Using Mass Spec etc to work out a compound and its structure - help!!!!!

Hi! I've been given a mass spec, infra-red and NMR spectrum for a compound and i need to work out what the compound is and its structure.

I get as far as find the infra-red values on a data booklet but the rest is beyond me!

Any help would be gratefully received!

(edited 9 years ago)

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Reply 1

Pigster

What functional groups does the IR suggest?

What is the molecular ion peak? (it's only ikle)

How many proton environments and total the integration values?

Try to work out the molecular formula.

There are only four isomers. Draw them. Which fits the splitting patterns on the NMR?

Reply 2

Ariella

Original post by Pigster

I think the mass spec shows 0-H or N-H and C-H functional groups (i thought the rest of the peaks were fingerprinting - no 100% sure though)

I presume as there 4 peaks on the NMR graph that there are 4 photon environments... but this is where I get a bit stuck. I read that "to determine the number of H’s in each signal, divide the integral height by the height for one proton and round to the closest integer" but Im not so sure how this is done... for example, do you simply take the peak of 4 and divide it by 1 to find that there are 4?!

Sorry, this is probably really simple.. once i've grasped the method i'll be sorted.

Reply 3

Pigster

The broad peak at 3300 on the IR suggests O-H. Using the molecular ion peak on the mass spec try to work out the molecular formula.

The integration values of 2,1,4,3 suggests it has a multiple of ten hydrogens. That should help you get the formula

Reply 4

Ariella

Original post by Pigster

Great - thank you! :smile:

So, MR is 56 because its the largest peak and if the number of Hs is a multiple of ten, I'm assuming (with an MR of 56) that there will be 5... and I already know there's an O.

So, 56-5-16=35

12x3=36 so there is probably going to be 2 C?

Therefore, the molecular formula is C3H5O? And there are 4 different hydrogen environments so that dictates the structure?

Reply 5

Pigster

56 is the base peak, not the molecular ion peak. The Mr is not 56.

Also, since when is five a multiple of ten?

Reply 6

Ariella

Original post by Pigster

56 is the base peak, not the molecular ion peak. The Mr is not 56.

Also, since when is five a multiple of ten?

Ok, its just difficult to think of a compound with an MR of 57 which has 10 hydrogens (57-10=47) and O (47-16=31) and then simply carbon (31/12=2.5???)

How can 10 hydrogens fit onto a compound with such a small MR ????

Reply 7

Pigster

Its not 57, either.

It is the right-hand most peak. I did warn you that it is only ikle. VERY ikle.

Reply 8

Ariella

Original post by Pigster

Its not 57, either.

It is the right-hand most peak. I did warn you that it is only ikle. VERY ikle.

Oh i see!!! Ok, I think its 2-methylpropan-1-ol (fingers crossed)

Thank you for your help!

Reply 9

Pigster

How many NMR peaks would 2-methylpropan-1-ol have? And what would the integration values be?

Reply 10

Ariella

Original post by Pigster

How many NMR peaks would 2-methylpropan-1-ol have? And what would the integration values be?

Completely forgot about NMR peaks - thanks! I think I'm ok with this one now.

I have a similar question however with another spectra (I completely understand if you don't want to help as you've already been so useful - so feel free to ignore this question if that's the case!!!)

Ive attached the spectra below and so far I've worked out that:

Infrared spectra:
• Peak at 3000cm-1 indicates C-H
Screen Shot 2014-05-11 at 09.59.48.png

NMR spectra:
• A multiple of 7 hydrogen atoms present
• 3 hydrogen environments
Screen Shot 2014-05-11 at 09.59.43.png

Mass spectra:
• Molecular peak of 79 (Mr is 79)
Screen Shot 2014-05-11 at 09.59.36.png

Working out the molecular formula:
• Mr is 79
• The only elements in this compound are C and H
• A ratio to fit this MR and the information from the NMR spectra is C6H7 (clearly this doesnt sound right and suggests ive done something wrong - Im conidering the idea that the fingerprinting below 1000 may indicate the presence of something other than just H or C)

Reply 11

Pigster

It would be interesting to know which exam board you're doing, not all boards would test molecules such as this.

There is a peak at 80, which is 1/3rd the size of the peak at 78. Similarly there are peaks at 63/65 and 49/51 which are 3:1 in size ratio.

How can you explain the peak at 3.3 on the NMR? What might the environment be?

Reply 12

Ariella

Original post by Pigster

Its a module at UoN :smile:

Im guessing that, therefore, its going to consist of three carbons bonded to 7 hydrogens with another atom attached to the last C ? No idea what the final atom is though (if that's even right!)

Reply 13

Pigster

Assume you are correct, how heavy is it? And how many bonds to carbon must it form? (or conversely, how many bonds does carbon have left unaccounted for?)

Then do a little research on relative (isotopic) abundances.

Voila.