Could someone help me please (preferably talk me through not just provide the answer) with the IUPAC name of phthalic anhydride? I'm sorry for not posting attempts but I'm afraid I have absolutely no idea how to begin here...

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    $\begingroup$ The preferred IUPAC name (PIN) of phthalic anhydride is 2-benzofuran-1,3-dione. Please add details to isolate your problem and to narrow the answer set. $\endgroup$ – Faded Giant Aug 20 '15 at 15:29
  • $\begingroup$ Wikipedia IUPAC names are in my experience about 50/50 , hence why I didn't just run with that. If that is indeed the correct name, how is it derived? $\endgroup$ – Some_Guy Aug 20 '15 at 15:35

Your approach is correct.

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

P-65.7.7 Cyclic anhydrides

P- Cyclic anhydrides formed from two acid groups attached to the same parent hydride structure are named in two ways:

(1) as heterocyclic pseudoketones;

(2) by changing the class term ‘acid’ to ‘anhydride’ in the systematic or retained name of the dibasic acid.

Method (1) generates preferred IUPAC names

Using Method (1), the first step is naming the heterocyclic component as follows.

P- Heteromonocyclic components fused to a benzene ring

Unless listed as a retained name (…), a benzene ring fused to a heteromonocycle of five or more members (a benzoheterocycle) is named by placing the locant(s) indicating the position(s) of the heteroatom(s) at the front of the name consisting of the fusion prefix ‘benzo’ followed by a retained name, a Hantzsch-Widman systematic name, or a name formed by skeletal replacement (‘a’) nomenclature (…). The locants cited correspond to the full bicyclic structure. As in Hantzsch-Widman names, locants are placed in the order corresponding to the order of citation of the heteroatoms in the heterocyclic component. The locant ‘1’ is always assigned to the atom of the heterocyclic component next to a fusion atom. Heteroatoms are allocated lowest locants as a set, without regard to kind; if there is a choice, lowest locants are assigned in accordance with the seniority of the ‘a’ prefixes (…). In general nomenclature locants may be omitted when the name is unambiguous; for preferred IUPAC names locants must be cited. The letter ‘o’ of the ‘benzo’ prefix is elided when followed by a vowel. Indicated hydrogen is placed at the front of the name, when required.

Accordingly, the name ‘benzofuran’ consists of the prefix ‘benzo’ followed by the retained name ‘furan’.


The locant ‘1’ is assigned to the atom of the heterocyclic component next to a fusion atom. The locant ‘2’ is assigned to the heteroatom and it is placed at the front of the name.


Finally, the corresponding pseudoketone is named using the suffix ‘one’ in accordance with rules expressed for ketones. Thus, the preferred IUPAC name (PIN) of phthalic anhydride is ‘2-benzofuran-1,3-dione’.


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Benzofuran is named by the position of the oxygen, hence 2-Benzofuran. (below is 1-benzofuran)

enter image description here

The =O s are then added on the 1 and 3 carbons, hence, 1-3-dione

Compare, cyclopentan-1,3-dione

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The end result is


enter image description here

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  • $\begingroup$ How about 1,3-dioxo-2-benzofuran? $\endgroup$ – busukxuan Aug 21 '15 at 10:07
  • $\begingroup$ I have absolutely no idea what the rules are governing which is appropriate, I'm just taking analogy from cyclopentanone not being called oxo-cyclopentane. But yeah, I have no idea :/ $\endgroup$ – Some_Guy Aug 21 '15 at 10:15
  • $\begingroup$ i think it doesn't matter, coz IUPAC nomenclature is designed such that each name refers to exactly one compound, but not necessarily the other way round. $\endgroup$ – busukxuan Aug 22 '15 at 7:55

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