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We were asked this question in class:

What is the IUPAC name of the compound $\ce{CHO-CH2-CH(CHO)-CH2-CHO}?$

I don't know for sure, but I think the answer should either be 3-formylpentan-1,5-dial or 2-(formylmethyl)-butane-1,4-dial. However, I am told that the correct answer is propane-1,2,3-tricarbaldehyde.

How can that be? I searched on the internet and found that we use 'carbaldehyde' as the suffix only when the $\ce{-CHO}$ group is attached to a ring. But here the given compound is an open chain compound, so I don't see why we're using 'carbaldehyde' as the suffix.

My course textbook mentions that this compound is named so "to give identical treatment to all aldehydic groups". But I haven't learnt that such a rule exists in IUPAC nomenclature.

What is the correct name of this compound and why?

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From the Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013, let's cite one of the rules for usage of the -carbaldehyde suffix, the relevant one:

P-66.6.1.1.2 The suffix ‘carbaldehyde’ is used when more than two $\ce{-CHO}$ groups are attached to an alkane.

Example:

$\ce{OHC-\overset{1}{C}H2-\overset{2}{C}H(CHO)-\overset{3}{C}H2-\overset{4}{C}H2-CHO}$
butane-1,2,4-tricarbaldehyde (PIN)

Thus the correct name is propane-1,2,3-tricarbaldehyde.

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