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I looked on wikipedia and the IUPAC name for citric acid is given as "2-hydroxypropane-1,2,3-tricarboxylic acid"

But I thought that the longest carbon chain is 5, wouldn't you count the first and last carbon atoms? alt text

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2 Answers

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The longest carbon chain is 3. We do not include the Carbon atoms on the carboxyl groups because these are functional groups that are already named under 'tricarboxylic acid'.

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Yes, if you weren't given that answer, wouldn't you start by identifying the longest carbon chain? It's a good question, I think! – Collin Li♦ Oct 19 at 12:20
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Good question. It boggled me enough to get me doing some research on it.

True, the very first rule in IUPAC nomeclature is:

  1. Identify the parent hydrocarbon chain (The longest continuous chain of carbon atoms)

However, there is an exception:

From Wikipedia:

If there are multiple carboxyl groups on the same parent chain, the suffix "-carboxylic acid" can be used (as -dicarboxylic acid, -tricarboxylic acid, etc.). In these cases, the carbon in the carboxyl group does not count as being part of the main alkane chain. The same is true for the prefix form, "carboxyl-". Citric acid is one example; it is named 2-hydroxypropane- 1,2,3-tricarboxylic acid, rather than 2-carboxy, 2-hydroxypentanedioic acid.

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