Fragmentation of Carboxylic Acids and Their Derivatives

In aliphatic esters the fragments typical for carboxylic acids are shifted by 14 u to higher mass upon transition from the free acid to the methyl ester and by further 14 u for the ethyl ester. [85,86,94,95] 

For carboxylic acid ethyl and longer aliphatic chain esters the McLafferty rearrangement can also occur on the alkoxy branch (R ) of the molecular ion. It then competes as a second alkene loss with the reaction at R  

Precursor Product Stracture Formula Accurate Mass [uf 

Precursor ProdiKt structure Formula Accurate mass [u]  

Example The base peak at m/z 88 in the mass spectrum of ethyl hexanoate results from butene loss from the molecular ion, M, m/z 144, via McLafferty rearrangement. This product ion may then undergo ethene loss to yield the fragment ion at m/z 60. The remaining fragments can be rationalized by propyl loss via y- 

---