbon with $-\text{CH}_3$ as C1, going counter-clockwise:
- Methyl is at position 1.
- Nitro is at position 3.
- Chlorine is at position 4.
- This gives the locant set: $(1, 3, 4)$.
- Scheme 3: Numbering starting from carbon with $-\text{NO}_2$ as C1:
- Nitro is at position 1.
- Chlorine is at position 2.
- Methyl is at position 5.
- This gives the locant set: $(1, 2, 5)$.
Comparing the locant sets $(1, 2, 4)$, $(1, 3, 4)$, and $(1, 2, 5)$, the lowest set of locants is $(1, 2, 4)$ because 2 is smaller than 3 at the second position of comparison.
Thus, the correct numbering is:
- 1-chloro
- 2-nitro
- 4-methyl
Next, we arrange these substituents alphabetically in the name:
- Alphabetical order: Chloro (C) $<$ Methyl (M) $<$ Nitro (N).
Combining these coordinates alphabetically gives:
1-Chloro-4-methyl-2-nitrobenzene.
