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Proton NMR ><

Okay, I decided to re-read through my book on the proton NMR section since I don't really understand it. But.. once again, I am insanely confused. First of all there was a question to outline the chemical shifts of different peaks, and hence find out what molecule it is (from the data sheet). I can easily read the chemical shifts, and if I see it as something from 3.5 - 4, the answer says it's something like 1 - 5, when the peak is clearly no where near TMS.

Second problem is to do with the peaks itself.

It's hard to read that, so refer to the chromatogram and I'll write out the text:

Molar Mass of C2H4O = 24 + 4 + 16 = 44 g/mol
Therefore the molecular formula isC4H8O2
The NMR spectrum shows three peaks, so there are three different proton environments.

Okay, nice and simple, that's easy to understand. Next bit:

The ratio between the peaks is 3 : 3 : 2. This adds up to 8, which indicates that the peak at 1.2 (closest to TMS) is a triplet, indicating that the proton is next to a CH2. The peak at 4.2 is a quartet, indicating that the proton is next to a CH3. THis indicates the presence of a CH3-CH2-grouping. The peak at 2.1 is a singlet suggesting that it is a CH3- and that the adjacent carbon has no hydrogen. The data sheet confirms that CH3- bonded to a carbonyl absorbs in the region 2.0-2.9ppm.

Hmm.. how is there a peak ratio of 3 : 3 : 2? I deduced 3 : 2 : 1, and even that is pushing it. There is no way that there are two peaks of equal height there. It can't be a typo either as the working out all seems to 'work' after it.

Can someone please enlighten me as to what I'm not understanding >< ?

(edited 13 years ago)

Reply 1

EierVonSatan

The integration trace (which isn't shown) tells you how many protons are in each signal, not the height of the peaks :smile:

Reply 2

username297163

Original post by ViralRiver

It's hard to read that, so refer to the chromatogram and I'll write out the text:

Okay, nice and simple, that's easy to understand. Next bit:

Hmm.. how is there a peak ratio of 3 : 3 : 2? I deduced 3 : 2 : 1, and even that is pushing it. There is no way that there are two peaks of equal height there. It can't be a typo either as the working out all seems to 'work' after it.

Can someone please enlighten me as to what I'm not understanding >< ?

Looks like george facer book and theres rarely any mistakes in those text books...what page is it?

Reply 3

ViralRiver

Original post by EierVonSatan

The integration trace (which isn't shown) tells you how many protons are in each signal, not the height of the peaks :smile:

Sorry, I don't quite understand what you mean =\ . What do the height of the peaks show? Where did they get 3 : 3 : 2 from,?

Reply 4

ViralRiver

Original post by Mr_Muffin_Man

Looks like george facer book and theres rarely any mistakes in those text books...what page is it?

http://www.amazon.co.uk/OCR-A2-Chemistry-Textbook-Older/dp/1844894355/ref=sr_1_6?ie=UTF8&qid=1290962622&sr=8-6

It's that book, page 99.

Reply 5

ViralRiver

Okay, I've gone over it over and over again. I simply cannot see where they got the 3 : 3 : 2 from =\ .

Reply 6

username297163

Original post by ViralRiver

http://www.amazon.co.uk/OCR-A2-Chemistry-Textbook-Older/dp/1844894355/ref=sr_1_6?ie=UTF8&qid=1290962622&sr=8-6

It's that book, page 99.

ah right it looked like another book

but as you posted before there is a CH3 group next to a CH2 group and a CH3 group next to an O but not an OH if the ratio is 3:3:2

but in very simple terms the reason its 3:3:2 is because there are 2 different CH3's (3:3) and a CH2 (3:3:2)

if it was a 3:2:1 as you said it would mean there would be something like an -OH group a CH2 group and a CH3 group

I found that the integration numbers tell you how many protons there are but im nto sure if you can say that for everything...for example if you had 1 tall peak of 9 you could see that it would mean 3 identical CH3 groups (3x3hydrogens)

but yh hope this post makes sense

Reply 7

ViralRiver

I understand that 3 : 2 : 1 wouldn't make sense with the final result, and why 3 : 3 : 2 works. But I'm just not sure how they worked it out. How can you find that peak ratio by looking at the peaks? oO - do the heights not mean anything?

Reply 8

username297163

Original post by ViralRiver

erm i think the heights just mean the absorption

but this is why i quite like nmr because you have to use all the information youve got to see what you have

basically you know that you have a CH3 singlet which means its not next to anything with hydrogens so you could draw a CH3-O-

and you can go on like that to see how many hydrogens you have from the molecule you worked out....alot of it is long and trial and error....thats the way i do it

if anyone else has a faster way id be glad to hear it though

Reply 9

EierVonSatan

Original post by ViralRiver

Sorry, I don't quite understand what you mean =\ . What do the height of the peaks show? Where did they get 3 : 3 : 2 from,?

The heights of the peaks don't tell you anything, the area underneath the peaks tell you how many protons are in that signal. The height of the integration trace (the squiggly line above the peaks *not shown on that spectrum*) tells you this.

Who got what from where? Surely it's given in the question (or there is a trace and I can't see it).

Reply 10

ViralRiver

The question just says Compound A has the empirical formula C2H4O and a molar mass of 88. Deduce the identity of compound A. The proton NMR spectrum of compound A is shown below, then the image thing, then the block of text for the answer.

So, without the trial and error method, how can I see that there is a 3 : 3 : 2 ratio in the peaks?