# Nomenclature of alkoxy and halo groups

CH3-O-CH2CH2Cl

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?

• This is rather weird. I was expecting your suggestion to be correct. – M.A.R. Jun 23 '15 at 19:59
• @M.A.Ramezani Are you sure there's no order of preference for prefix functional groups? – agdhruv Jun 23 '15 at 20:00
• 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/… – user17037 Jun 23 '15 at 20:14
• @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. – agdhruv Jun 23 '15 at 20:21
• According to the IUPAC name generator at web.chemdoodle.com/demos/iupac-naming, the "correct" name is "1-Chloro-2-methoxyethane". – Curt F. Jun 23 '15 at 23:05

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:

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.

• 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. – Martin - マーチン Jun 24 '15 at 13:34
• IUPAC has to make regular changes to the rules, or else no one would have to buy the books they publish. IUPAC's constant changes and refusal to provide open access to their published "standards" also serve the important goal of continually confusing new generations of o-chem students, and also driving lots of traffic to here at chem.se. Thanks IUPAC! – Curt F. Jun 24 '15 at 14:04

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

P-14.5 ALPHANUMERICAL ORDER

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’.

Furthermore,

P-14.4 NUMBERING

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.

• So, you mean the book is wrong? – agdhruv Sep 5 '15 at 16:24
• @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. – Loong Sep 5 '15 at 17:48