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Topic: Create the structure for C11H14 based off this NMR spectrum  (Read 4049 times)

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Offline PaulSzczesniak

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Create the structure for C11H14 based off this NMR spectrum
« on: November 30, 2014, 02:45:16 PM »
This is what I have thus far. C11H14 has an IHD of 5. The multiplet way downfield is a pretty good indication of a mono-substituted benzene ring so let's start with that (5 hydrogens, 6 carbons used = C5H9 left). The benzene ring consists of three pi bonds and, obviously, one ring therefore occupying 4 of the 5 pi bonds/rings = 1 pi bond must be made. This is probably the double bond required for the acid catalyzed hydration. With respect to the spectrum, specifically integration number (#H's at each signal), # of signals, and splitting pattern I believe this concludes:

Four separate signals must be present (excluding the multiplet):

A). Quartet ≈5.4ppm (1H): 3 neighboring hydrogens
B). Doublet ≈2ppm (3H): 1 neighboring hydrogen....(I'm thinking this signal to be a deshielded methyl group due to benzene ring?)
C). Quartet ≈2ppm (2H): 2 equivalent H's with 3 neighboring hydrogens
D). Triplet ≈1ppm (3H): another possible methyl group considering the expected ppm...2 neighboring hydrogens

So I'm just having trouble trying to piece these carbons and hydrogens together while maintaining a double bond somewhere in the midst of that. Any hints/help appreciated. Thanks in advance.

Offline kriggy

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Re: Create the structure for C11H14 based off this NMR spectrum
« Reply #1 on: November 30, 2014, 04:01:47 PM »
Well the best way is to draw the structures from data you have.
You know you have monosubstitued aromatic ring, how many carbons/hydrogens are in that substituent? You know you have 2 methyl groups, what they connected to? (you can realize that from splitting of the signals). Then, what are those connected to from the "other side"? (again you can tell from splitting of those how many hydrogens are on the other substituent). Then

Offline PaulSzczesniak

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Re: Create the structure for C11H14 based off this NMR spectrum
« Reply #2 on: November 30, 2014, 05:10:43 PM »
Well the best way is to draw the structures from data you have.
You know you have monosubstitued aromatic ring, how many carbons/hydrogens are in that substituent? You know you have 2 methyl groups, what they connected to? (you can realize that from splitting of the signals). Then, what are those connected to from the "other side"? (again you can tell from splitting of those how many hydrogens are on the other substituent). Then

I got:

CH3-CH2-[BENZENE RING]-CH=CH-CH3

I keep getting this but it doesn't seem consistent with the spectrum. The two hydrogens among the C=C double bond are nonequivalent, no? My thought process was that they are enantiotopic hydrogens therefore there is no signal difference.


Offline OrgXemProf

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Re: Create the structure for C11H14 based off this NMR spectrum
« Reply #3 on: November 30, 2014, 10:10:47 PM »
1. What structural information can be gleaned from the fact that Compound A undergoes acid catalyzed hydration to afford Compound B?

2. The 5H multiplet located between delta 6.8-7.4 in the proton NMR spectrum for Compound A argues against a disubstituted benzene ring.

3. One vinyl proton at delta 5.4 appears to be coupled to a CH3 group.

4. CH3CH2 absorptions are located upfield; the CH2 quartet is located ca. 1 ppm at lower field than the CH3 triplet; what structural feature in Compound A does this suggest?

5. The chemical shift of a CH3 doublet is located very close to that of a CH2 quartet, which suggests that these two substituents are situated in similar chemical/magnetic environments.

6. Glue the pieces together to form a complete C11H14 structure. Then, some additional points may bear further consideration:

(a) Is the NMR spectral data sufficient to permit you to arrive at a single structure conclusively?

(b) Are stereochemical considerations potentially important?

(c) If two or more structures appear to fit the given NMR data, which NMR experiment(s) might be performed to distinguish among possible structures for Compound A?


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