If you are given a mass spectrum for a molecule, you may be asked to draw the structures for certain molecular fragments based on where they appear in the mass spectrum. For example, here is a sample question of the type you may see on an exam: The mass spectrum for 2-pentanone is shown here. Draw the fragments responsible for the peaks in the mass spectrum at m/z 71, 58, and 43.
The main piece of information to remember in answering these types of questions is that each of the structures you draw must be positively charged. Neutral fragments are discarded and don’t reach the detector.
Usually the best way to start is by looking for likely breaks in the molecule. Can you get an alpha cleavage (a break next to a heteroatom like O or N)? Can the molecule break to form a benzylic fragment or allylic fragment? Can a tertiary cation be made? If it’s a carbonyl compound, does it have a gamma hydrogen that could be involved in a McLafferty rearrangement?
This example has neither a benzene ring nor a carbon-carbon double bond. It doesn’t have the capacity to make a tertiary cation, either. But it does have a heteroatom (oxygen). So, alpha cleavage would definitely be a possibility. Additionally, because it’s a carbonyl compound and has a hydrogen in the gamma position, you’ll probably see a peak for the McLafferty rearrangement.
Start with alpha cleavage. The molecule can split on the left of the carbonyl group to make the resonance-stabilized cation that has a molecular weight of 71 amu. You may have already noticed that this peak with an m/z of 71 is 15 mass units below the molecular ion peak, suggesting a loss of a methyl group. This cleavage is shown here.
Or the molecule could split via an alpha cleavage on the right side of the carbonyl to make a cationic fragment that weighs 43 amu, as shown in the next figure.
Another possible alpha cleavage.
Now try the McLafferty rearrangement. The molecule is shown here to make it easy to see which bonds get made and which bonds get broken. The McLafferty rearrangement gives the piece that weighs 58 amu, as shown in the next figure.