IUPAC name, phthalic anhydride

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...


[![enter image description here](https://i.stack.imgur.com/Ttxtim.png)](https://i.stack.imgur.com/Ttxtim.png)




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


[![enter image description here](https://i.stack.imgur.com/wq6q0.png)](https://i.stack.imgur.com/wq6q0.png)


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


Compare, cyclopentan-1,3-dione 


[![enter image description here](https://i.stack.imgur.com/Ycy5Im.png)](https://i.stack.imgur.com/Ycy5Im.png)


The end result is 


2-benzofuran-1,3-dione


[![enter image description here](https://i.stack.imgur.com/c5Cxcm.png)](https://i.stack.imgur.com/c5Cxcm.png)


 5

Your approach is correct.


The current version of *[Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book)](http://dx.doi.org/10.1039/9781849733069-fp001)* reads as follows:



> 
> **P-65.7.7** Cyclic anhydrides
> 
> 
> **P-65.7.7.1** 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-25.2.2.4** 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’.


[![furan](https://i.stack.imgur.com/nfLfA.png)](https://i.stack.imgur.com/nfLfA.png)


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.


[![2-benzofuran](https://i.stack.imgur.com/o2dGZ.png)](https://i.stack.imgur.com/o2dGZ.png)


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’.


[![2-benzofuran-1,3-dione](https://i.stack.imgur.com/h418d.png)](https://i.stack.imgur.com/h418d.png)


 3