How will this compound be named? Both, methoxy and chloro are given equal preference, since they are prefix functional groups. The answer given in the book is 2-chloro-1-methoxyethane, but why is it not 1-chloro-2-methoxyethane, as the C of chlorine comes before M in the alphabetical order?

  • $\begingroup$ This is rather weird. I was expecting your suggestion to be correct. $\endgroup$
    – M.A.R.
    Commented Jun 23, 2015 at 19:59
  • $\begingroup$ @M.A.Ramezani Are you sure there's no order of preference for prefix functional groups? $\endgroup$
    – agdhruv
    Commented Jun 23, 2015 at 20:00
  • $\begingroup$ It might be that there is an order of preference after all. According to the following link, ethers have a priority over halo groups: masterorganicchemistry.com/2011/02/14/… $\endgroup$
    – user17037
    Commented Jun 23, 2015 at 20:14
  • 1
    $\begingroup$ @AnthonyKelesidis Well, I am quite sure there is no order of preference here. Refer to the answer by to this question of mine : chemistry.stackexchange.com/questions/33122/… . It makes complete sense, and also gives me the correct answer. $\endgroup$
    – agdhruv
    Commented Jun 23, 2015 at 20:21
  • 2
    $\begingroup$ According to the IUPAC name generator at web.chemdoodle.com/demos/iupac-naming, the "correct" name is "1-Chloro-2-methoxyethane". $\endgroup$
    – Curt F.
    Commented Jun 23, 2015 at 23:05

2 Answers 2


Your name, 1-chloro-2-methoxyethane, is correct.

Here is a link to the relevant page of the 1993 edition of the IUPAC Blue Book - Nomenclature of Organic Chemistry, which is published online by ACD Labs with permission of IUPAC.

Here are some example ethers and sulfides from that page, and we see a similar case to yours:

example IUPAC ether nomenclature

There is no difference in preference or priority between halo and alkoxy substituents (or any of the other prefix only substituents). Thus, in a case where there are two such groups and assignment of locants generates the same combination either way, you choose to assign the lower locant number to the alphabetically first.

  • 2
    $\begingroup$ Please be aware, that there is a new version of the blue book from 2013 and we have already had some questions, where there were significant differences. However, in that case I believe the rule still holds. $\endgroup$ Commented Jun 24, 2015 at 13:34

The current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book) reads as follows:


Alphanumerical order has been commonly called ‘alphabetical order’. As these ordering principles do involve ordering both letters and numbers, in a strict sense, it is best called ‘alphanumerical order’ in order to convey the message that both letters and numbers are involved

Alphanumerical order is used to establish the order of citation of detachable substituent prefixes (not the detachable saturation prefixes, hydro and dehydro), and the numbering of a chain, ring, or ring system when a choice is possible.


P-14.5.1 Simple prefixes (i.e., those describing atoms and unsubstituted substituents) are arranged alphabetically; multiplicative prefixes, if necessary, are then inserted and do not alter the alphabetical order already established.

Therefore, the correct alphanumerical order of citation in your example corresponds to the name ‘x-chloro-y-methoxyethane’.



When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:


(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;

(g) lowest locants for the substituent cited first as a prefix in the name;


Note that Rule f takes precedence over Rule g.
According to Rule f, the name of the given compound could be ‘1-chloro-2-methoxyethane’ as well as ‘2-chloro-1-methoxyethane’ since both names correspond to the locant set ‘1,2’ (all prefixes considered together in a series of increasing numerical order).
According to Rule g, however, the compound is named as ‘1-chloro-2-methoxyethane’ rather than ‘2-chloro-1-methoxyethane’ since ‘chloro’ is cited first as a prefix in the name.


  • $\begingroup$ So, you mean the book is wrong? $\endgroup$
    – agdhruv
    Commented Sep 5, 2015 at 16:24
  • 2
    $\begingroup$ @ag_dhruv The name “2-chloro-1-methoxyethane” given in your book is wrong in so far as it is not the preferred IUPAC name; it is not even a valid name for general (IUPAC) nomenclature. Nevertheless, this name is unambiguous, and interpretation of this name would yield the correct compound. Therefore, it is not completely wrong. $\endgroup$
    – user7951
    Commented Sep 5, 2015 at 17:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.