Hydrocarbons - Alkane or Alkene?

Alkanes and alkenes are made up of hydrogen and carbon atoms only - they are hydrocarbons. They are very difficult to tell apart using spectra, but there are a few tell-tale signs to help us.

IR Spectra

Have a look at these two spectra. One if for cyclohexane (an alkane) and one is for cyclohexene (an alkene, with a C=C double bond).

The bottom IR spectrum is for cyclohexene. We know this because it has a peak at around 3100cm^-1. This is absent in alkanes.

¹³C-NMR Spectra

Alkenes also have a characteristic peak when we look at the ¹³C-NMR spectrum. A peak between 115-145ppm is unique for alkenes. If it is absent, there are no C=C double bonds. If it is present, you can be very confident there is a C=C double bond in the structure.

Here is the spectrum for cyclohexene which shows the characteristic peak, shifted to 127ppm due to the C=C bond: