NMR question

On one of the NMR question I found the structure using hydrogen NMR and infrared spec
I found it was ester and I throught it would be C2H5COOCH3 but the answer was
CH3COOC2H5 and on the mark scheme it says don't accept C2H5COOCH3. How do I distinguish it from NMR spec. Do any of you guys understand the term peak being furthest downfield cuz it says there must be another peak furthest downfield for it to be
C2H5COOCH3. I really need help on this..

Reply 1

EierVonSatan

21

You can tell between those two using the ppm values and the integration of the peaks.

So for CH3COO- you would expect a peak at about 2.1 ppm with integration 3.

furthest downfield = highest ppm

Reply 2

BJack

19

You can tell from the splitting pattern (singlet vs quartet) and peak integral (3 vs 2 protons) which way round the alkyl groups are attached. The highest-shift protons (i.e. the furthest downfield) will be the ones on the carbon bonded to oxygen. This is because oxygen is electron-withdrawing, so the closer oxygen is to the protons, the more deshielded the protons are.

Reply 3

Merdan

OP

10

Original post by BJack

You can tell from the splitting pattern (singlet vs quartet) and peak integral (3 vs 2 protons) which way round the alkyl groups are attached. The highest-shift protons (i.e. the furthest downfield) will be the ones on the carbon bonded to oxygen. This is because oxygen is electron-withdrawing, so the closer oxygen is to the protons, the more deshielded the protons are.

Its also got integration patterns on which are 23-29-30 for 3 different environments. How do you derive 3 vs 2 protons and what does it tell you? Sorry I didn't study integration part of the nmr spec as I haven't got it in my notes so would be great if you go into detail pls?

Reply 4

BJack

19

Original post by Merdan

Its also got integration patterns on which are 23-29-30 for 3 different environments. How do you derive 3 vs 2 protons and what does it tell you? Sorry I didn't study integration part of the nmr spec as I haven't got it in my notes so would be great if you go into detail pls?

The peak integral helps you work out how many protons there are in a particular environment: the ratio of the peak integrals is the ratio of the number of protons in different environments. It's not 100% accurate, so you will often need to round sensibly. In this case, you can reasonably divide by 10 and round to say that your integrals are 2:3:3 — these are the number of protons you have in each environment.

In this case, if the two proton integral has highest shift, you can tell that the -C₂H₅ chain is attached to an oxygen. If the highest shift corresponds to a three proton peak, you can conclude that -CH₃ is attached to the oxygen.

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