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What is the IUPAC rule for naming a bicyclo compound with a fused benzene ring? Are you required to state the number of double bonds by adding something like 2,4,6-ene to the name or do benzene rings have a specific name ?

For example:

enter image description here

What will this compound be named? Will it be bicyclo[4.3.0]non 1,2,4 triene or is there some specific rule for fused benzene rings, as I stated before?

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    $\begingroup$ This is, derived from the stem compound, indene, $\text{2,3-dihydro-1}​\textit{H}\text{​-indene}.$ $\endgroup$
    – Buttonwood
    May 14, 2021 at 4:49
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    $\begingroup$ One follows the rules for fused rings. Years ago there were different acceptable approaches. And many structures can have traditional name (in the discussion at least, for identification, as it goes with data such as mp, H-NMR, elemental analysis,... perhaps one has to be stricter. But name as carbazole, naphthalene, indene, etc are OK. $\endgroup$
    – Alchimista
    May 14, 2021 at 9:14
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    $\begingroup$ Please don't answer in comments. $\endgroup$
    – Loong
    May 14, 2021 at 10:50
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    $\begingroup$ Also known as indane $\endgroup$
    – user10186832
    May 16, 2021 at 5:44

2 Answers 2

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Bicyclic compounds with a fused benzene ring may be named correctly in two or more ways in accordance with the several methods recommended by IUPAC.

  1. If there is a retained name (e.g. naphthalene) for the parent ring components listed in Chapter P-25, this name is preferred.
  2. If there is no retained name, a fusion name (e.g. benzo[8]annulene) is preferred for a benzene ring fused to another cycle of five or more members.
  3. In the remaining cases, a von Baeyer name (e.g. bicyclo[4.1.0]hepta-1,3,5-triene) is used.

The following list shows examples for benzene rings with simple bridges from 1 to 6.

bicyclo[4.1.0]hepta-1,3,5-triene
bicyclo[4.2.0]octa-1,3,5-triene
2,3-dihydro-1H-indene
1,2,3,4-tetrahydronaphthalene
6,7,8,9-tetrahydro-5H-benzo[7]annulene
5,6,7,8,9,10-hexahydrobenzo[8]annulene
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Beside systematic nomenclature about organic structures, there are other systems in use. One of them is the by Hantzsch and Widman which departs from a number of stem structures. Some of these archetypical motifs are named systematically, others (still) retain their traditional name which does say little to you about the structure (e.g., pyridine) until you learn them. One (incomplete) compilation of these names, for example, is this one.

This being said, you either look up, or over time (i.e., training) recognize that the parent structure of your molecule is indene:

enter image description here

except that the double bond in the five-membered ring of the structure you showed is completely saturated.

You remember reactions on benzene, and recall about the importance to denote the sites in organic structures. The approach to use numbers equally exists for larger compounds, too; a few positions are labeled. While there are cases where you may shorten 1H-indene to indene, this no longer is the case to describe unambiguously the compound shown by you, which is 2,3-dihydro-1H-indene.

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  • $\begingroup$ I didn't say I apply Hantzsch-Widman here. I followed @Long's comment "Please don't answer in comments". $\endgroup$
    – Buttonwood
    May 19, 2021 at 21:04

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