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.
The IR stretching frequency of a C–H bond depends on the hybridization of the carbon atom.
An sp3 hybridized carbon displays the lowest C–H stretching frequency. The frequency increases for the C–H with an sp2 carbon, followed by an sp carbon.
This is because an sp-hybrid orbital has maximum s character, with its electron density very close to the nucleus.
This makes the sp C–H bond shorter and stronger than the sp2 and sp3 C–H bonds, which is reflected in the observed stretching frequency trend.
For instance, the IR spectra of alkanes, alkenes, and alkynes, in the 2600 to 3400 cm−1range, show the alkane C–H stretch below 3000 cm−1. The C–H stretches for alkenes and alkynes appear around 3100 cm−1 and 3300 cm−1, respectively.
Tetra substituted alkenes, however, do not have a band at 3100 cm−1due to a lack of C–H bonds.
Similarly, internal alkynes do not exhibit a band at 3300 cm−1.
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