NMR spectra of alkynes and small rings

You have also seen the same argument used in Chapter 8 to justify the unusuai acidity of C-H protons on triple bonds (such as alkynes and HCN), and alluded to in Chapter 3 to explain the stretching frequency of the same C-H bonds. Like alkynes, three-membered rings are also unusually easy to deprotonate in base. 

Here is an example where deprotonation occurs at a different site in two compounds identical except for a C-C bond closing a three-membered ring. The first is an ortholithiation of the type discussed in Chapter 24. 

Now what about the NMR spectra of alkynes By the same argument, protons on alkynes ought to appear in the NMR at quite high field because the Catom is sp hybridized, so it makes its a bonds with sp orbitals (i.e. 50% s character). Protons on a typical alkene have 6h about 5.5 ppm, while the proton on an alkyne comes right in the middle of the protons on saturated carbons at about 5 2-2.5 ppm This is rather a large effect just for increased s character and some of it is probably due to better shielding by the triple bond, which surrounds the linear alkyne with n bonds without a nodal plane. 

A more interesting exampie comes from the base-catalysed addition of methanoi to buta-1,3-diyne (diacetylene). The compound formed has one double and one triple bond and the 13C NMR shows clearly the greater deshielding of the double bond. 

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