Confused with NMR spectroscopy.

I had a question in a past paper I did, and the 13C spectrum showed 4 peaks - so 4 environments (correct)

I then worked out all the other information in the question, and couldn't figure out a structure. So I guessed, ethyl benzanoate, and managed to get this correct. But I was skeptical of this. Wouldn't this have 5 environments? I will send an image of question, mark scheme and an annotation i made to see why I thought five.

Reply 1

willwillwill1

Reply 2

hwhejfjejsjx

Bump

Reply 3

TypicalNerd

Original post by willwillwill1

I would argue the mess around δ = 130 ppm looks more like four peaks that are just really close together. These are your arene peaks and I would expect there to be four carbon environments within the benzene ring rather than just 2.

If we call the carbon to which the carbonyl is bound carbon 1, then number the other carbons clockwise up to 6, you should identify four environments:

Carbon 1 is its own environment

Carbon 2 and carbon 6 make a second environment

Carbon 3 and carbon 5 make a third environment

Carbon 4 is its own environment

All the carbons you’ve labelled “1” have differing distances from the carbonyl group and so the effect of the carbonyl group on the shift for each one is different. Just because they are arene carbons that each have a hydrogen on them doesn’t mean they are all the same environment.

(edited 8 months ago)

Reply 4

Pigster

Original post by willwillwill1

I had a question in a past paper I did, and the 13C spectrum showed 4 peaks - so 4 environments (correct)
I then worked out all the other information in the question, and couldn't figure out a structure. So I guessed, ethyl benzanoate, and managed to get this correct. But I was skeptical of this. Wouldn't this have 5 environments? I will send an image of question, mark scheme and an annotation i made to see why I thought five.

You're getting H-NMR and C-NMR confuzzled.

Here, there are a total of 7 C-NMR peaks: there are four different peaks at around 130. On the H-NMR, there are a total of three peaks: the one of right is a triplet with a shift value between 0.5-2 (caused by the environment you have labelled 5 on your diagram), there is the quartet with a shift between 3-4.3 (which you have labelled 4) and there is the heavily split third peak (yes, I know it looks like 2 peaks) with a shift between 5.3-8, which is in the benzene environment.

C-NMR shows a separate peak for each C env. in the benzene ring.
H-NMR shows ONE peak for the entire benzene ring, always found between 5.3-8 and often heavily split.

I think you're doing OCR A, in which case, the spec. says: "Learners will be expected to identify aromatic protons from chemical shift values but will not be expected to analyse their splitting patterns."