Carbon-hydrogen stretch alkenes

Alkanes, alkenes, and alkynes also have characteristic C — H stretching frequencies. Carbon-hydrogen bonds involving sp3 hybrid carbon atoms generally absorb at frequencies just below (to the right of) 3000 cm-1. Those involving sp2 hybrid carbons absorb just above (to the left of) 3000 cm-1. We explain this difference by the amount of 5 character in the carbon orbital used to form the bond. The s orbital is closer to the nucleus than the p orbitals, and stronger, stiffer bonds result from orbitals with more s character. Even if an alkene s C=C absorption is weak or absent, the unsaturated C—H stretch above 3000 cm-1 reveals the presence of the double bond. 

Bands due to carbon-carbon stretching may appear at about 1500 and 1600 cm for aromatic bonds, at 1650 enr for double bonds (shifted to about 1600 cm i by conjugation), and at 2100 cm for triple bonds. These bands, however, are often unreliable. (They may disappear entirely for fairly symmetrically substituted alkynes and alkenes, because the vibrations do not cause the change in dipole moment that is essential for infrared absorption.) More generally useful bands are due to the various carbon-hydrogen vibrations. 

---