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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: lucas89 on April 01, 2009, 07:02:36 PM

Title: Proton NMR for 1,5-diphenyl-1,4-pentadien-3-one
Post by: lucas89 on April 01, 2009, 07:02:36 PM

Hey Chemforums.

I'm doing my organic lab write up, and I'm given the NMR spectra for 1,5-diphenyl-1,4-pentadien-3-one. The spectra has 7 spikes, indicating seven chemically different protons in the molecule. However, I can only count out five. I'm probably making a stupid mistake here somewhere.

The protons I have accounted for are:
-On the benzene rings, 2 groups of 2 and a lone proton, giving three spikes there (since the molecule is symmetric, I'm assuming that the hydrogens on the other benzene group are chemically equivalent).
-A proton on each side of the double bond, giving two more protons, for a complete total of five.

My guess would be the that the protons on the benzene ring on either side of the single bond to the rest of the molecule may not be chemically equivalent, due to the oxygen?

Attached is a paint drawing of the molecule in question. Speaking of which, if anyone can tell me of any websites or programs that will able to draw molecules (nicely) on the computer, it'd be greatly appreciated!

Title: Re: Proton NMR for 1,5-diphenyl-1,4-pentadien-3-one
Post by: gfunk on April 01, 2009, 09:55:55 PM

Are you sure that there are seven peaks?  Weird splitting patterns might make it seem as though there are more peaks than there really are.  You're right that there are pretty much only five unique protons.  Weird conformations of molecules can make two different protons on the same carbon atom resonate at different frequencies, but this molecule is fairly simple.

An NMR simulation of this molecule yielded five unique protons.  Looking on SDBS, they identify four unique peaks (merging the meta and para protons into one huge multiplet).  Maybe scan/take a picture of your spectrum?

Anyhow, ChemDraw is my personal choice for drawing molecules on the computer.  It also does NMR simulations.  But it's not free.  ChemSketch is free, though.  That's a popular choice for students.  I think there are other free alternatives, like ISIS Draw?  I think.

Title: Re: Proton NMR for 1,5-diphenyl-1,4-pentadien-3-one
Post by: lucas89 on April 01, 2009, 10:11:53 PM

Ah, I have discovered the problem! There are only five unique protons, and hence peaks. However, the scale is quite zoomed in, allowing for more spacing between peaks, and appearing to be unique. I can take this from here :P

Thanks!