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Topic: Organic nomenclature (why treat a group as a keytone, not aldehyde?)  (Read 6145 times)

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Offline JohnGraham

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Hi there - I have a question about IUPAC nomenclature. In my text, I stumbled up on the structure NC(=O)CCC=O named as 4-oxobutanamide (and chemicalize agrees).

The part I don't understand is the 4-oxo- prefix. Since the CHO group on the fourth carbon atom (i.e. at the end of the chain), I'd expect this to be treated as an aldehyde group, but instead it's being treated as a keytone group. I'd therefore expect the name to be formylbutanamide (no number because it must be at the other end of the chain to the amide group). However, when I put this into chemicalize I get this, 5-oxopentanamide, which not only forces the prefix from formyl- back to oxo-, but also adds an extra carbon onto the butane chain. It seems it wants to treat the carbon in the CHO group as separate from the main chain, even though it's connected directly to it.

What have I got wrong? Is there some convention I'm not aware of here?

Offline Dan

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Re: Organic nomenclature (why treat a group as a keytone, not aldehyde?)
« Reply #1 on: September 29, 2011, 12:40:06 PM »
Formyl = -CHO, the carbon is included in the prefix formyl.

You could say 3-formylpropanamide, but then you have broken the rule of naming the compound after the longest parent chain.

In order to name this as an aldehyde, you'd have to call it 3-carbamoylpropanal, but this breaks convention. When naming a compound, you choose a principal functional group. In this case your options are an aldehyde or an amide - carboxylic acid derivatives are higher priority than aldehydes, so the principal functional group is an amine. The suffix of the compound is therefore -amide, and the parent compound is butanamide. The wikipedia article is good, I'd suggest reading it.

The prefix oxo is used to indicate =O in cases where that carbonyl is not the principal group, and is used for aldehydes and ketones alike.

4-oxobutanamide has an aldehyde group
3-oxobutanamide has a ketone group

This is analogous to using the prefix -hydroxy where an alcohol is not the principal functional group. Example: 3-hydroxypropanoic acid is taken over 2-carboxyethanol.
« Last Edit: September 29, 2011, 02:40:00 PM by Dan »
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Offline JohnGraham

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Re: Organic nomenclature (why treat a group as a keytone, not aldehyde?)
« Reply #2 on: September 29, 2011, 01:49:07 PM »
Hi Dan - thanks for the reply. Most of my confusion stems from the fact that my text lists formyl-, not oxo- as the prefix for a non-principal aldehyde group, so I assumed I should use this.

On the other hand, now I have another point of confusion - Wikipedia says here that I should always use oxo- unless the carbon in that group can't be included in the chain, but here it says that oxo- is only for when the compound is a "natural product or a carboxylic acid".

I had a look at Wikipedia's references on the page for Aldehyde, and section R-5.7.1.2.1 on this page gives an example of a compound named both 3,6-dioxohexanoic acid and 5-formyl-3-oxopentanoic acid, but the text says that oxo- is used for an =O substituent and formyl- is used for a -CHO substituent. Also, the end of the first chapter in this other page referenced from Wikipedia says to use formyl- except in names of natural products. I guess carboxylic acids (and derivatives) count as this? I'm not sure what does...

Anyway, now my head's spinning. I guess I'll ask my tutor which I should use when.

Thanks very much for your help.
« Last Edit: September 29, 2011, 01:59:26 PM by JohnGraham »

Offline fledarmus

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Re: Organic nomenclature (why treat a group as a keytone, not aldehyde?)
« Reply #3 on: September 29, 2011, 02:10:46 PM »
There is a subtle difference between oxo and formyl. An oxo group is just the =O part, while a formyl group is -CHO. This is why in your initial structure, the name is 4-oxo: it is the double-bonded oxygen which is a substituent on carbon number 4 following IUPAC numbering.

The difference between when you use oxo and when you use formyl - you use oxo when the =O is on the carbon chain you are naming, and formyl when it is a substituent on some other group that you are naming. For example, a benzene ring that has a formyl group attached directly to the ring would be formylbenzene (assuming it had another substituent of higher priority, otherwise it would be benzaldehyde). However, a benzene ring with a three carbon chain terminating in an aldehyde would be (3-oxo-1-propyl)benzene.

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