8
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Why is 4-ethyl-1-fluoro-2-nitrobenzene (structure is in the attached picture) called so ?

Shouldn't it be called as 1-ethyl-4-fluoro-3-nitrobenzene since the precedence of ethyl is greater than halogens?

4-ethyl-1-fluoro-2-nitrobenzene

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

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.

Furthermore,

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; (…)

Therefore, the example is named as 4-ethyl-1-fluoro-2-nitrobenzene rather than 1-ethyl-4-fluoro-3-nitrobenzene since the locant set ‘1,2,4’ is lower than ‘1,3,4’.

4-ethyl-1-fluoro-2-nitrobenzene

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  • $\begingroup$ I see that the locant set of 1,2,4 is lower than 1,3,4 but the precedence of ethyl is greater than fluorine .. so shouldn't ethyl get the lowest number ? $\endgroup$ – user126190 Feb 7 '15 at 15:19
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    $\begingroup$ @user126190 The prefixes (‘ethyl’, ‘fluoro’, and ‘nitro’) are ‘all considered together in a series of increasing numerical order’. $\endgroup$ – Loong Feb 7 '15 at 15:36
  • $\begingroup$ But this website link and many other websites too say alkanes are before halogens .. $\endgroup$ – user126190 Feb 8 '15 at 11:33
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    $\begingroup$ @user126190 Your link is about the seniority of functional classes in functional class nomenclature, which is not related to your question. In your example, the functional class is already given; it is neither alkane nor alkyl halide – it is benzene. $\endgroup$ – Loong Mar 28 '15 at 17:21
  • $\begingroup$ @Loong is correct. As far as numbering of locants is concerned, alkyls, alkoxys, halos, and nitros all has the lowest priority because they are substituent only functional groups. $\endgroup$ – Ben Norris Jun 1 '15 at 18:38

protected by andselisk Jun 7 at 5:25

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