The molecular ion peak is weak but usually observable.
An important fragmentation pattern involves a-cleavage (breaking either bond to the carbonyl carbon) as shown in figure 43.
Figure 43. a-cleavage.
Loss of the alkyl group as a free radical, leaving CO2H+, also occurs as shown in figure 44.
Figure 44. Loss of alkyl radical.
With acids having g hydrogens, the principal pathway for rearrangement is the McLafferty rearrangement (see figure 45).
Figure 45. McLafferty rearrangement of an acid.
The complete mass spectrum for pentanoic acid is given in figure 46, which illustrates most of these points.
Figure 46. Mass spectrum for pentanoic acid.
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