Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Spectroscopy => Topic started by: el quimo on April 24, 2014, 08:46:48 PM
-
Hello, new here.
I hope someone can help interpret this spectra, I would be very grateful.
Starting from α-methyl styrene and bromoform, I synthesized 1-methyl-2,2-dibromocyclopropylbenzene. Using MeLi in dry ether under N2 via a Skattebol rearrangement this turned into 1-methylpropa-1,2-dienylbenzene (allene). The remaining MeLi was quenched with water. The product was identified via proton NMR. Impurities were detected and I performed a flash column (98 hexane:1 dichloromethane:1 ethyl acetate). Same impurities were detected again. I performed a TLC plate using same solvent ratio. It produced two bands with RF=.75 and RF=.24. The first band from the bottom (.25) was trace amounts of acetophenone while the second band was the desired product. From HNMR it can be seen that the impurity remains. I performed plate once again. Same result.
I think that the allene might be being oxidized by air, this would also explain why acetophenone is also detected.
The NMR is a cryptic, from the integration values of the aromatic protons I can see that the impurity is also aromatic. But I cannot account for the rest of the peaks.
Spectra is attached. Last attachment is structure of the allene.
Doublet at 5.0ppm is CH2 from allene. Triplet at 2.1 ppm is methyl from allene.
Singlet at 5.3ppm is solvent (methylene chloride)
-
I'll look at the NMR when I have a bigger screen to look at.
Do these degrade in a GC injector?
If you think aerial oxidation is occurring this is easy to spot by TLC.
Take a 2 x longer plate than you would usually use and run it with your crude to 1/2 way up the plate and pencil mark your solvent front and your product. Remove it and allow the solvent to evaporate completely. Then return the plate and run the full length. Again mark the solvent front at the end.
If you have an air sensitive product it will oxidise on the plate while it dries and taking the mark where your product was after the first run as the new baseline you may see another spot appear as the purity. I got this a lot and it took me ages to work out that my product wasn't impure, just sensitive.
-
Thanks for the reply. I'll try doing that and report back if it works.