(3) Heteroatoms, such as nitrogen, oxygen and sulphur, promote cleavage of an adjacent carbon–carbon
single bond by forming an ion in which the lone-pair electrons participate in resonance stabilization:
This produces prominent peaks at m/z 43, 57, 71 etc.
(4) Fragmentation may result in prominent peaks with masses not apparently related to the original
molecule. Rearrangements resulting in the elimination of neutral molecules are common as are those
due to the migration of hydrogen atoms in molecules containing a heteroatom. The peaks may
sometimes be recognized from the fact that if derived from an even-weight parent ion, they are
themselves even-numbered.
The interpretation of mass spectra and the validity of the above rules may be demonstrated by
examining the spectra of several compounds. Returning to the spectrum of n-hexadecane shown in
Figure 9.53, it will be seen that it is characterized by a small parent peak (Rule 1) and clusters of peaks
14 mass units (CH 2 ) apart. The largest peak in each cluster is CnH 2 n + 1, the most intense groups are those
at C 3 and C 4 , and the decrease in intensity of successive groups is smooth. Contrast this with a spectrum
of the isomeric 5-methylpentadecane (Figure 9.55) which shows similar clusters but a prominent group
at C 6 because of branching (Rule 2). The lack of a smooth decrease in the intensity of successive groups
differentiates branched chain from straight chain compounds.
Figure 9.55
Mass spectrum of 5-methylpentadecane.The spectrum of benzyl acetate is shown in Figure 9.56. The parent peak at m/z 150 is prominent (Rule
- as is the tropylium ion peak at m/z 91 (Rule 2). The base peak at m/z 108 is due to a rearrangement
(Rule 4) after cleavage of the acetyl group which itself gives a prominent peak at m/z 43