[UHStudent1138]

Hello all,

I am working on the spectra from the following link:
http://www.chem.ucla.edu/cgi-bin/webspectra.cgi?Problem=bp2

For the most part, I understand how the spectra available contribute to the answer, but the left (upfield) side of the proton NMR is proving confusing.  I see how the methyl group around 4ppm and the lone aldehyde hydrogen around 10.5 ppm produce their respective patterns, but the remaining 4 hydrogens on the left side of the benzene ring mystify me.  To be more specific in my confusion:

1. Why does the 7.0 ppm shift have a multiplicity of two?  Given that each hydrogen is a different distance from the either and aldehyde, shouldn't each hydrogen have a different peak?

2. What could account for the two-multiplicty 4-peak split between 6.9-7.0 ppm?  Since this is a benzene ring, each hydrogen should be adjacent to a maximum of two hydrogens, no?  One on each carbon on each side of the bond?

I understand the other two shifts within the 7-8 ppm range, but not how they might fit in with the one above.  What am I not seeing?

Thanks for your help.

[blaisem]

The quartet at 7 ppm is an overlap of a doublet and a triplet.  Does that answer your questions?

[UHStudent1138]

blaisem, thanks for that tip.  Does that suggest that the proton at 7ppm is adjacent to two different groups - one group of one hydrogen and one group of two hydrogens?

[Dan]

blaisem, thanks for that tip.  Does that suggest that the proton at 7ppm is adjacent to two different groups - one group of one hydrogen and one group of two hydrogens?

Notice the relative integration is 2. The multiplet you see at 7 ppm is two different overlapping signals - the low symmetry of the multiplet suggests two different overlapping signals and the relative integration indicates 2H.

[blaisem]

As Dan said, a normal quartet exhibits 1:3:3:1 splitting.  If you go through the binomial expansion, you'll see a symmetry between the peak intensities of a multiplet.  This peak shows very little symmetry, suggesting something else is going on.  Furthermore, as in any peak analysis, it's important to consider the integration in addition to the chemical shift and multiplicity.  In this case, you have two hydrogens for that peak.

The bottom line is that it is possible for two different hydrogens to have nearly the same chemical shift, so that their signals overlap.  Take a look at the solution molecule provided in your link and consider what you expect its spectrum should look like.  You already implied the correct expectations in your original questions.  Use these expectations, along with what Dan and I have said, to understand what is going on with that mystery multiplet.