Spectroscopic Properties of Alkynes

The infrared spectrum of a monosubstituted alkyne such as ethynylbenzene, C6H5C≡CH (Figure 10-4), has a strong band near 3300cm−1, which is characteristic of the carbon-hydrogen stretching vibration in the grouping ≡C−H. At a lower frequency (longer wavelength) around 2100cm−1, there is a band associated with the stretching vibration of the triple bond (also see Figure 9-36). Therefore the presence of the grouping −C≡CH in a molecule may be detected readily by infrared spectroscopy. However, the triple bond of a disubstituted alkyne, R−C≡C−R, is detected less easily because there is no ≡C−H absorption near 3300cm−1, and furthermore the C≡C absorption sometimes is of such low intensity that it may be indiscernible. Raman spectroscopy  or chemical methods must then be used to confirm the presence of a triple bond.

Figure 10-4: Infrared spectrum of ethynylbenzene in carbon tetrachloride solution.

Alkynes, like alkenes, undergo electronic absorption strongly only at wavelengths in the relatively inaccessible region below 200nm200nm. However, when a triple bond is conjugated with one or more unsaturated groups, radiation of longer wavelength is absorbed. To illustrate, ethyne absorbs at 150nm150nm and 173nm, whereas 1-buten-3-yne (CH2=CH−C≡CH) absorbs at 219 nm and 227.5 nm.

The proton nuclear magnetic resonance spectrum of ethynylbenzene is shown in Figure 10-5. The peaks near 435 Hz and 185 Hz correspond to resonances of the phenyl and ≡C−H protons, respectively. The difference in chemical shift between the two types of protons is considerably larger than between alkenic and aromatic protons come into resonance at higher magnetic fields  than alkenic or aromatic protons. In fact, the ≡C−H protons of alkynes have chemical shifts approaching those of alkyl protons. (Also see Figure 9-36.)

Figure 10-5: The proton nmr spectrum and integral of ethynylbenzene at 60 MHz relative to TMS as 0.00. This spectrum also illustrates the use of nmr for detection of small amounts of impurities. The almost imperceptible peaks around 6ppm6ppm are in the correct locations for alkene hydrogens. The integral indicates that the ratio of alkene to ethyne hydrogens is on the order of 1:15. The substance most likely to give rise to the peaks is ethenylbenzene (styrene, C6H5CH=CH2) and, if so, it is present to the extent of about 2%.

The mass spectra of alkenes and alkynes usually give distinct molecular ions; however, the fragmentation is often complex and not easily interpreted