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Wikipedia states IUPAC name of the above compound as 1,1'-methylenebis(4-isocyanatobenzene) .

I have a confusion. Shouldn't isocyanate group be given preference over the methylene group? I assume the name as 4,4'-methylenebis(1-isocyanatobenzene) .

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Similar is the case with DDT. Wikipedia states its IUPAC name as 1,1'-(2,2,2-Trichloroethane-1,1-diyl)bis(4-chlorobenzene) .

Why is the numbering of carbon with three chlorine atoms done $1$ ? Moreover, can it be $1,1'$ in the first place in place of $4,4'$ ? I guess the $\text{diyl}$ term indicates that it is connected to two different carbons(of benzene) .

To sum up, I want to know which chain should I select as parent chain and why .

  • 2
    $\begingroup$ Related: What is the rationale of the IUPAC name for DDT? $\endgroup$
    – user7951
    Commented Sep 2, 2018 at 7:12
  • $\begingroup$ @Loong is methylene diisocyanato diphenyl also related? $\endgroup$ Commented Sep 2, 2018 at 7:14
  • $\begingroup$ Methylene (or xx-diyl) is not a substituent. Having two outgoing bonds, they must be part of the core structure, which takes precedence. Simply speaking. $\endgroup$
    – Karl
    Commented Sep 2, 2018 at 8:54
  • $\begingroup$ It's of course not a duplicate, but most of the quoted text in Loong's answer (linked above) is applicable to your case. $\endgroup$ Commented Sep 2, 2018 at 14:05
  • $\begingroup$ @Loong.. Would you please simplify your answer? Some terms and language in the quoted section of your answer seems to be confusing to me (that's not vague, but the language is too unfathomable to me as I'm a non-english ) ... Would you please provide an answer as to why it's 1,1' etc...in a simple language? $\endgroup$ Commented Sep 2, 2018 at 15:35

2 Answers 2


The isocyanato group is named by prefix (like halogeno-), it's not a principal characteristic group named by suffix. It can be said that these groups have lower priority than the “joint” position.

See the different numberings (examples from the IUPAC nomenclature of organic chemistry, 2013):



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1,1′-oxybis(4-bromobenzene) (PIN)

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4,4’-oxybis(2-bromobenzoic acid) (PIN)

Therefore it is named 1,1′-methylenebis(4-isocyanatobenzene), not 4,4′-methylenebis(1-isocyanatobenzene).

(You will probably argue that here, the oxy might have higher priority than bromo group or methylene connector, but “connector priority” is not taken into account.)

See also more rigirous answer to Q: What is the rationale of the IUPAC name for DDT?


One rule of nomenclature is that you should get the simplest name and, in both of your examples, the simplest is to acknowledge the symmetry of each molecule. So the "parent hydride" is methane in your first example and ethane in the second one because there is no simple way to describe an "alkyl group with 2 substituted phenyls attached".


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