[wwilder24]

I need help deciphering my NMR for an aldol condensation. We were given two unknowns (an aldehyde and a ketone) and performed an aldol condensation. The possible aldehydes are o-anisaldehyde, benzaldehyde, p-anisaldehyde, 3,4-dimethoxybenzaldehyde, 4-chlorobenzaldehyde, and p-tolualdehyde. The possible ketones are acetone, acetophenone, cyclopentanone, 2-pentanone, and 2-hexanone.


Above is the NMR obtained, the peak at 7.26 is probably CDCl3, there is also another singlet upfield at about 1.5 ppm which is probably water. From this I can tell I have two phenyl rings but, I am having trouble piecing the other parts together. The doublet at around 7.75 ppm I believe is an alkene. My integrations are also confusing me and I am not sure just how correct they are. I am leaning toward 4-chlorobenzaldehyde and acetophenone but I can't explain the doublet at 7.1 ppm.

[Nosterius]

From the integration, I would say you have a symmetric molecule with two alkenes since the two doublets at 7.75 and 7.10 ppm couple together and have an integration of 2 each.

The multiplets at 7.63 and 7.42 are integrated narrowly. I would expect them to be 4 and 6 respectively.

What would happen if both sides of acetone reacted?

[wwilder24]

I just don't know how to explain the one that integrates to 5 (or as you say 6), I guess looking at it more it wouldn't be acetophenone because I lack proper splitting for that in the multiplets. Let me see if my thoughts are correct in narrowing down my choices. Since I don't have any singlets (besides the one at 7.26 that is CDCl3 and 1.5 ppm that is water) couldn't i eliminate everything but benzaldehyde and the 4-chlorobenzaldehyde? The methoxy and methyl substituents would give me singlets right?

[Nosterius]

Right.

If two benzaldehyde molecules reacted with one molecule of acetone, how many aromatic proton would you have? And how many alkene protons?

[wwilder24]

That would give me 4 alkene protons and 10 aromatic protons I believe. Also I'm not so good with the whole doublet of doublet and triplet of doublets when it comes to aromatic rings, I feel like with benzaldehyde I would be seeing triplets of doublets.

[Dan]

Aromatic rings can give surprisingly complicated multiplets as a result of magnetic inequivalence and overlapping shifts. I wouldn't read too much into the patterns of the aromatic shifts, the integration is the only clue for the shifts at 7.63 and 7.43 ppm - and as has been said the two multiplets in the aromatic region have not been fully integrated and should be 4 and 6.

[Nosterius]

It's ok; NMR spectroscopy is something you develop over time, when you work with them often. You'll eventually get a feeling of the structure of a molecule by taking a glimpse of its spectra. Coupling patterns for substituted aromatic rings can be fairly complicated, especially for mono-substituted ones, where long-range coupling can be problematic.

I often saw monosubstituted benzenes having two sets of multiplets, one accounting for its meta protons, and the other being a mixture of ortho and para proton. The ratio would then be 2:3.

With these hints, can you provide a structure (or a name spelled by Chemdraw). I will let you know if it is correct.

[wwilder24]

I'm hoping this is right, so what is being said is that the peak integrating to 4 would account for my 4 meta protons and the other one integrating to 6 contains the 4 ortho and 2 para protons? And then for the two alkene peaks one set would be related to the pair of protons close to the benzene ring and the other to the pair closer to the ketone?

[Nosterius]

This seems fine to me.

This is in fact a classic ligand in palladium catalyzed chemistry. You might want to look at "dibenzylideneacetone" on the aldrich website. There is an NMR spectra of the molecule which seems to corroborate your findings.

[wwilder24]

Thank you very much for the help ! (interesting I can't put the ! next to the word help...)

[orgopete]

I don't think that is it. I don't think it fits the NMR integral.