This idea that small rings have more p character in the ring and more s character outside the ring also explains the effects of small rings on proton NMR shifts. These hydrogens, particularly on three-membered rings, resonate at unusually high fields, between 0 and 1 ppm in cyclopropanes instead of the 1.3 ppm expected for CH₂ groups, and may even appear at negative δ values. High p character in the framework of small rings also means high s character in C–H bonds outside the ring and this will mean shorter bonds, greater shielding, and small δ values.

Three-membered rings and alkynes

 You have also seen the same argument used in Chapter 8 to justify the unusual 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 com pounds 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 fi eld because the C atom is sp hybridized, so it makes its σ bonds with sp orbitals (i.e. 50% s character). Protons on a typical alkene have δH about 5.5 ppm, while the proton on an alkyne comes right in the middle of the protons on saturated carbons at about δH 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 π bonds without a nodal plane. This means that the carbon atoms also appear at higher field than expected, not in the alk ene region but from about δC 60–80 ppm. The s character argument is important, however, because shielding can’t affect IR stretching frequencies, yet C≡C–H stretches are strong and at about 3300 cm⁻¹, just right for a strong C–H bond. A simple example is the ether 3-methoxyprop-1-yne. Integration alone allows us to assign the spectrum and the ¹H signal at 2.42 ppm, the highest field signal, is clearly the alkyne proton. Notice also that it is a triplet and that the OCH₂ group is a doublet. This 4JHH is small (about 2 Hz) and, although there is nothing like a letter ‘W’ in the arrangement of the bonds, coupling of this kind is often found in alkynes.

A more interesting example comes from the base-catalysed addition of methanol 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.

You may have noticed that we have drawn the double bond with the cis (Z) configuration. We know that this is true because of the proton NMR, which shows a 6.5 Hz coupling between the two alkene protons (much too small for a trans coupling; see p. 295). There is also the longer-range coupling (4J = 2.5 Hz) just described and even a small very long-range coupling (5J = 1 Hz) between the alkyne proton and the terminal alkene proton.

مواضيع ذات صلة

Alkenes react with bromine

Effects of substituents on CH2 groups

Effects of functional groups

Identifying compounds from nature

Squares and cubes: molecules with unusual structures

Reactive intermediates can be detected by spectroscopy

Identifying products spectroscopically An ambiguous reaction product

Why is there no coupling to protons in normal 13C NMR spectra

Coupling in carbon NMR spectra

Interactions between different nuclei can give enormous coupling constants

Proton NMR distinguishes axial and equatorial protons in cyclohexanes

Simple calculations of C=O stretching frequencies in IR spectra