Spectroscopy help

Help I don’t get why for the Mark 3 (M3 on markscheme) they combined them both into one… I mean for the peak at 2.6 (quartet) I got confused as to if it’s a ketone or benzene with a CH on it because on the proton NMR sheet it’s like this (for ocr a) I’m so confused 😭 plz help me

for part b… and also for part a why is my answer incorrect I though D2O can do that too

(edited 11 months ago)

Reply 1

ghostyblop

Hi, so for the peak at 2.6 (quartet), the shift is too high for it to be a ketone so that rule's out one of your options.
If it was a methyl group attached to benzene you would expect to see a singlet as there are no adjacent protons on the next carbon, so that rule's out another one of your options.

Often when you see a quartet and a triplet together around 1-2ppm it often represents an ethyl group, although this might be something that is easier to see with practice. The reason the shift of the quartet high, is because it is attached to the benzene as you rightly said above it gives a larger shift. Because of this you can deduce that there is an ethyl group attached to the benzene.

It might be easier to think about it by starting with the benzene shift. You know that there are 4 protons attached to benzene that are in the same environment, this must mean you need to find 2 more functional groups that can attach to the benzene. We've worked out that one is an ethyl group, the other is a triplet aldehyde, which must mean it is connected to CH2, this can be verified by the doublet at 3.7ppm (the reason it has a higher normal shift is because it is attached to an aldehyde and benzene group so the protons are more deshielded.) So there must also be and ethanal group attached.

For part a your answer is incorrect. For one there isn't an alcohol fg in the molecule, if there was you should see the shift representing the alcohol completely disappear it wouldn't appear at zero. TMS gives a shift at zero to act as a standard.

I hope that was helpful, it's kind of hard to explain this stuff without drawing it out for you, if you have any questions/ I didn't explain it well enough feel free to ask again, it's late so I may have made a mistake lol :biggrin: