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Hydrocarbons such as alkanes, alkenes, and alkynes show characteristic C–H stretching absorption bands. These IR stretching frequencies depend on the hybridization of the involved carbon atom and can be explained in terms of the s character of each hybridized atomic orbital.
Among the sp, sp2, and sp3 hybridized orbitals, sp orbitals have the maximum s character (50%). Consequently, the electrons are held more closely to the nucleus, resulting in stronger and shorter C–H bonds that stretch at a higher frequency compared to sp2 and sp3 hybridized carbon atoms. Indeed, the observed C–H stretching frequencies are 3300 cm−1 (sp), 3100 cm−1 (sp2), and below 3000 cm−1 (sp3). It is worth noting that sp2 C–H and sp C–H stretching absorption bands are not observed for tetra-substituted alkenes and internal alkynes, respectively.
From Chapter 13:
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