November 27, 2020, 03:28:40 AM
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Topic: Help with analysis IR & NMR spectra for radical bromination experiment  (Read 331 times)

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Offline ilyem46

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This is for a 2nd semester organic chemistry lab, i wasn't sure if i should post in this forum or the Ochem forum, but chose this because it's purely an analytical question. So triphenylmethane was mixed with a solution of bromine in DCM and reacted using light to create bromotriphenylmethane. We ran IR, GC-MS, and proton NMR to analyze the product(s). I was able to figure out the products using solely the GC-MS spectra but the other spectra aren't making sense to me so i'm having a hard time writing my results and interpreting these results because of this. The GC-MS showed three different compounds present in our final product, the first spectra shows a mass peak of 244.1g/mol which is our starting material (triphenylmethane). The other compound is triphenylmethanol with mass peak at 260.1, which i assume was produced by some autooxidation or something... i'm going to figure that out later. The third compound had some fishiness to it. This compound is triphenylethanol with a mass peak of 288.1g/mol. My professor confirmed that he suspected one of our reagents had ethanol in it by mishaps, so that peak should have been bromotriphenylmethane so we're all clear on interpreting these results.

What i'm confused about is my IR spectra shows no peak for a hydroxyl group, and my NMR spectra just shows one singlet peak at 7.4 ppm. The only significant speak on the IR is a position of 3058/cm with medium intensity. I would consider it sharp and not broad at all, but i'm overall having a hard time assigning the peaks on the IR spectra period. I even ran the IR twice just to make sure and the spectra were identical. The singlet peak for the proton NMR i suspect is the proton on the oxygen of triphenylmethanol, but why aren't there any other peaks representing any of the other hydrogens? Or peaks representing the different hydrogen's on the different compounds even?
Here's images of the IR & NMR spectra: 
I'd appreciate any advice or input on how to interpret these in my report! thanks

Offline mjc123

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Re: Help with analysis IR & NMR spectra for radical bromination experiment
« Reply #1 on: February 17, 2020, 04:49:39 AM »
No peak for a hydroxyl? What have you drawn pencil lines to?
You have nothing between 2900-3000 cm-1. What does that tell you?

Offline clarkstill

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Re: Help with analysis IR & NMR spectra for radical bromination experiment
« Reply #2 on: February 17, 2020, 06:02:13 AM »
That's a very weak 1H NMR spectrum - you shouldn't be able to see anywhere near that amount of baseline noise if your sample is of a reasonable concentration. The single peak also looks suspiciously like CDCl3 (7.26 ppm) and definitely not like DMSO as the metadata states (you'd expect peaks at 2.5 and ~3.3 in DMSO). There's something fishy about that spectrum.

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