Acylium ions, [RCO]⁺ : Peak Heights and Stability

Ions with the positive charge on the carbon of a carbonyl group, C=O, are also relatively stable. This is fairly clearly seen in the mass spectra of ketones like pentan-3-one.

The base peak, at m/z=57, is due to the [CH₃CH₂CO]⁺ ion. We've already discussed the fragmentation that produces this.

Using mass spectra to distinguish between compounds

Suppose you had to suggest a way of distinguishing between pentan-2-one and pentan-3-one using their mass spectra.

pentan-2-one		CH3COCH2CH2CH3
pentan-3-one		CH3CH2COCH2CH3

Each of these is likely to split to produce ions with a positive charge on the CO group. In the pentan-2-one case, there are two different ions like this:

• [CH₃CO]⁺
• [COCH₂CH₂CH₃]⁺

That would give you strong lines at m/z = 43 and 71. With pentan-3-one, you would only get one ion of this kind:

• [CH₃CH₂CO]⁺

In that case, you would get a strong line at 57. You don't need to worry about the other lines in the spectra - the 43, 57 and 71 lines give you plenty of difference between the two. The 43 and 71 lines are missing from the pentan-3-one spectrum, and the 57 line is missing from the pentan-2-one one.

The two mass spectra look like this:

As you've seen, the mass spectrum of even very similar organic compounds will be quite different because of the different fragmentation patterns that can occur. Provided you have a computer data base of mass spectra, any unknown spectrum can be computer analyzed and simply matched against the data base.