I was presented with the following question in an online test:

screenshot of test question with organic species whose IUPAC name is to be identified

To avoid any confusion, here are the arranged options:

A. 2-Chlorotoluene
B. 1-Chloro-2-methylbenzene
C. B and D
D. O-Chlorotoluene

Correct answer: (C) B and D.
Explanation: Chloro is given priority so numbered as 1

My question is: Are the ortho, meta and para prefixes part of the official IUPAC nomenclature or not?

I chose B, what should be the correct answer to this question?

  • $\begingroup$ .. if taking typography into account, all names in the image are wrong. $\endgroup$ – mykhal Jul 20 '18 at 17:04
  • $\begingroup$ @mykhal: No, options b and c (in the image) are correct. $\endgroup$ – Rahul Verma Jul 20 '18 at 17:17

According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book),

The locants o, m, p are no longer recommended; the numerical locants ‘1 ,2-‘, ‘1 ,3-‘, and ‘1,4-‘ must be used in substitutive names. However, as an exception, the three isomers of xylene are still recognized as o-, m-, and p-xylene in general IUPAC nomenclature (see P-22.1.3).


The letters o, m, and p have been used in place of ortho, meta, and para, respectively, to designate the 1,2-, 1,3-, and 1,4- isomers of disubstituted benzene. This usage is strongly discouraged and is not used in preferred IUPAC names.

The preferred IUPAC name for the compound that is given in the question is 1-chloro-2-methylbenzene.


The name 2-chlorotoluene may be used in general nomenclature.

  • $\begingroup$ How comes only o-chlorotoluene is correct, but not 2-chlorotoluene? I think none of them is an IUPAC name. If one is correct, then the other should be as well. $\endgroup$ – Mathew Mahindaratne May 29 '18 at 20:48
  • $\begingroup$ @MathewMahindaratne, Why 2-chlorotoluene is incorrect? $\endgroup$ – Rahul Verma May 30 '18 at 3:30
  • $\begingroup$ That's my question too. If they marked o-chlorotoluene as correct, then they should mark 2-chlorotoluene correct as well (@Loong). $\endgroup$ – Mathew Mahindaratne May 30 '18 at 14:50
  • 1
    $\begingroup$ @MathewMahindaratne, as Loong's answer has already very clearly laid out, the IUPAC Blue Book states 1-chloro-2-methylbenzene is the preferred name and 2-chlorotoluene is acceptable (see also chemistry.stackexchange.com/q/75668/16683). The name o-chlorotoluene is strongly discouraged. So, the answer to the multiple-choice question is clearly not in accordance with latest IUPAC nomenclature guidelines. Unfortunately, there is not a lot that we can do about that: we can only teach the correct things, we cannot teach people how to guess which wrong answer will satisfy an online system. $\endgroup$ – orthocresol May 30 '18 at 23:48
  • 1
    $\begingroup$ And your comments are also not quite spot on. Nowhere is it said that "if one is correct then the other should be". In fact this is already directly refuted, as we see that one (2-chlorotoluene) is acceptable and the other (o-chlorotoluene) not, at least if we go by what the Blue Book states. As for the question setter, we can only speculate what they think the Blue Book states, which is likely a futile exercise. $\endgroup$ – orthocresol May 30 '18 at 23:54

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