Spectroscopy Tutorial: Examples

Example 15


MW 82

Calculate the degree of unsaturation: the answer is 2. The molecule has two double bonds and/or rings.

IR Spectrum

The IR spectrum below doesn't show much of interest. We would expect two carbon-carbon double bonds (1680-1640), or one carbon-carbon triple bond (2260-2100). Could it be a ring, or two rings? The C-H stretching vibrations are all under 3000, indicating that there are no hydrogens on a carbon-carbon double bond or on a carbon-carbon triple bond - it is possible that the compound is a internal alkyne.

NMR Spectrum

The above spectrum indicates two identical -CH2CH3 groups. The degree of unsaturation of 2 both hints at an alkyne, although the carbon-carbon triple bond stretch 2260-2100 is not seen. This does not rule out the compound being an alkyne, because the normally weak carbon-carbon triple bond stretch is not observed in a symmetrically substituted alkynes. If we put the carbon-carbon triple bond in the middle of a straight carbon chain molecule, we get this:

This compound has two equivalent ethyl groups and a symmetrically substituted carbon-carbon triple bond. The C-H stretch is not seen in the IR because it is not a terminal alkyne (and therefore does not have a C-H on the triple-bonded carbon. This is how it correlates with the NMR spectrum:


Example 15 is 3-hexyne: