Alkanes carbon-hydrogen stretching

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... 

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. 

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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