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 solve the problem, we need to determine the correct IUPAC name for the given compound, which is a benzene ring with a methyl group (CH\(_3\)) and a chlorine (Cl) substituent.

1. Identifying the Structure:
The compound is a benzene ring with two substituents: a methyl group (CH\(_3\)) and a chlorine atom (Cl). From the diagram, the methyl group and chlorine are positioned on the benzene ring, and we need to determine their relative positions.

2. Determining the Positions:
In the benzene ring, the positions of substituents are numbered 1 through 6. Let’s assume the methyl group is at position 1 (as is common in IUPAC naming to give the lowest possible numbers). The diagram shows the methyl group and chlorine in a para relationship (opposite each other, 1,4-positions). Thus, if the methyl group is at position 1, the chlorine is at position 4.

3. Naming the Compound:
In IUPAC nomenclature for a disubstituted benzene, the substituents are named in alphabetical order, and the lowest possible numbers are assigned to their positions. Here, the substituents are:
- Chloro (Cl)
- Methyl (CH\(_3\))
Alphabetically, "chloro" (C) comes before "methyl" (M), so "chloro" is assigned the lowest possible number. If methyl is at position 1, chlorine is at position 4, making the name 1-methyl-4-chlorobenzene. Alternatively, if chlorine is at position 1, methyl is at position 4, giving 1-chloro-4-methylbenzene. Since "chloro" precedes "methyl" alphabetically, 1-chloro-4-methylbenzene is preferred.

4. Evaluating the Options:
- Option A: Methylchlorobenzene
This name does not follow IUPAC rules, as it lacks positional numbers and is not a systematic name. Incorrect.
- Option B: Toluene
Toluene is methylbenzene (C\(_6\)H\(_5\)CH\(_3\)), with only a methyl group and no chlorine. Incorrect.
- Option C: 1-Chloro-4-Methylbenzene
This matches our deduced name: chlorine at position 1, methyl at position 4, following alphabetical priority. Correct.
- Option D: 1-Methyl-4-Chlorobenzene
This is the same compound as option C but prioritizes "methyl" over "chloro," which violates the alphabetical rule in IUPAC naming. Incorrect.

Final Answer:
The correct IUPAC name for the compound is 1-chloro-4-methylbenzene (option C).