NMR symmetry

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yellowjellybean

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What exactly are they talking about when they say the spectrum shows symmetry in the structure? If a a structure had perfect plane symmetry (like say in a benzene) would there only be half the number of signals on the NMR spectrum?
 
What exactly are they talking about when they say the spectrum shows symmetry in the structure? If a a structure had perfect plane symmetry (like say in a benzene) would there only be half the number of signals on the NMR spectrum?

Only unique hydrogens will have unique signals. For benzene, while you have six hydrogens, an NMR spectrum will only show one signal because ALL of the hydrogens are the same (same spatial orientation relative to other atoms). So not necessarily half the number of signals.

If you can give me more info about the problem/passage you were having trouble with, I can probably help more.
 
Oh ok, so what would the integration be for that one signal for a benzene? Just 1H?
So for a benzene with two chlorine substituents para to each other would have a 2H singlet but I guess I am concerned if they ever showed me a spectrum of half a symmetric molecule, how would I be able to know how to draw the structure?

One more question. If we want to determine the splitting of Ha. But the atom adjacent to the carbon of Ha is NOT a carbon, but an oxygen that has Hb. Will Hb still split Ha?

Thank you so much
 
Oh ok, so what would the integration be for that one signal for a benzene? Just 1H?
So for a benzene with two chlorine substituents para to each other would have a 2H singlet but I guess I am concerned if they ever showed me a spectrum of half a symmetric molecule, how would I be able to know how to draw the structure?

One more question. If we want to determine the splitting of Ha. But the atom adjacent to the carbon of Ha is NOT a carbon, but an oxygen that has Hb. Will Hb still split Ha?

Thank you so much

Think about it like this: all the hydrogens are exactly the same so when you look at an NMR spectrum, they all have a signal but since the signals all show up at the same exact field strength, it just looks like one signal.

Integration would be 1H. The integral is useless alone as far as i know since there'd be nothing to compare it too, it's really just a relative measure between areas of the different peaks.

Benzene with two para Cl: All hydrogens are the same so only expect one peak, again 1H singlet. If they just provided you with this NMR, there's no way of telling what the compound would be (well, if you really dive into the ppm you may have a shot). You'd most likely be given additional spectra or info that will tell you # of carbons, degrees of unsaturation etc.

Your last question, are you asking if the hydrogen on an adjacent oxygen will play a role in the splitting? It will and for this reason, CDCl3 is used rather than CHCl3. The splitting will be more complex than a simple doublet though depending on the other adjacent carbons. Usually, the oxygen's hydrogen will appear as a much broader peak though (due to h-bonding leading to rapid switching) so I wouldn't read much into the splitting on that hydrogen itself.

If you are unsure about stuff like this, a good way to go about it is to google for the NMR spectrum of simple compounds that will illustrate your question.
 

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