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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?
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:
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.
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.
