June 05, 2020, 04:40:05 PM
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Topic: How to identify different carbon and hydrogen environments!!  (Read 18106 times)

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Offline sameeralord

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Hello guys  :)

I'm back ;D. I hope you guys can remember me. I should rethank everyone who has helped me so far because my understanding of chemistry is now a lot better :). Any ways we are doing spectroscopy now. This is NMR and I have trouble with identifying different carbon and hydrogen environments.

For example

CH3CH2CH3

CH3CH2CH2OH

I'm confused as how many different hyrdogen environments are in here. Also in NMR with peak splitting lets say a carbon is between two carbons and those carbons have 3,4 hydrogen repectively how do I know there would be N+1 4 or 5 peaks.

Any help would be as always well appreciated ;)

Offline sjb

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Re: How to identify different carbon and hydrogen environments!!
« Reply #1 on: May 08, 2008, 10:35:25 AM »
One way to look for different environments is to replace (for instance) the each hydrogens with a dummy element X, and see if you have different compounds as a result.

So how may are there in propane? CH2XCH2CH3, CH3CHXCH3, CH3CH2CH2X (but this last one is the same as the first), so there are only two environments here.

How many in propan-1-ol (your second structure)?

Your second question is a little more involved. Typically, if the environments are the same, then n protons will split your signal into n+1 peaks, but if they're different, then each will theorectically split in it's own right - but you may not be able to see each of the individual peaks.

So in propane (again), the methyl protons (on the end) see the methylene (in the middle), and as there are 2 methylene, you observe a triplet. Similarly, each of the methylene protons sees 6 methyl protons, and you should be able to see a septet, but here the end peaks are relatively weak - do you know about the actual intensities of the splitting pattern - so you may not see them.

At one level, if for instance you had 1-bromopropane, the situation is similar for the 3-methyl, and the 1-methylene - both splitting into a triplet, but as the two neighbouring environments for the 2-methylene are different, you might observe a quartet of triplets (or vice versa) but the J-values may be very close, so you might not get resolved peaks

S

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