Question

Which is the correct IUPAC name for
 

• A. Methylchlorobenzene
• B. Toluene
• C. 1-Chloro-4-Methylbenzene
• D. 1-Methyl-4-Chlorobenzene

Hint:

In aromatic compounds with more than one substituent, the substituents are numbered to give the lowest possible locants, with the substituent listed first in alphabetical order.

Answer 1

The Correct Option is C

To determine the correct IUPAC name, we analyze a benzene ring substituted with a methyl group (CH\(_3\)) and a chlorine atom (Cl).

1. Structure Identification:
The compound features a benzene ring with methyl and chlorine substituents. Their relative positions on the ring are crucial.

2. Position Determination:
Benzene ring positions are numbered 1 through 6. Assuming the methyl group occupies position 1 for systematic numbering, the diagram indicates a para (1,4) relationship between the methyl and chlorine groups. Therefore, if methyl is at position 1, chlorine is at position 4.

3. Naming Convention:
For disubstituted benzenes, IUPAC nomenclature requires listing substituents alphabetically, with priority given to the lowest possible numbering. The substituents are:
- Chloro (Cl)
- Methyl (CH\(_3\))
"Chloro" (C) alphabetically precedes "Methyl" (M). Thus, "chloro" should receive the lower position number. If methyl is at position 1 and chlorine at position 4, the name would be 1-methyl-4-chlorobenzene. Conversely, if chlorine is at position 1 and methyl at position 4, the name is 1-chloro-4-methylbenzene. Adhering to alphabetical order, 1-chloro-4-methylbenzene is the preferred IUPAC name.

4. Option Evaluation:
- Option A: Methylchlorobenzene
This name lacks positional locants and is not systematic according to IUPAC rules. Incorrect.
- Option B: Toluene
Toluene is exclusively methylbenzene (C\(_6\)H\(_5\)CH\(_3\)) and does not contain a chlorine substituent. Incorrect.
- Option C: 1-Chloro-4-Methylbenzene
This aligns with our derived name, with chlorine at position 1 and methyl at position 4, respecting alphabetical priority. Correct.
- Option D: 1-Methyl-4-Chlorobenzene
This represents the same compound as option C but incorrectly prioritizes "methyl" over "chloro" alphabetically. Incorrect.

Final Determination:
The correct IUPAC nomenclature for the compound is 1-chloro-4-methylbenzene (Option C).