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. $\endgroup$ 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.


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