The molecular ion peak is usually very weak or absent for alcohols.
The most important fragmentation process for alcohols involves the loss of an alkyl group. The largest alkyl group is preferentially lost as shown in figure 25.
Figure 25. Loss of alkyl group in alcohol fragmentation.
A second common mode of fragmentation involves dehydration. The importance of this fragmentation process increases with increasing chain length. Loss of water (M - 18) is very indicative of an alcohol functionality (see figure 26).
Figure 26. Dehydration of an alcohol.
Phenols can lose the elements of carbon monoxide to give strong peaks at M - 28.
Phenols can also lose the elements of the formyl radical (HCO·) to give strong peaks at M - 29.
The complete mass spectrum for 2-pentanol is given in figure 27, which illustrates most of these points.
Figure 27. Mass spectrum for 2-pentanol.
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