[qw098]

Hi,

Is it possible for a true sextet to exist in an H-NMR spectrum of a compound that only contains carbon, hydrogen and oxygen?

I tried drawing a structure that would show a true sextet... and I ended up being able to draw such a compound... look below. Does that mean that it is indeed possible to have a true sextet with a compound that only contains carbon, hydrogen and oxygen?!

Thanks.

[Honclbrif]

Beta protons of butyric acid look like a sextet to me.

http://www.sigmaaldrich.com/spectra/fnmr/FNMR009383.PDF

[qw098]

Wouldn't that be a triplet of quartets and not a true sextet?

Also, I just realized my structure would be a quartet of doublets of doublets, no?

[fledarmus]

To get a true sextet, you would need five protons on adjacent atoms in identical magnetic environments - I can't think of a structure that would fit that requirement.

[Honclbrif]

Would a triplet of quartets look the same as a sextet if J1 and J2 were the same?

[qw098]

Yes, I think it would.

But it still wouldn't be a "true" sextet, no?

[Dan]

To get a true sextet, you would need five protons on adjacent atoms in identical magnetic environments - I can't think of a structure that would fit that requirement.

It is mathematically impossible due to the valency of carbon.

[Goala]

Interesting

[qw098]

Would you mind expanding on why it is "mathematically impossible due to the valency of carbon"?

[Honclbrif]

If it fits the Pascal's Triangle ratio 1:5:10:10:5:1, isn't it a "true" sextet?

[Dan]

It depends on how we are defining a "true" sextet. I read it to mean, as fledarmus said, a proton split by the presence of five identical adjacent hydrogens - that is impossible.

The reason it's not possible is that five adjacent H must be spread over at least 2 carbons (because the valency of carbon is 4). In order to be 5 identical H, they would have to be spread over 5 carbons (5 being the lowest factor of 5) - but these 5 carbons cannot be connected to the same carbon (because the valency of carbon is 4). I hope that makes sense.

If the 5 H are not identical, but the coupling constants are the same by coincidence (e.g. butyric acid example given before), you do get a sextet - but I'm not sure it's a "true" sextet. What's a non-true (fake?, apparent?) sextet in that case?

[Honclbrif]

Where does it say the protons have to be identical or on the same carbon, and is there an IUPAC definition of a "true" sextet? So long as all the J values are the same who cares if the protons are on different carbons?

[orgopete]

If you have a spectrum and the J-values are the same, it will appear as a sextet.

If you have a compound, you can argue that the J-values will not be identical because the hydrogens cannot be identical.

[fledarmus]

My interpretation of the original question was that a true sextet was being distinguished from some other arrangement of peaks which just appeared to be a sextet. A true sextet would appear to be a sextet under whatever magnetic strength you were using; an overlapping doublet of quartets (for example) which appeared to be a sextet at 200 MHz wouldn't appear to be one at 400 MHz or at 60 MHz, because the chemical shifts change (in Hz) with the magnetic strength, but the coupling constants do not.

The only ways I can think of to have five identical protons split a single proton would be if the proton was attached to an atom which could make six bonds, and those bonds would be in the shape of a pentagonal pyramid. The typical pattern of six bonds to a single atom wouldn't lead to identical coupling between any one group and all five of the other groups. Another possibility would be an atom which had five bonds to protons and a sixth bond to another atom with a proton, where all five of the bonds to protons could rapidly interchange.

If you try to do it with just hydrogen and carbon, you end up with different numbers of protons on adjacent carbons - 3 and 2, or 2,2, and 1. These would not be in identical magnetic environments.

[qw098]

Well... if I look at this compound shown below... I would see a septet for the central hydrogen no? But it is impossible for me to get a true sextet right?

And by true sextet, I mean nuclei (A) coupling with five other nuclei (X) that are chemically distinct from A.