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Hello, I am trying to determine the chemical formula for the following compound. From the IR spec, I see there is a carbonyl present. Moving on to the mass spec, I see an M+ peak of 142, with what appears to be no Cl/Br present.

This brings me to a chemical formula of C10 H6 O (12 x 10 + 6 + 16 = 142), with a degree of unsaturation of 8, which seems off.

I feel I am doing something wrong and any help would be greatly appreciated, thank you!

  • $\begingroup$ Related cross post homework $\endgroup$ – Todd Minehardt Apr 25 '19 at 2:06
  • $\begingroup$ Look at your proton NMR integrals.This restricts the possible formulae with respect to the number of hydrogen atoms. $\endgroup$ – Nicolau Saker Neto Apr 25 '19 at 2:57
  • $\begingroup$ @JakeL The molecular mass which you have calculated is correct, but the formula instead of $\ce{C10H6O}$, it should be $\ce{C9H18O}$ with a degree of unsaturation $1$ , which corresponds to your ketone. Now your proton integrals add up to $9$, but here there are $18$, so the integrals now can be scaled up as $4:8:6$. Now it has to be a straight chain compound, and symmetric also... little thinking can give you the answer....it's $\text{Nonan-5-one}$. $\endgroup$ – Soumik Das Apr 25 '19 at 3:02
  • $\begingroup$ Ahh gotcha, makes sense now! It's been awhile since I've had to interpret spectra and I completely forgot about integral scaling in the hnmr. Thanks so much! $\endgroup$ – Jake L Apr 25 '19 at 4:04

What I was trying to say was that firstly the molecular formula has to be determined correctly. From the mass spectrum of the given molecule, the molecular ion peak ($\ce{M+}$) was determined as $\frac{m}{z}=142$, which corresponds to the molecular formula $\ce{C9H18O}$. Note that, it can also correspond to $\ce{C10H6O}$, but in your $^1\mathrm H$ NMR spectra, a total integration of at least $9$ is there, thus the latter consideration is anyway not possible to be the correct formula.

After determining the chemical formula, it's good to look for index of hydrogen deficiency(i.e. degree of unsaturation) of the molecule. If the formula is $\ce{C9H18O}$, the degree of unsaturation is $1$. From the IR spectra also, we can see a sharp distinct $\ce{C=O}$ stretch near $1700\ \mathrm{cm^{-1}}$, and also no aldehydic proton peak near $9.5{-}10\ \mathrm{ppm}$, therefore we can conclude there is a keto group present in the molecule.

Now, we come to analyse the $^1\mathrm H$ NMR spectrum, which shows only total relative integral of $9$, but remember the integrals in proton NMR are always relative, they are never absolute. So, you can scale them up or down relative to your choice. Here to match the total protons in our system, we have to scale it up by a factor of $2$. Therefore the integrals now correspond to $4,8$ and $6$ protons respectively, and also by this scaling we can also say that the molecule is symmetric in nature.

Now peak at $\delta_\mathrm H=2.5\ \mathrm{ppm}$ is splitted into a triplet and these correspond to the protons attached to your carbonyl, resulting in the most downfield shift, near $\delta_\mathrm H =1.5\ \mathrm{ppm}$, two peaks merge and therefore they are actually the methylene protons, and lastly near $\delta_\mathrm H\approx0.9\ \mathrm{ppm}$, the triplets correspond to methyl proton peaks and thus we arrive at the final most likely possible structure as,

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Analysis of the splitting pattern to find out the $^3J$ values, I leave it to you...

  • $\begingroup$ I disagree vehemently with "that firstly the molecular formula has to be determined correctly." You don't jump to conclusions with sort of a problem you need to gather evidence, then decide how that evidence fits into the puzzle. $\endgroup$ – MaxW Apr 25 '19 at 17:34
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    $\begingroup$ @MaxW Without the molecular formula in your hand, how do you proceed further ? How can you decide then whether there are one keto, or two keto or one ester etc. ? $\endgroup$ – Soumik Das Apr 26 '19 at 3:20
  • $\begingroup$ What I am trying to say is that you don't just look at the mass spec and try to work out the formula first before doing anything else. The hydrogen ratios in the NRM are one of the key clues. Also as I said it is an iterative process. There isn't any nice flowchart for this sort of problem. $\endgroup$ – MaxW Apr 26 '19 at 5:34
  • $\begingroup$ I think we're really having a violent agreement. I totally agree that figuring out the empirical/molecular formula is a very important step. I just disagree that it is the first thing to do. $\endgroup$ – MaxW Apr 26 '19 at 5:56

A more by the numbers approach is this. Gather some evidence before speculating about the molecular formula.

Do a first pass to see what "jumps out" of each of the spectra.

From the IR spectra:

  • The bands below 3000 are C-H indicting that this is a totally unsaturated hydrocarbon.
  • The strong peak at 1700 indicates that there is a carbonyl group in the compound. So this is either an aldehyde or ketone.

From the Mass spectra:

  • The $m/z$ peak is at 142 and pretty clean, no other major peaks around it. So this is a "small" molecule.

From the NMR spectra:

  • There is no aldehyde proton at 9-10, so the molecule is a ketone, not an aldehyde.

  • There are three sets of peaks with integrals 2:4:3. So there must be 9, 18, 27, ... hydrogen atoms.

  • The fact that all the peaks are at low values indicates that there is no hydrogen bonded to a double or triple bonded carbon.

Now guess about a molecular formula

Now there is enough evidence to speculate an alkane with a ketone group.

Guess a formula, see how the pieces fit with a further analysis of IR, NMR and mass spec data.

It's an iterative process...


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