[KLAV]

http://jasper.kaptest.com/content/me....ospq09g01.gif

Click the image for the spectrum

I keep getting confused/frustrated at these questions. I know it's Propanal but I'm trying to work out why the singlet at the far left isn't a doublet.

CH3-CH2-CH=O

I understand that it's the H in O=C-H but why doesn't it follow the N+1 rule and become a doublet? (plus I thought that it should be one because it's next to a Carbon with 2 hydrogens?)

[EierVonSatan]

It's a low resolution spectrum, the coupling constant is small for aldehydes and so you don't see it on low quality spectra

It should be a triplet. It's mean of an A-level question to put that in front of you tbh.

Edit: actually this isn't a spectrum at all, it's a drawing by someone who doesn't know anything about NMR - the quartet should be more downfield than the triplet due to the electron withdrawing effect of the carbonyl.

[KLAV]

(Original post by EierVonSatan)
It's a low resolution spectrum, the coupling constant is small for aldehydes and so you don't see it on low quality spectra

But yes, it should be a doublet. It's mean of an A-level question to put that in front of you tbh.

Edit: actually this isn't a spectrum at all, it's a drawing by someone who doesn't know anything about NMR - the quartet should be more downfield than the triplet due to the electron withdrawing effect of the carbonyl.

So it should be Doublet -> Quartet -> Triplet? That's how I would've answered it if it was it Propanal.

[EierVonSatan]

(Original post by KLAV)
So it should be Doublet -> Quartet -> Triplet? That's how I would've answered it if it was it Propanal.

It's a touch more complicated than that unfortunately:

CH₃CH₂CHO

The red protons, will give rise to a triplet at around 1 ppm due to being next two blue protons.

The blue protons however see both green and red protons. The simple n+1 rule would mean you'd get a quintet (5 peaks) around 2.5 ppm.*

The green proton would be a triplet between 9 and 10 ppm.

All in all, a bad question

*this isn't what a real spectrum would give, you would get something called a doubet of quartets, but I don't think you need that for this level AFAIK

[charco]

(Original post by EierVonSatan)
It's a touch more complicated than that unfortunately:

CH₃CH₂CHO

The red protons, will give rise to a triplet at around 1 ppm due to being next two blue protons.

The blue protons however see both green and red protons. The simple n+1 rule would mean you'd get a quintet (5 peaks) around 2.5 ppm.*

The green proton would be a doublet between 9 and 10 ppm.

All in all, a bad question

*this isn't what a real spectrum would give, you would get something called a doubet of quartets, but I don't think you need that for this level AFAIK

You meant to say that the green proton would appear as a triplet, EVS.

[EierVonSatan]

(Original post by charco)
You meant to say that the green proton would appear as a triplet, EVS.

I can't believe I did that Thanks