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According to my chemistry textbook, 2-hydroxy-1,2,3-propanetricarboxylic acid is represented as:

2-hydroxy-1,2,3-propanetricarboxylic acid

But 5-bromoheptanoic acid is given as:

5-bromoheptanoic acid

In the former naming, propane is considered in the base chain while the COOH carbons are not. In the latter one, the COOH carbons are considered in the base carbon chain.

On checking NCBI Pubchem, I found the following:

https://pubchem.ncbi.nlm.nih.gov/compound/16217547#section=Top https://pubchem.ncbi.nlm.nih.gov/compound/5-Bromovaleric_acid#section=Top

Why are the carbons of carboxylic acids excluded in the former name but included in the latter?

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    $\begingroup$ Related: IUPAC name for citric acid $\endgroup$ – Loong Jan 9 at 14:35
  • $\begingroup$ I don't remember the specific recommendations but taking pentane or pentanoic ac. as parent compound would be rather messy. Keep in mind that except for reporting a new isolated molecule following recommendations is very important but less crucial than what it might seem to beginners (assuming you are). Perhaps both examples adhere to recommendations and/or others will answer. I just want to point out that you should not over think if you can univocally name a structure (or draw from name). $\endgroup$ – Alchimista Jan 9 at 14:41
  • $\begingroup$ @Alchimista Yeah, thanks. Found that dicarboxylic acids (such as glutaric acid; structure: en.m.wikipedia.org/wiki/Glutaric_acid) have the COOH carbon considered in the base name in most cases but in tri/more carboxylic acids, this rule doesn't follow. $\endgroup$ – Kartik Jan 9 at 15:02
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When identical groups are present such that neither can be preferred over the other then, such nomenclature is preferred to emphasise on the equivalence of the groups such as in the one we have here. We cannot name in such a way that includes the three groups together. So, we name it that way instead.

As such, the rules of IUPAC Nomenclature are themselves arbitrary therefore, it is just something that is the way it is. For more information you might like to see https://old.iupac.org/publications/books/principles/principles_of_nomenclature.pdf, Page 34 onwards.

Edit: I almost forgot that a compound can have more than one name, even by the IUPAC nomenclature rules. As such, the names used commonly as IUPAC names are the preferred ones.

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According to IUPAC Nomenclature rules, the carboxylic acid functional group carbon is included in the base aliphatic chain if one or two carboxylic acid functional groups are present and the standard suffix '-oic acid' is used. For three or more carboxylic acid groups in an aliphatic chain, the carboxylic acid carbons are excluded and the suffix 'carboxylic acid' is used instead.

Suffixes such as carbonitrile and carbaldehyde (When is the carbaldehyde suffix correct?, an example for cyclic compounds) are used similarly to denote the exact positions of the functional groups to avoid ambiguity.

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