Recently I carried out alkyne formation reaction. However, I have a doubt whether only alkyne is the product or allene is also get formed? I just took simple 1H NMR (500 MHz). It shows me a predominantly a alkyne but few peaks of allene are also there. Please suggest me the exact method to detect the presence of both alkyne and allene. Is there any instrumentation technique available to detect the relative abundance of allene and alkyne in a mixture?
The best spectral handle is the carbon-13 NMR resonance for the sp hybridized carbon of the allene, which will resonate above 200 ppm. See the attached file which was obtained from: http://www.chem.wisc.edu/areas/reich/nmr/c13-data/cdata.htm.
Joseph Sloop sir,
Yes sir, it will show peak above 200 ppm if I would go for C13 NMR. However, how to calculate their relative amount, as it is mixture of allene and alkyne?
The 1H Chemical Shifts of allenes are around 5.6 ppm but can go up to 6.5 ppm. However, Coupling Constants is no more than 6 Hz.
Therefore we can assign the signals from each compound analyzing the Coupling Constants. Of course it always depends on each particular compound.
you can use 1H NMR if the alkynes you mention are terminal alkynes (triple bond at the end of a chain). then the H bound to the sp-carbon will have a chemical shift approx. between 2 and 3 ppm, quite different from the allene case (5 to 7 ppm).
If you are able to identify their proton signals the alkyne/allene ratio should be easy to calculate from their proton integrals. Although I would repeat the 1H experiment with a longer relaxation delay to get more accurate integrals.
If you can dissolve a large amount of sample and can get a lot of NMR time you could even try a C13 inverse gated experiment and integrate the carbon signals.
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