Fragmentation Patterns of Other Nitrogen Compounds

As is true of amines, nitrogen-bearing compounds such as amides, nitriles, and nitro compounds must follow the Nitrogen Rule (explained more completely in Section 3.6) If they contain an odd number of nitrogen atoms, they must have an odd-numbered molecular weight. 

The mass spectra of amides usually show observable molecular ion peaks. The fragmentation patterns of amides are quite similar to those of the corresponding esters and acids. The presence of a strong fragment ion peak at miz = 44 is usually indicative of a primary amide. This peak arises from a-cleavage of the following sort. 

Copyright 2013 Cengage Learning. AH Rights Reserved. May not be copied, scanned, or duphcated, in whole or in part. 

Aliphatic nitriles usually undergo fragmentation so readily that the molecular ion peak is too weak to be observed. However, most nitriles form a peak due to the loss of one hydrogen atom, producing an ion of the type R—CH=C=N. Although this peak may be weak, it is a useful diagnostic peak in characterizing nitriles, hi the mass spectrum of hexanenitrile (Fig. 4.59), this peak appears at tniz = 96. 

When the alkyl group attached to the nitrile functional group is a propyl group or some longer hydrocarbon group, the most intense peak in the mass spectrum results from a McLafferty rearrangement  

---