What is the difference between oxo and formyl? both represent aldehyde when used as a substituent(correct me if I am wrong). But when should I use formyl and when oxo?

This answers my problem and also says that oxo is also used for aldehyde.


If they get priority in the name they get the suffix ‘al’ or ‘carbaldehyde’ As substituent prefixes ‘oxo’ and ‘formyl’ are used. The prefix ‘formyl’ is used in preferred IUPAC names, except for a -CHO group at the end of an acyclic chain.

However he did not give any sources.


2 Answers 2


Oxo is the substituent name for the $\ce{=O}$ group of an aldehyde or a ketone if that group is found on the longest continuous carbon chain that contains a higher priority group.

e.g., $\ce{OHC-CH2-CH2-CH2-CH2-CO2H =}$ 6-oxohexanoic acid

If the aldehyde carbon is not part of the longest continuous chain, then it becomes a formyl substituent.

e.g., $\ce{CH3-CH2-CH(-CHO)-CH2-CH2-CO2H =}$ 4-formylhexanoic acid

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    $\begingroup$ Welcome to our site! Could you perhaps add a few referenced (preferably from the IUPAC Blue Book), to back your answer up? Nomenclature systems are defined as standards, hence the requirement of references. $\endgroup$ May 28, 2020 at 4:38

"Oxo" refers to a substitution of a single oxygen atom on a carbon atom by a double bond. "Formyl" refers to the substitution of a three atom HCO group onto a different carbon or other atom.

I'll leave it to others to explain the differences in IUPAC nomenclature.

So for example 3-oxopentanal has five carbons just like pentanal, but it has one extra oxygen atom, double bonded to carbon #3. (This means it has two less hydrogens, a fact that the nomenclature elides.) This is a case where "oxo" refers to a ketone, not an aldehyde.

"Formyl" is sometimes used as a substituent, but not always. "Formyl benzene" is a incorrect name for benzaldehyde, but 4-formylbenzoic acid does exist and is used in practice as the name of chemical.


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