1h NMR spectrum of an aldehyde C5H8O?
i need to know the structure of this aldehyde.
only 1h nmr is given
there is a 3H with 3 peaks at 1ppm, 2H Quartet at 2ppm, 1H-2peaks at 6ppm, 1H-6peaks at 6.8ish ppm, and 1H 1peak at 9.8ish ppm I get the part where the 9.8ish ppm being the aldehyde group and 1ppm and 2ppm being CH2-CH3-R but i do not understand the ones at the 6's ppm
- CubbiLv 710 years agoFavorite Answer
im is right, but I'll add that this is likely /trans/ CH3-CH2-CH=CH-CHO.
the reason is, you said you see 6 peaks from the proton on C3, and I bet the intensities of that multiplet are 1:2:1:1:2:1, which is a doublet of triplets with triplet splitting 6-7Hz (CH2-CH) and the doublet splitting ~15Hz (trans CH=CH). A cis isomer would have had cis coupling closer to 8 Hz which would mess with your 7Hz aliphatic coupling giving you something that looks closer to a quartet. But I don't have a reference spectrum of the cis isomer to look at, so I can't be sure (those cis couplings can range anywhere from 0 to 12 hz, after all)
But the database chemical shifts for the trans isomer match your description :)
- AlisonMLv 410 years ago
OK, I've tried to work this out, but I'm not too sure I've got it. A couple of questions - at what frequency was the spectrum run? Also, what are the relative peak heights in the 6 line multiplet at 6.8 ppm?
It's likely that the compound is something like i) H2C=C(CHO)CH2CH2 or ii) CH3CH2CH=CHCHO - the alkene protons in these would come somewhere between 5.5 and 7 ppm. The problem I'm having is the couplings - in the case of ii above, the aldehyde signal would probably appear as a doublet, while there might also be some additional couplings in the case of i.
EDIT: Is there any way you can post a picture of the spectrum?
- Anonymous10 years ago
C5H8O has (5 - 8/2 + 1) = 2 degrees of unsaturation, so the molecule has either a ring or a double bond. There aren't enough DOU for an aromatic ring, so the signals at 6 and 6.8 are most likely a double bond,
Since 6.8 is a bit far downfield for an alkene, this molecule is probably conjugated. Most likely structure is
with C3 further downfield than C2 because of resonance.
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- Anonymous10 years ago
The 2H at 2 ppm is supposed to have five peaks, not four. I have the same question.
- GraceLv 44 years ago
I did not see the spectrum , But i suppose it is the second compound since in the first the sufaces would be equivalent