1,2-ethanediol Proton Splitting

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letaps

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Hey,

For, 1,2-ethanediol would there be one triplet and one doublet?
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The NMR spectrum should only have a triplet and a singlet. The hydrogen on the alcohol, is a bit too far away (it's 4 atoms away) to be split by the hydrogen on the carbon.

Side note: Any long range coupling that might occur should be too small to see anyway, and I don't think that's on the MCAT.
 
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spatel said:
what about the hydrogen from the alcohol group?
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It would be a triplet (due to the hydrogens attached to the carbon) and a singlet (due to the hydrogen on the OH group). Under usual conditions, an OH proton does not split the NMR signal of adjacent protons. Also, the signal due to an OH proton is not split by adjacent protons. So it would be a triplet and a singlet not triplet and doublet.
 
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The two protons on the alcohol are in the same chemical environment, therefore its a singlet.

But aren't the hydrogens on the carbons in the same chemical environment as well? Why are they splitting eachother? because they aren't splitting the alcohol proton.
 
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herewego said:
The two protons on the alcohol are in the same chemical environment, therefore its a singlet.

But aren't the hydrogens on the carbons in the same chemical environment as well? Why are they splitting eachother? because they aren't splitting the alcohol proton.
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The triplet represents all the chemically equivalent hydrogens, and since they are chemically equivalent, there will only be one triplet for those hydrogens.
 
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Right, but what is splitting those chemically equivalent protons on the carbons? There are 4 chemically eq on the carbons, and the two on the OH. And they aren't splitting the OH proton right? I'm really having a brain freeze I'm usually fine with NMR.

Why isn't it singlet for the two hydrogens on the OH, singlet for all the rest of the hydrogens on the carbon, because they are chem equivalent and there's nothing left to split them.
 
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herewego said:
Right, but what is splitting those chemically equivalent protons on the carbons? There are 4 chemically eq on the carbons, and the two on the OH. And they aren't splitting the OH proton right? I'm really having a brain freeze I'm usually fine with NMR.
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They don't split the OH proton-nothing really splits the OH protons in NMR because (from what I remember anyway) the proton undergoes rapid exchange. It doesn't really stay on the oxygen too long for the NMR to detect splitting/coupling.
 
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So where is the triplet coming from..where are 2 extra protons that are splitting the 4 hydrogens on the carbons?

There are 6 hydrogens total yes? 2 are the OH protons. They aren't split and do not split. Therefore there are 4 hydrogens left. All 4 of those hydrogens are chemically equivalent.

SO

shouldn't there be a singlet for the OH protons, singlet for the 4 hydrogens on the carbons?
 
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herewego said:
So where is the triplet coming from..where are 2 extra protons that are splitting the 4 hydrogens on the carbons?

There are 6 hydrogens total yes? 2 are the OH protons. They aren't split and do not split. Therefore there are 4 hydrogens left. All 4 of those hydrogens are chemically equivalent.

SO

shouldn't there be a singlet for the OH protons, singlet for the 4 hydrogens on the carbons?
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Yes, there is a singlet for the OH protons, and no there is not a singlet for the 4 hydrogens. Just because there are chemically equivalent hydrogens doesn't mean that they do not split each other-it just means that there will be one signal representing them all-I think that's what's causing the confusion. So we already established that the OH hydrogen does not split, and so there is 1 signal represenitng BOTH OH hydrogens. There are two -CH2-groups, each splitting the other (2 H + 1=3), hence the triplet. Since there are two chemically equivalent -CH2- groups, there will be ONE signal representing them both (just like how there's 1 signal representing BOTH OH protons)-the only difference is the methylene hydrogens split each other. Don't confuse chemical equivalency with splitting.
 
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There's a mirror plane that I've depicted with a blue dotted line. The two red hydrogens are equivalent as are the green ones (they both have the same respective environments). The Red Hydrogens are split by the two green ones (2 H nearby + 1 =3), and vice versa. But since there's a plane of symmetry, they will have the same chemical shift so it will appear to be one big triplet of 4 H's. The alcohol hydrogen is too far away to be split, and will be a singlet (0 H + 1= 1). And since there is an axis of symmetry the other alcohol hydrogen has the same chemical shift.
 
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Consider an isopropyl group as another example: CH3-CH-CH3 (obviously it's not linear-it's sort of a v shaped molecule). So, a septet and doublet are characteristic of an isopropyl group. The hydrogens on the two methyl groups are chemically equivalent, and so they both split the hydrogen on the CH group (1+1=2, there's your doublet)-you'll only see ONE doublet to represent BOTH methyl groups. They are chemically equivalent, yes, but there's still splitting. The CH will split both methyl groups (6+1=7, there's your septet).
 
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pookiez88 said:
Yes, there is a singlet for the OH protons, and no there is not a singlet for the 4 hydrogens. Just because there are chemically equivalent hydrogens doesn't mean that they do not split each other-it just means that there will be one signal representing them all-I think that's what's causing the confusion. So we already established that the OH hydrogen does not split, and so there is 1 signal represenitng BOTH OH hydrogens. There are two -CH2-groups, each splitting the other (2 H + 1=3), hence the triplet. Since there are two chemically equivalent -CH2- groups, there will be ONE signal representing them both (just like how there's 1 signal representing BOTH OH protons)-the only difference is the methylene hydrogens split each other. Don't confuse chemical equivalency with splitting.
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I don't think you're right here. You only split when there isn't chemical equivalence. Chemically equivalent protons do not split. For example, here's the spectrum for 1,2-dichloroethane. There's no coupling.

http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi
 
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When 3 people were coming up with different answers than me I was getting worried..then I saw a diagram and then I thought "Holy .... this is serious."
 
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so what's the final word on 1,2-ethanediol: one triplet and one singlet?
 
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i don't understand this, how are there two singles: isn't there one singlet from the hydrogen from the -OH groups and than a triplet from the -CH2 group?
 
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So is this correct for the following two examples:

CH3-CH2-CH2-CH2-CH3 : two triplets and one quartet

CH3-CH2-CH2-CH3: two singlets
 
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spatel said:
So is this correct for the following two examples:

CH3-CH2-CH2-CH2-CH3 : two triplets and one quartet

CH3-CH2-CH2-CH3: two singlets
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No, pentane has a triplet for the terminal carbons, a sextet for the second carbons in, and a quintet for the middle carbon.
http://www.muhlenberg.edu/depts/chemistry/chem201woh/1Hpentane.html

Butane: a quartet and a triplet

The quartet is from CH2 being split by the terminal CH3
The triplet is from CH3 being split by CH2.
The CH2s don't split each other because they're equivalent.
They're messy, but I'm pretty positive that's a quartet and a triplet
 
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so, pentane has two triplets, two sextets, and one quintet and was i correct for butane (2 singlets)?
 
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Okay, this really pissed me off. I took AAMC#10 this morning only to find that this is an EXACT QUESTION from it. Seriously, can you not read the thread instructing you to only post AAMC questions in designated threads??
 
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I know! For $35 I expect an unspoiled test. The OP has posted literally at least a dozen individual threads with AAMC questions. But at least those warned you in the subject line!
 
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Seriously, can we ban this guy? capn is right, he has made a dozen other threads, after repeatedly being told not to.
 
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