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Proton NMR

I really am struggling with this question, could anyone walk me through it?
https://www.ocr.org.uk/Images/171722-unit-h432-02-synthesis-and-analytical-techniques-sample-assessment-materials.pdf
Its number 21.
I can analyse the spectra but I just can't seem to make the compound match up to the spectra
il try and explain it on here :smile:

so it gives you the empirical formula which is C3H6O, but the amount of hydrogens represented on the H NMR is 12, so the molecular formula needs to multiplied by 2, so is C6H12O2 (so start thinking of types of molecules that contain 2 oxygens (carboxylic acids, esters etc)
if you look at the d20 spectrum, anything on their that has disappeared from the one above is the OH group, as there is only 1 disappeared it so only has one OH group so other oxygen must be a C=O

so the peak at 3.7ppm = OH
peak at 1ppm = is a doublet, but 6 in environment and adjacent to a carbon with 1 hydrogen so CH(CH3)2
peak at 2.7ppm = multiplet, but only one in environment, so makes me think its the H from the CH(CH3)2 - but as it is 2.7 I think it is the carbon that is next to the C=O
so at the moment the molecule looks like - CH(CH3)2C=O
peak at 3.1 = triplet, and 2 in environment, CH2 - CH2
and peak at 3.8 ppm = triplet, 2 in environment CH2 - CH2 - O
so, adding that along with the OH should look like this:
CH(CH3)2C=OCH2CH2OH

then check that the number of carbon environments matches up to the 5 on the spectra, and that should work, hopefully that helped!
(edited 1 year ago)
Reply 2
Original post by Harriet234
il try and explain it on here :smile:

so it gives you the empirical formula which is C3H6O, but the amount of hydrogens represented on the H NMR is 12, so the molecular formula needs to multiplied by 2, so is C6H12O2 (so start thinking of types of molecules that contain 2 oxygens (carboxylic acids, esters etc)
if you look at the d20 spectrum, anything on their that has disappeared from the one above is the OH group, as there is only 1 disappeared it so only has one OH group so other oxygen must be a C=O

so the peak at 3.7ppm = OH
peak at 1ppm = is a doublet, but 6 in environment and adjacent to a carbon with 1 hydrogen so CH(CH3)2
peak at 2.7ppm = multiplet, but only one in environment, so makes me think its the H from the CH(CH3)2 - but as it is 2.7 I think it is the carbon that is next to the C=O
so at the moment the molecule looks like - CH(CH3)2C=O
peak at 3.1 = triplet, and 2 in environment, CH2 - CH2
and peak at 3.8 ppm = triplet, 2 in environment CH2 - CH2 - O
so, adding that along with the OH should look like this:
H(CH3)2C=OCH2CH2OH

then check that the number of carbon environments matches up to the 4 on the spectra, and that should work, hopefully that helped!

Thank you! This helped a lot :smile: I think I get confused with the chemical shifts, so for example when you looked at the triplet at 3.1ppm, on the proton nmr shifts it says that should be HC-O, but it's actually HC-R right? I don't know if you get what Im saying there I might just be going mad, but that's what trips me up a lot
Original post by Carm15796
I really am struggling with this question, could anyone walk me through it?
https://www.ocr.org.uk/Images/171722-unit-h432-02-synthesis-and-analytical-techniques-sample-assessment-materials.pdf
Its number 21.
I can analyse the spectra but I just can't seem to make the compound match up to the spectra

Harriet's work above is good, but there a couple of other pointers that could help.

1. The M+ ion from the mass spec is 116 and you can see that the empirical formula is C3H6O, which has a formula mass of 58. This gives you the molecular formula rapidly. C6H12O2

2. It is often useful to do an Index of Hydrogen Deficiency on the molecular formula. With 6 carbons you would expect 2n + 2 hydrogen atoms (the oxygen has no effect) = 14 hydrogen atoms. There are only 12, so you must have either a ring or a double bond in the structure.

3. The expanded signal at 2.7 shows 7 absorptions, this can only be due to (CH3)2CH- this group must go at the 'end' of the molecule.

4. The heptet (above) is matched with the doublet due to the CH above.

5. The two triplets have to be -CH2-CH2-

6. The D2O shake shows one acidic hydrogen atom (either OH, or COOH), but the signal at 3.7 is not possible for carboxylic acid, therefore alcohol.

7. There is no integral of 3, therefore no CH3 group at the other end.

(CH3)2CH-CO-CH2CH2OH

This also fits with the 13C resonance of 5 signals
Original post by Carm15796
Thank you! This helped a lot :smile: I think I get confused with the chemical shifts, so for example when you looked at the triplet at 3.1ppm, on the proton nmr shifts it says that should be HC-O, but it's actually HC-R right? I don't know if you get what Im saying there I might just be going mad, but that's what trips me up a lot

no I do !!! I used to swell, my teacher told me its about electronegativity, so the closer an atom gets to the more electronegative one (in this case oxygen) it gets moves along the spectrum more (sometimes in papers they will tell you this) but glad it helped :smile:
Reply 5
Original post by Harriet234
no I do !!! I used to swell, my teacher told me its about electronegativity, so the closer an atom gets to the more electronegative one (in this case oxygen) it gets moves along the spectrum more (sometimes in papers they will tell you this) but glad it helped :smile:

Ohhhh that makes so much more sense! I was getting so confused when 3 peaks were all due to HC-O lmao. One final thing, on a proton nmr question in an exam, how would you present the information to get marks - like in terms of wording how would you say that x chemical shifts corresponds to x structure
Original post by Carm15796
Ohhhh that makes so much more sense! I was getting so confused when 3 peaks were all due to HC-O lmao. One final thing, on a proton nmr question in an exam, how would you present the information to get marks - like in terms of wording how would you say that x chemical shifts corresponds to x structure

so I tend to annotate the graphs before I try and write it up, just so when I actually write it I know what i am talking about. I tend to go in the order the information is given just so its easy to follow, and il say for example ' on the H NMR for X, the peak at 3.2ppm is a doublet, therefore due to N+1 rule is adjacent to 1 hydrogen, there are 3 in the environment, then underneath it I will just bullet point possible structures I think it could be, then when drawing it up will circle the correct ones. and just repeat that for all the peaks/graphs it gives you etc !
I'ts also important to mention if your ruling out a certain compound to say why it is not this one, as that often gets marks too !!
Reply 7
Original post by Harriet234
so I tend to annotate the graphs before I try and write it up, just so when I actually write it I know what i am talking about. I tend to go in the order the information is given just so its easy to follow, and il say for example ' on the H NMR for X, the peak at 3.2ppm is a doublet, therefore due to N+1 rule is adjacent to 1 hydrogen, there are 3 in the environment, then underneath it I will just bullet point possible structures I think it could be, then when drawing it up will circle the correct ones. and just repeat that for all the peaks/graphs it gives you etc !
I'ts also important to mention if your ruling out a certain compound to say why it is not this one, as that often gets marks too !!

I've started annotating the graph now and you're right it makes it a lot easier, I'll try to stick to what you said with structuring it :smile: thank you so much for all the help!!

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