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 is 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 characteristic 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:

Mass Spectra

Mass spectra help us identify branching (the presence of side chains). The structure of different fragments have different relative stabilities:

primary < secondary < tertiary

Applying the logic of this to fragmentation patterns, it means that a split which produces a secondary fragment is going to be more likely than one producing a primary one. A split producing a tertiary fragment will be even more likely. Let's look at the mass spectrum of 2-methylbutane. 2-methylbutane is an isomer of pentane - isomers are molecules with the same molecular formula, but a different spatial arrangement of the atoms.

Look first at the very strong peak at m/z = 43. This is caused by a different ion than the corresponding peak in the pentane mass spectrum. This peak in 2-methylbutane is caused by:

The ion formed is a secondary fragment - it has two alkyl groups attached to the carbon with the positive charge. As such, it is relatively stable. The peak at m/z = 57 is much taller than the corresponding line in pentane. Again a secondary fragment is formed - this time, by:

You would get the same ion, of course, if the left-hand CH3 group broke off instead of the bottom one as drawn. In these two spectra, this is probably the most dramatic example of the extra stability of a secondary fragment.