Aldehydes and NMR

[computerdorkdan]

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Do aldehydes get split by neighbors in HNMR and do aldehydes split neighbors?

For example, would CH3CHO show:
- two singlets
- a quartet and a doublet
- a quartet and a singlet
- a singlet and a doublet
- something else?

 

[loveoforganic]

For some reason, I remembered from class that aldehyde protons didn't couple. However, both berkeley review and my study buddy from organic say differently, so I'd say quartet and doublet.

 

[Doodl3s]

I thought they didnt couple either.... I know -OH and acid COOH don't split... i thought aldehydes didnt either... but its one or the other... if you see a peak at 9... if it is split, its splitting the neighbors as well. If its not split, the neighbors aren't either. So its either both or none. But i can't say for sure.

 

[computerdorkdan]

For some reason, I remembered from class that aldehyde protons didn't couple. However, both berkeley review and my study buddy from organic say differently, so I'd say quartet and doublet.

I remember them not coupling from class either. EK tells me they do (through a problem in the 1001 series).

If I go look up acetaldehyde in SDBS (http://riodb01.ibase.aist.go.jp) it looks like the proton is not being split.

 

[loveoforganic]

http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi

I see a coupling constant

 

[computerdorkdan]

http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi

I see a coupling constant

What molecule were you looking at? The link is a generic one.

If you're talking about acetaldehyde I can't tell if it's splitting or just not a perfect singlet.

 

[loveoforganic]

Oops, not sure how that happened. I had pulled up acetaldehyde. The spectrum wasn't clear, but in the peak assignment it listed a coupling constant.

 

[phEight]

Why wouldn't it split? It would be a quartet and a doublet. Protons on adjacent carbons split each other. The reason COOH / OH don't split is because the O is in the way. In an aldehyde the protons are adjacent. Look more closely at the structure. Don't let the bond angles fool you into thinking the H is far and thus not adjacent.

 

[boaz]

I second that. H's on adjacent carbons do split.

The reason COOH and OH don't split is not because it's oxygen instead of carbon but rather because of "fast exchange", i.e. the NMR machine "sees" only an "averaged" proton because of its acidity (protons constantly being exchanged among the molecules).

 

[ImDiene0412]

I also thought they didn't couple, but am having the same problem with Berkeley Review. The adjacent carbon is a carbonyl though, so I don't see how it has protons to couple with? My textbook says "Splitting is observed only for nonequivalent protons on the same carbon or adjacent carbons."

 

[aldol16]

I also thought they didn't couple, but am having the same problem with Berkeley Review. The adjacent carbon is a carbonyl though, so I don't see how it has protons to couple with? My textbook says "Splitting is observed only for nonequivalent protons on the same carbon or adjacent carbons."

You need to draw out the structure of acetaldehyde. The aldehyde proton is directly attached to the carbonyl carbon - not adjacent to it. Therefore, there is no reason it wouldn't split the signal.