[electrogeek]

Hi everyone,

I have been given a Proton NMR spectrum from my lab practical this week from my synthesis of 4-phenylbenzoic acid (I drew a quick skeletal formula of this on the NMR spectrum). I have attached a picture of the spectrum below. I've looked at my spectrum and noticed that I had a peak missing at about 12 ppm, but I believe this is down to proton exchange between my product and the solvent (the solvent being used here is DMSO - d6 in heavy water), which would explain the broad peak at 3.29 ppm which I have labelled.

I know all the other peaks at 7 - 8 ppm is due to the protons on the aromatic rings (not acidic and so wouldn't be exchanged with any deuterium), but I don't know what the peak at 3.13 ppm is due to? I was wondering if this was due to proton exchange with the DMSO-d6 instead of the heavy water, and would this explain the tiny signal at about 1 ppm because of I = 1 for deuterium, leading to the spin states being 1, 0 and -1?

Thank you - any help in identifying the peak at 3.13 ppm will be greatly appreciated!

[wildfyr]

What was the synthesis route you used to get to your product?

[electrogeek]

What was the synthesis route you used to get to your product?

I started off with a mixture of sodium carbonate, deionized water, 4-bromobenzoic acid and phenylboronic acid. I then added palladium (II) acetate once everything was dissolved (the palladium catalyst was within a solution of sodium hydroxide). This was heated for an hour at 80°C. Once the flask cooled, I added hydrochloric acid and isolated crude product via vacuum filtration and washed with water.

To purify the product, I performed a recrystallization with ethanol. The purified product was removed via vacuum filtration again and washed with a small amount of cold ethanol this time.

The NMR sample featured 5 mg of the purified product in DMSO-d6.

Thinking about it a bit more I was wondering if actually it was a proton exchange between the heavy water (or DMSO-d6) and the ethanol where it didn't dry fully? I wondered this because the tiny peak at about 1 ppm is a triplet, and EtOD would produce a triplet in that area, but there isn't a quartet for the EtOD so I wasn't sure... If this is the case, would there be a favoured exchange between one of the deuterated solvents and the hydrogen in the alcohol group of the ethanol, or is it equally likely that they'll both do the exchange with the ethanol?

[wildfyr]

No, there aren't any ethanol peaks in there. You would see the CH3CH2 quartet and triplet. It could be acetate from the palladium acetate.

[electrogeek]

I thought that as well, but I had a look at the proton NMR for the palladium acetate and the chemical shift seemed a bit low (the one I looked at says it's about 2.2 ppm)...

Could it be just water in that case then? Because if HDO is produced then surely the other deuterium atom can be transferred to give deuterated 4-phenylbenzoic acid?

[wildfyr]

No. A deuterium peak won't show up, that's the whole point of using deuterated solvent.

What do you think of accidental contamination from dirty glassware, especially the tube itself?

[electrogeek]

What I meant was that maybe the 4-phenylbenzoic acid exchanges it’s proton with the deuterium of HDO to give water (as in H₂O water, which would produce a singlet) and then a partially deuterated form of 4-phenylbenzoic acid (meaning no peak at 12 ppm observed because of the OD group that would be present due to the exchange, which would be occurring with the heavy water and 4-phenylbenzoic acid anyway and is why it’s not in my spectrum above).

The NMR tube and cap were brand new from the lab office. The only thing that maybe contaminated it if anything would be something on the spatula, but it was washed with acetone and dried before weighing out the 5 mg of product (though could be a possibility I guess).

Thanks for the help so far by the way.

[electrogeek]

I've had another idea about this peak at 3.13 ppm. Could this be due to an interaction of the COOH group with the DMSO-d6, and the OH peak hasn't actually disappeared at all, but has just moved to a lower ppm value?

I don't know if this can happen, but I don't think all of the 4-phenylbenzoic acid converted to a deuterated form of 4-phenylbenzoic acid.

[wildfyr]

That peak is so sharp, I really don't think it's some exchangeable proton of any kind. It's a true impurity.

Otherwise the literature spectra would have it.

[electrogeek]

Ah okay - would you get any proton deuterium exchange in the first place anyway? I can’t seem to find anything about carboxylate acids doing this...

I think I have sussed it - I also ran a GC/MS and I’m wondering if a small amount of methanol somehow got in to the sample from setting up the GC/MS sample? I looked at the paper linked and it says there would be a singlet in the presence of heavy water at 3.16 ppm. So maybe a trace amount somehow got in to the NMR tube when I dissolved my product in the DMSO-d6 (maybe I forgot to swap the tip of my Gibson pipette before I measured the DMSO-d6 aliquot, for example)?

https://www3.nd.edu/~hgao/JOC1997p7512.pdf