Animated Solution for Chemistry - Organic Chemistry: The IUPAC name(s) of the following compound is(are)

When we first look at a substituted benzene ring, it might seem like there is only one strict way to name it. However, the IUPAC nomenclature system is surprisingly elegant and flexible, allowing for multiple correct names depending on how we define the "parent" structure.

Let's dive into this specific molecule, which features a central benzene ring adorned with a chlorine atom ($-Cl$) and a methyl group ($-C{H}_3$) positioned directly opposite each other.

Benzene as the Parent

If we treat the simple benzene ring as our parent chain, we have two substituents to deal with: a chloro group and a methyl group.

According to IUPAC rules, when multiple substituents are present on a ring, we must list them in strict alphabetical order. 'C' for chloro comes before 'M' for methyl. This alphabetical priority doesn't just dictate the written name; it also dictates the numbering of the carbon atoms in the ring.

Because chloro comes first alphabetically, we assign the carbon attached to it as position $1$. We then count around the ring towards the methyl group to give it the lowest possible locant. Counting either way, the methyl group lands on carbon $4$.

Putting it all together, we get our first perfectly valid IUPAC name:

This confirms that Option (C) is absolutely correct.

Toluene as the Parent

Now, here is where the flexibility of IUPAC shines. The system officially retains certain common names as valid parent structures. A benzene ring with a methyl group attached is universally recognized as Toluene.

If we choose to use Toluene as our parent structure, the rules for numbering change slightly. By definition, the carbon atom of the ring that is attached to the methyl group is strictly designated as carbon $1$.

With the methyl-bearing carbon locked in at position $1$, we count around the ring to find the position of our remaining substituent, the chloro group. It falls on carbon $4$.

Naming this substituent on our new parent gives us our second valid IUPAC name:

This confirms that Option (B) is also correct.

It is just as important to understand why the other options are incorrect.

Option (A) suggests $4\text{-methylchlorobenzene}$. While "chlorobenzene" is a valid chemical, it is not a retained parent name with priority like Toluene. If benzene is the parent, the substituents must be listed alphabetically.

Option (D) suggests $1\text{-methyl-}4\text{-chlorobenzene}$. This completely violates the alphabetical order rule, placing 'methyl' before 'chloro'.

Therefore, the only correct IUPAC names provided are $4\text{-chlorotoluene}$ and $1\text{-chloro-}4\text{-methylbenzene}$.