I'm not sure how to even start these two problems. How do you predict the structure of something when its molecular formula isn't provided to you? Also how do you calculate percent purity from this given data?
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1 Answer
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Look first at the C13 spectrum which is labelled up to show you have one methyl group, two CH2 groups in very different environments and a quaternary C. Then look at the proton spectrum which shows a methyl group with very small coupling with a shift suggesting it is attached to a double bond, 2 protons with a shift suggestive of being on a double bond, and a singlet CH2 with a shift suggesting that the carbon it is on has a heteroatom attached. The MS shows that the molecule contains Cl. This is not compatible with 3-Cl-1-butene.
I would suggest that the CH2 at 4ppm is due to a -CH2Cl group. The two protons at 5ppm are due to =CH2 which is why they show little coupling. The very small coupling shown by the Me group is suggestive of allylic coupling. If you draw out the possible structures of C4H7Cl the one that best fits is 3-Cl-2-methyl-propene.
The purity of the sample is most easily measured by comparing the integration of the Me group with that of the small doublet beside it which is the material named on the bottle, 3-Cl-1-butene.
answered May 22, 2018 at 10:53
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$\begingroup$ Ahhhh that's what I missed, I didn't see that there was a Cl from the mass spec! Thank you! $\endgroup$ Commented May 22, 2018 at 11:02
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$\begingroup$ One other question actually, why do we look at the integration of the methyl group and compare it to the small doublet beside it when it comes to analyzing purity? This was a "challenge" problem assigned by the professor and I don't think he went over this in our ochem2 class... $\endgroup$ Commented May 22, 2018 at 12:00
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$\begingroup$ The integration is a quantitative measure of how much material is present in the mixture. If the value of the integral of the small doublet is 5% of the main Me peak then the sample is 95% pure. $\endgroup$ Commented May 22, 2018 at 12:28