IUPAC Nomenclature of Substituted Aromatic Hydrocarbon

To determine the correct IUPAC name(s) for the given compound, we analyze its chemical structure:

The compound consists of a benzene ring substituted with a methyl group ($-\text{CH}_3$) and a chlorine atom ($-\text{Cl}$) at positions para ($1,4$) to each other.

1. Systematic IUPAC Nomenclature (Benzene as Parent)

• When a benzene ring is disubstituted with two non-principal functional groups (both treated as prefixes, i.e., "chloro" and "methyl"), the numbering of the ring is decided by alphabetical order.
• Since "chloro" comes alphabetically before "methyl" ('c' before 'm'):
  • The carbon attached to chlorine is assigned locant 1.
  • The carbon attached to the methyl group is assigned locant 4.
• Therefore, the systematic IUPAC name is 1-chloro-4-methylbenzene.

2. IUPAC Nomenclature using Retained Names

• IUPAC allows the use of certain retained special names, such as toluene for methylbenzene.
• When toluene is used as the principal parent hydride:
  • The carbon atom attached to the methyl group is inherently assigned position 1.
  • The chlorine atom attached at the para-position receives locant 4.
• Therefore, another valid IUPAC name is 4-chlorotoluene.

3. Evaluation of Options

• (A) 1-chloro-4-methylbenzene: Correct systematic IUPAC name based on alphabetical locant priority.
• (B) 4-chlorotoluene: Correct IUPAC name using the accepted retained parent name "toluene".
• (C) 4-methylchlorobenzene: Incorrect nomenclature.
• (D) 1-methyl-4-chlorobenzene: Incorrect numbering, because alphabetical priority dictates that "chloro" should receive locant 1 over "methyl".

Thus, the correct options are A and B.