6.12 Fragmentation of Carboxylic Acids

         The molecular ion of straight chain carboxylic acids is weak but usually present.  The McLafferty rearrangement results in a prominent peak and often accounts for the base peak. 

         Short chain carboxylic acids give prevalent peaks at [M – OH] and [M – CO₂H].  In larger carboxylic acids these peaks are less prevalent.  Long chain carboxylic acids are better identified by the fragments at C_nH_2n-1O₂ (Figure 6.17).  There is also the presence of the hydrocarbon fragment at C_mH_2m+1 illustrated in Section 6.9.

Figure 6.17. Fragmentation of a Carboxylic Acid. Spectra from the NIST/EPA/NIH Mass Spectral Library.  Reprinted with permission from NIST.

Aromatic acids have a more prominent molecular ion peak but undergo similar fractionation to short chain hydrocarbons.  They produce large peaks at [M – OH] and [M – CO₂H].  Aromatic acids can also lose water if an ortho group contains an abstractable hydrogen atom.