I have a question about proton nmr, or more specifically the proton nmr of 1,4-diethylbenzene. as part of a presentation we were asked to predict the proton nmr of 1,4-diethylbenzene which I did. However our lecturer disagreed with the predicted nmr, and while I agreed with his point then (not good practice to argue with the person marking you) I am not convinced his point was correct. For the four protons on the benzene ring I predicted (as did most other people) that you would get a 4 proton tall doublet at around 8. The lecturer says it would be a singlet. Which is it?. I had considered that it may be a singlet since all four protons are essentially identical but the book I used very briefly suggested a doublet. Even though the protons are all the same I personally thought they would still be split by the protons on adjacent carbons. Would they? Thatnks in advance Stephen

I have a question about proton nmr, or more specifically the proton nmr of 1,4-diethylbenzene. as part of a presentation we were asked to predict the proton nmr of 1,4-diethylbenzene which I did. However our lecturer disagreed with the predicted nmr, and while I agreed with his point then (not good practice to argue with the person marking you) I am not convinced his point was correct. For the four protons on the benzene ring I predicted (as did most other people) that you would get a 4 proton tall doublet at around 8. The lecturer says it would be a singlet. Which is it?. I had considered that it may be a singlet since all four protons are essentially identical but the book I used very briefly suggested a doublet. Even though the protons are all the same I personally thought they would still be split by the protons on adjacent carbons. Would they? By two mirror planes of symmetry and two C2 axes there is only one kind of proton on the ring in the fast exchange limit (reasonable temps, isotropic medium).  Is the aromatic proton singlet split into a triplet by the equivalent allylic methylenes three carbons down? Yes, but - given distance - is the averaged coupling constant detectable at machine resolution?

nmr of 1,4-diethylbenzene. as part of a presentation we were asked to predict the proton nmr of 1,4-diethylbenzene which I did. However our lecturer disagreed with the predicted nmr, and while I agreed with his point then (not good practice to argue with the person marking you) I am not convinced his point was correct. For the four protons on the benzene ring I predicted (as did most other people) that you would get a 4 proton tall doublet at around 8. The lecturer says it would be a singlet. Which is it?. I had considered that it may be a singlet since all four protons are essentially identical but the book I used very briefly suggested a doublet. Even though the protons are all the same I personally thought they would still be split by the protons on adjacent carbons. Would they? Thatnks in advance Stephen In 1,4-diethylbenzene all four aromatic protons are identical because you can flip one into the others by symmetry operations. Four indistinguishable protons cannot give a doublet.

The lecturer says it would be a singlet. Which is it?. I had considered that it may be a singlet since all four protons are essentially identical but the book I used very briefly suggested a doublet. Even though the protons are all the same I personally thought they would still be split by the protons on adjacent carbons. Would they? By two mirror planes of symmetry and two C2 axes there is only one kind of proton on the ring in the fast exchange limit (reasonable temps, isotropic medium).  Is the aromatic proton singlet split into a triplet by the equivalent allylic methylenes three carbons down? Yes, but - given distance - is the averaged coupling constant detectable at machine resolution? Jeez.  And here I am...brain-damaged, out of the shop for two years, and all that, thinking that symmetry arguments are only valid for predictions of vibrational spectra! Fast exchange....Al...it's the hot peppers...right?  Care to provide some discussion of exchange of equivalent benzenoid protons or pointers to the literature? Long-range coupling may be out of the realm of this particular question.  While there is no NMR spectrum available from the NIST WebBook (or elsewhere on the Web, AFAIK), the IR spectrum does suggest some complexity in the modes for those ring protons....  Assuming lo-res NMR shows singlets, what is observed at hi-res?   Mark

: I have a question about proton nmr, or more specifically the proton : nmr of 1,4-diethylbenzene. : For the four protons on the benzene ring I predicted (as did most : other people) that you would get a 4 proton tall doublet at around 8. : : The lecturer says it would be a singlet. : By two mirror planes of symmetry and two C2 axes there is only one : kind of proton on the ring in the fast exchange limit (reasonable : temps, isotropic medium).  Is the aromatic proton singlet split into a : triplet by the equivalent allylic methylenes three carbons down? : Yes, but - given distance - is the averaged coupling constant : detectable at machine resolution? Uncle Al (inter alia) seems to have forgotten the difference between chemical and magnetic equivalence.  In principle, the protons on the ring would show second-order spectra; in practice, all you'll see is a singlet, although I guess if you did really spiffy tuning, you might see some broadening. As Uncle Al points out, the ring protons are identical by symmetry, and hence chemically equivalent.  However, a ring proton experiences different couplings to the methylene protons on the ethyl substituents, since it is ortho to one and _meta_ to the other.  Theoretically, these kinds of things lead to complicated second-order spectra.  For example, 1,4-difluorobenzene has a single peak in the proton NMR spectrum, but it is not a singlet, a doublet, or a doublet of doublets.  For 1,4-diethylbenzene, however, the coupling constants between the ring protons and the methylene protons are expected to be quite small (<1 Hz), and so any effect on the spectrum would be inside the linewidth in the sense that unless one is very careful, one doesn't get linewidths that small in the spectrum.  And if you check the Aldrich catalog, you will find that the ring proton resonance appears as a singlet.

I have a question about proton nmr, or more specifically the proton nmr of 1,4-diethylbenzene. [snip...] The lecturer says it would be a singlet. Which is it?. I had considered that it may be a singlet since all four protons are essentially identical but the book I used very briefly suggested a doublet. Even though the protons are all the same I personally thought they would still be split by the protons on adjacent carbons. Would they? By two mirror planes of symmetry and two C2 axes there is only one kind of proton on the ring in the fast exchange limit (reasonable temps, isotropic medium).  Is the aromatic proton singlet split into a triplet by the equivalent allylic methylenes three carbons down? Yes, but - given distance - is the averaged coupling constant detectable at machine resolution? Jeez.  And here I am...brain-damaged, out of the shop for two years, and all that, thinking that symmetry arguments are only valid for predictions of vibrational spectra! Fast exchange....Al...it's the hot peppers...right?  Care to provide some discussion of exchange of equivalent benzenoid protons or pointers to the literature? Long-range coupling may be out of the realm of this particular question.  While there is no NMR spectrum available from the NIST WebBook (or elsewhere on the Web, AFAIK), the IR spectrum does suggest some complexity in the modes for those ring protons....  Assuming lo-res NMR shows singlets, what is observed at hi-res? If you put a highly symmetric set of aromatic protons in an anisotropic (liquid crystal) medium you get a literal forest of lines.  All sorts of stuff is being averaged out by tumbling.  Merely taking an NMR in deuterobenzene can gave you anomalous stuff because of slow pi-interactions between solvent and host. The temperature is important.  Rotational averaging eases off as the temp drops.  All the elegant symmetry arguments start to lift as the symmetric methylenes start caring about relative conformation on a spectral time scale. The problem is more acute in high field machines with their much higher resolution and narrower temporal windows for taking spectral snapshots.  None of this is big news.  We deal with it every day.