Description: Alkynes have a very interesting structure as they are essentially b
ID: 545251 • Letter: D
Question
Description: Alkynes have a very interesting structure as they are essentially built out of two double bonds that are stacked 90 degrees from one another. This is similar to the structure of an allene, which is a carbon with two double bonds but each double bond is going out at different directions (see below). Interestingly, although these structures look similar in terms of orbitals, the protons attached to them drastically differ in their H NMR shifts. The allene proton shift is around 4.5 ppm whereas the alkyne shift is around 2.1.
Instructions: Your task for this case study is to look up the three dimensional orbital diagram for both an alkyne and an allene, and draw them. Then, propose a reason for why these two functional groups have such drastically different NMR shifts (hint, look up the concept of ring currents). Please explain your reasoning
2.1 ppm 4.5 ppm CEC-H H C C-c Alkyne AlleneExplanation / Answer
Generally NMR peaks comes to alkyne - 2 to 3.5ppm and alkene 4 to 6ppm
alkyne- acetylene comes to 2.1ppm
alkene- allene comes to 4.5ppm
Acetylene high shieling than allene. Allene deshielding in NMR (van der waal's deshielding) so resonate at slightly higher value of ppm then expected in the absence of the effect.Acetylene is (diamagnetic shielding effect) protons induced field oppose theapplied field so NMR peaks comes to low value of ppm.