[Anish Saxena]

Hello, I need help with the following question:


Please remember:
1. More than one options are possible.
2. IUPAC nomenclature is to be followed.

Background:
This question was asked in a major entrance examination (JEE Advanced) in India recently.

Thank you in advance!

[Arkcon]

The IUPAC rules are published online, so you can check them.  You wont have them on an examination that you can't take home, but you can memorize some simple rules to help.

Lets work together, with this multiple choice, to see if we can solve this one.  Note: This is what we do here, give hints, to help you help yourself.  We dont dump answers.

IUPAC naming is meant to remove the confusion of common names.  Do you see a common name being used?  You can exclude that one.

IUPAC naming is meant to be unambiguous.  Is there a name that could mean more than one molecule?  You can exclude that one.

Hints such as these will help you answer this problem, or at least let you guess better.

[Anish Saxena]

Thanks for the reply Arkcon,

I should've gone through the forum rules and given you a better picture. I did attempt to solve it, in examination hall as well as afterwards. I'd like to make you aware that the examination authority does accept common names accepted by the IUPAC as answers. Now to the question:

In following IUPAC rules we give more priority to halogens than saturated hydrocarbon substituent if benzene is the main carbon chain (please correct me if I'm wrong). Hence carbon attached to chlorine will be given first position. And then by numbering we give 4 to methyl group. So perfect IUPAC name would be (considering rule related to alphabetical order as well):

1-Chloro-4methylbenzene that is, Option C.

Methyl benzene on the other hand is also referred to as toluene and is accepted by IUPAC. Hence 4-Chlorotoluene also makes sense. That is, Option B.

However, 1-Chlorobenzene can also be referred to as simply Chlorobenzene to reduce redundancy. therefore 4-Methylchlorobenze should also be fine as methyl group is a substituent afterall. That is, Option A.

Option D is incorrect because numbering is wrong.

Hence answer should be A,B and C. Is there something I'm missing here? According to most answer keys I've checked so far, Option A has been omitted, why?

Thanks for any help, in advance!

[Arkcon]

Methyl benzene on the other hand is also referred to as toluene and is accepted by IUPAC. Hence 4-Chlorotoluene also makes sense. That is, Option B.

I didn't know that one.  I thought toluene was a common name, and would not be accepted by IUPAC naming.  It is typical, in common names, you add an IUPAC prefix to another common name, for example, the drug para-amino benzoic acid.

Option D is incorrect because numbering is wrong.

So you know, according to IUPAC rules, we number halogens first, then aliphatic constituents.  I wasn't sure about that one.

So what's the difference between A and C?

[Anish Saxena]

I didn't know that one.  I thought toluene was a common name, and would not be accepted by IUPAC naming.  It is typical, in common names, you add an IUPAC prefix to another common name, for example, the drug para-amino benzoic acid.

I know it is an accepted IUPAC name by a reliable source, my textbook (NCERT-Chemistry class 12th), much like aniline. Yes, we can treat common names (IUPAC accepted) as root word (sort of) and add substituents as prefix. So Option B should be correct.

So you know, according to IUPAC rules, we number halogens first, then aliphatic constituents.  I wasn't sure about that one.

Yes, again referred from my textbook.

So what's the difference between A and C?

I find none, and hence argue that both should be the answer along with B, that is, A,B and C. Most answer keys however skipped option A from the answer. I couldn't find any reason why.

[Arkcon]

There's nothing you can see about option A that is an ambiguous name?  The methyl is in the 4 position.  How are you sure about the chloro?  Is chlorobenzene defined as 1-chloro?

Can you not call it 4 methy chlorobenzine, if the chloro is 2 or 3?  Hmmm ...I guess, you can't.  But still, I think the name is ambiguous by IUPAC rules, even if it won't really cause confusion.

[Anish Saxena]

Chlorobenzene implies we have selected Chlorine at first position. Hence 4-Methylchlorobenzene implies methyl is at position 4 wrt chlorine. The only refutation I see is that while toluene is an accepted IUPAC name, writing 1-Chlorobenzene as Chlorobenzene may not be. And maybe that is why it has been omitted. If you think it looks ambiguous by IUPAC rules, it might well be. Thank you for clearing up this doubt!

[discodermolide]

Toluene is an approved IUPAC name so B is correct.
If you wish to name it as a benzene then it is 1-chloro-4-methylbenzene.
The substituents are placed in alphabetical order.