What is the rationale of the IUPAC name for DDT?

The structure of DDT is 


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


One of the benzene molecules should be the parent chain as it has a greater number of carbon atoms than the straight chain (which has just two carbon atoms). Taking benzene as the parent chain, I think that the name of the compound should be 1-Chloro-4-((1'-(4"-chlorophenyl))-2',2',2'-tricholoroethyl) benzene.


But the Wikipedia article states that the IUPAC name of the compound is 
1,1,1-Trichloro-2,2-bis(4-chlorophenyl)ethane.


Why is the ethyl chain taken as the parent chain when it has lesser number of carbon atoms than the benzene ring?




According to the current version of *[Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book)](http://dx.doi.org/10.1039/9781849733069-fp001)*, a structure with two or more identical units that are linked by di- or polyvalent groups can be named in two ways:


* by ordinary **substitutive nomenclature**, in which one of the parent structures is chosen as the senior parent structure and the remainder of the structure is expressed by substituent prefixes
* by **multiplicative nomenclature**, in which two or more parent structures are connected by symmetrical or unsymmetrical single or concatenated substituent groups


Both methods yield correct names, which may be used in general nomenclature. However, the IUPAC recommendations define a **preferred IUPAC name** (PIN), which is the name preferred among two or more names generated from two or more IUPAC recommendations.



> 
> **P-45.1** MULTIPLICATION OF IDENTICAL SENIOR PARENT STRUCTURES
> 
> 
> **P-45.1.1** Multiplicative nomenclature is senior to substitutive nomenclature for generating preferred IUPAC names to express multiple occurrences of identical senior parent structures, other than alkanes, in the name of the parent structure (see P-51.3.1). In most cases multiplicative names are shorter than regular substitutive names. A preferred IUPAC name is generated by multiplicative nomenclature when the following criteria for its use are met (see P-51.2.3).  
> 
> (1) the linking bonds (single or multiple) between the central substituent group of the multiplicative group and all subsequent structural units are identical; and  
> 
> (2) the multiplicative groups, other than the central multiplicative group, are symmetrically substituted; and  
> 
> (3) the locants of all substituent groups on the identical parent structures, including suffix groups, are identical.
> 
> 
> When these conditions are not met, substitutive nomenclature generates preferred IUPAC names.
> 
> 
> 


The given structure of the DDT molecule fulfils these requirements; therefore, the preferred IUPAC name is generated by multiplicative nomenclature as follows.



> 
> **P-15.3.1.3** Multiplicative name formation  
> 
> Multiplicative names are formed in accordance with the number of occurrences of identical structural units as defined in P-15.3.1.1 and the relationship of the linking multiplicative substituent group to the identical structural units.  
> 
> When a compound contains identical structural units as defined in P-15.3.1.1 linked by a symmetrical simple, compound, complex, or concatenated multiplicative group (a di- or polyvalent substituent group), it is named by stating successively:
> 
> 
> (a) the locants for the positions of substitution of the linking multiplicative substituent atom or group to the identical parent structural unit (the locant 1 is omitted when alone in the name of a mononuclear parent hydride);
> 
> 
> 


Here: 1,1'



> 
> (b) the name of the linking multiplicative substituent atom or group;
> 
> 
> 


Here: 2,2,2-trichloroethane-1,1-diyl



> 
> (c) the numerical prefix ‘di’, ‘tri’, etc.; and/or ‘bis-‘, ‘tris-‘, etc., with no elision of the final vowel before the name of the identical parent structural unit;
> 
> 
> 


Here: bis



> 
> (d) the name of one of the identical structural units including the principal characteristic group and substituents, if any, enclosed in appropriate enclosing marks (see P-16.5).
> 
> 
> 


Here: 4-chlorobenzene



> 
> The numbering of the identical parent structural unit is retained and, when there is a choice, the locants of the point of substitution by the linking multiplicative substituent groups on the identical parent structure are as low as possible.
> 
> 
> 


Here: 1,1'-(…)bis(4-chlorobenzene) (not 4,4'-(…)bis(1-chlorobenzene))


Therefore, the preferred IUPAC name is **1,1'-(2,2,2-trichloroethane-1,1-diyl)bis(4-chlorobenzene)**


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In short
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The rule for identifying the parent chain isn't just "longest".


In fact, "longest" is the fallback for when no other means are appropriate. The most important decider is the number of suffix substituent groups, which in this case is (chloro-)phenyl.


It's not a benzene compound, because each benzene ring only has one (n-ethyl), whereas the ethane chain has two.


In long
-------


To understand why this should be so; let's consider the actual precedence list for naming the parent chain (from [Wikipedia](https://en.wikipedia.org/wiki/IUPAC_nomenclature_of_organic_chemistry#Basic_principles)):


1. It should have the maximum number of substituents of the suffix functional group. By suffix, it is meant that the parent functional group should have a suffix, unlike halogen substituents. If more than one functional group is present, the one with highest precedence should be used.
2. It should have the maximum number of multiple bonds
3. It should have the maximum number of single bonds.
4. It should have the maximum length.


For DDT, we can use rule one: The ethane in the middle has two phenyls stuck to it, but each phenyl only has one ethyl; so the ethane wins.


For a simpler example of the same principle at work, consider 1,4-dioctylbenzene: A benzene ring with two octyl tails coming off of it. There are again two candidates for the parent chain, one making it a benzene, one making it an octane, 1-(4'-octylphenyl)-octane.


But because choosing the benzene gives a parent chain with two substituents, it becomes 1,4-dioctylbenzene.


Edit:
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There seems to be some confusion as to whether the rules presented above are the currently recommended ones. A careful reading of the 2013 Blue Book would be necessary to figure that out, and the text isn't online yet.


However: The priority rules above were the rules used to arrive at 1,1,1-trichloro-2,2-bis(4-chlorophenyl)etane.


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