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I've been playing around with ChemDraw and found that it assigns the priorities of substituents in a way I wouldn't have expected, depending on the length of the alkyl substituent. I'm not a nomenclature buff, so I'd like to know if it's correct and if so, why.

substituted benzenes

  • $n = 1$: 1-ethyl-2-iodobenzene
  • $n = 2$: 1-iodo-2-propylbenzene
  • $n = 3$: 1-butyl-2-iodobenzene
  • $n = 4$: 1-iodo-2-pentylbenzene

What determines the priorities of the alkyl group and the iodo group?

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You correctly figured out that the prefixes that represent the substituent groups are arranged alphabetically. However, note that the order of citation and the numbering are laid down in two different rules.

With regard to the alphabetical order of citation of substituent prefixes, 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 alphanumerical order in your examples is correct:

x-ethyl-y-iodobenzene
x-iodo-y-propylbenzene
x-butyl-y-iodobenzene
x-iodo-y-pentylbenzene

The relevant rules concerning the numbering of locants for substituent prefixes are:

P-14.3.5 Lowest set of locants

The lowest set of locants is defined as the set that, when compared term by term with other locant sets, each cited in order of increasing value, has the lowest term at the first point of difference; (…)

and

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).

In accordance with Rule (f), your first example could be named as 1-ethyl-2-iodobenzene as well as 2-ethyl-1-iodobenzene since both names correspond to the locant set ‘1,2’. However, according to Rule (g), this example is named as 1-ethyl-2-iodobenzene rather than 2-ethyl-1-iodobenzene since ethyl is cited first as a prefix in the name.

Hence, the numbering in your examples is correct:

1-ethyl-2-iodobenzene
1-iodo-2-propylbenzene
1-butyl-2-iodobenzene
1-iodo-2-pentylbenzene

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