Alkynes are compounds that have a carbon-carbon triple bond (–C≡C–). The –C≡C– stretch appears as a weak band from 2260-2100 cm-1. This can be an important diagnostic tool because very few organic compounds show an absorption in this region.
A terminal alkyne (but not an internal alkyne) will show a C–H stretch as a strong, narrow band in the range 3330-3270 cm-1. (Often this band is indistinguishable from bands resulting from other functional groups on the same molecule which absorb in this region, such as the O-H stretch.)
A terminal alkyne will show a C–H bending vibration in the region 700-610 cm-1.
Summary:
The spectrum of 1-hexyne, a terminal alkyne, is shown below. Note the C–H stretch of the C–H bond adjacent to the carbon-carbon triple bond (3324), the carbon-carbon triple bond stretch (2126), and the C–H bend of the C-H bond adjacent to the carbon-carbon triple bond (636). The other bands noted are C–H stretch, scissoring, and methyl rock bands from the alkane portions of the molecule.
