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Topic: Structure from Mass Spectra Info and IR Info  (Read 5176 times)

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Offline bones

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Structure from Mass Spectra Info and IR Info
« on: May 27, 2018, 11:24:10 AM »
So, I've been reading in my book, online, and watching videos, and I've gotten very close to figuring this problem out but I'm having trouble getting the final structure(s).  I've determined the correct chemical formula for the compound, given the information below; C5H11N  I know the IR spectra info is supposed to be used to help determine the structure of the compound but I can't seem to figure it out.  I think maybe the nitrogen may be surrounded by three carbons, because it says there are no IR peaks in the 3000 area where one would expect to see an N-H stretching.  If someone could please give an explanation it would be greatly appreciated.

The mass spectrum of compound A shows the molecular ion at m/z 85, an M + 1 peak at m/z 86 of approximately 6% abundance relative to M, and an M + 2 peak at m/z 87 of less than 0.1% abundance relative to M.

Assuming that compound A has only C, H, and one N atoms, determine the molecular formula, and then draw a possible structure if compound A has IR absorption at 1620-1680 cm-1 but not at 3010-3090 cm-1 .

If more than one structure satisfies the given information, draw them all.
Specific stereochemistry not required.

Offline wildfyr

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Re: Structure from Mass Spectra Info and IR Info
« Reply #1 on: May 27, 2018, 03:58:59 PM »
Well I can think of a few isomers build around the same core that fulfill these parameters.

What do we know right off the bat about the types of bonds present from just the molecular formula C5H11N. Additionally, what functional groups give bonding between 1620 and 1680? Ignore the 3010-3090 for a minute.

Offline bones

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Re: Structure from Mass Spectra Info and IR Info
« Reply #2 on: May 27, 2018, 08:08:47 PM »
Well, since the degree of unsaturation is zero, there can be no C-C double bonds.  The 1620-1680 fits C-N double bond on the correlation chart, so it could be an imine, because there's no peak that would indicate an N-H bond.

So right now I have butan-2-imine and butan-1-imine that could be the correct structure (not sure if that's the right nomenclature; we haven't gotten to imines in class yet). 2-ethyl imine propane and 1-ethyl imine propane work (again just making up nomenclature to convey my structures).  I can also swap where the double bond is in both cases to the shortest carbon chain, leaving me with 8 isomers that fit those parameters.

EDIT:  It was due tonight so I went ahead and entered these structures, and it says the ONLY possible structure was 3-methyl-2-buten-2-amine.  But there's no IR where a N-H stretch should be, so this is confusing.
« Last Edit: May 27, 2018, 10:47:56 PM by bones »

Offline wildfyr

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Re: Structure from Mass Spectra Info and IR Info
« Reply #3 on: May 28, 2018, 12:30:38 AM »
Hm.... well your conclusion that there was no unsaturation is incorrect, there was 1 degree of unsaturation. I think the key is wrong if there are no N-H stretches. However N-H stretches typically do happen higher than 3100, so that didnt really rule anything out.

My conclusion was that it was N-(Methyl)(1-methyl)propyl-1-Imine. That gives no N-H, but does give C=N stretch at about 1650, 1 degree unsaturation, and correct of atoms.

Offline bones

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Re: Structure from Mass Spectra Info and IR Info
« Reply #4 on: May 28, 2018, 10:50:42 AM »
How are you getting the unsaturation of 1?  My calculation was (5+1)-(11/2)+(1/2) = 6 - 12/2 = 0.

Offline wildfyr

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Re: Structure from Mass Spectra Info and IR Info
« Reply #5 on: May 28, 2018, 12:42:15 PM »
I frankly didn't do the math, but with no unsaturation it's C5H13N. Like diethylmethylamine.

N adds 1 proton. Since you have H11 that implies unsaturation=1.

I don't really follow your arithmetic. 2n+2 rules applies for converting # of carbons to hydrogens. Nitrogen adds 1 hydrogen, halides remove 1H. Oxygen and sulfur do not affect the number of hydrogens. For every H2 you are missing after this math its 1 unsaturation.
« Last Edit: May 28, 2018, 01:15:32 PM by wildfyr »

Offline bones

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Re: Structure from Mass Spectra Info and IR Info
« Reply #6 on: May 28, 2018, 05:37:51 PM »
I use the formula they taught me in organic 1, which is (C+1)-((H+X)/2)+(N/2). When I do (2n+2)-(our compounds hydrogen’s)+N/2 it never seems to work out.

Offline wildfyr

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Re: Structure from Mass Spectra Info and IR Info
« Reply #7 on: May 28, 2018, 06:21:05 PM »
Try a few simple molecules (such as chlorobenzene, nitroethane, acrylonitrile, 1-decene, pentanoic acid, isobutane) and see how the calculations work out when you already know the molecule.

The formula I know is degree of unsaturation=[2C+2+N-halides-H]/2.

This site discusses it in more detail https://chem.libretexts.org/Core/Organic_Chemistry/Alkenes/Properties_of_Alkenes/Degree_of_Unsaturation
« Last Edit: May 28, 2018, 06:39:46 PM by wildfyr »

Offline hypervalent_iodine

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Re: Structure from Mass Spectra Info and IR Info
« Reply #8 on: May 28, 2018, 07:27:49 PM »
How are you getting the unsaturation of 1?  My calculation was (5+1)-(11/2)+(1/2) = 6 - 12/2 = 0.

I use that equation for DBE as well, and get 1. You aren't doing the math correctly. Remember you are dealing with -11/2, not +11/2, so you cannot simplify -11/2+1/2 to give 12/2. Put another way, if you wrote it as 1/2-11/2, what do you get?


Offline bones

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Re: Structure from Mass Spectra Info and IR Info
« Reply #9 on: May 29, 2018, 05:44:23 PM »
Yes I see now where I made the mistake.  The way I've been looking at it is (C+1)-((H+X)/2+N/2), but now I realize that the N/2 is not bound to the H+X/2 in the way I do the equation.

And wildfyr, I am using that equation, I just learned it in an expanded form where you divide each term by 2 individually, so you get C+1 instead of 2C+2.

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