Carbon-hydrogen stretch alkanes

A. Carbon-hydrogen stretching vibrations in branched alkanes... 

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. 

ANSWER (a) In the C—H stretch region (3350-2850 cm ) almost all the bands are below 3000 cm indicating that there are only alkane carbon-hydrogen bonds. No hydrogens are attached to double or triple bonds, because such carbon-hydrogen... 

Figure 4.3 Principal modes of vibration between carbon and hydrogen in an alkane (a) symmetrical stretching, (b) asymmetrical stretching and the bending vibrations, (c) scissoring,...

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