Fragmentation Patterns
Molecular ion peak: M + e
-
→ M
+
• + 2 e
-
1. Cleavage is favored at branched carbon atoms: tertiary, secondary, primary,
with the positive charge staying with the branched carbon (the more stable
carbonium ion).
2. Double bonds favor cleavage beta to the double bond.
[R-CH
2
-CH=CH
2
]
+
·
→
R· + [
+
CH
2
-CH=CH
2
]
3. A substance having a strong molecular ion peak often contains a ring, and the
more stable the ring the larger the peak.
4. Ring compounds usually contain peaks at the mass number characteristic of the
ring (e.g C
6
H
5+
at m/z = 77 for aromatics and C
7
H
7+
at m/z = 91 for alkyl
aromatics).
5. Saturated rings lose side chains at the alpha carbon. The peak corresponding to
the loss of two ring atoms is much larger than for the loss of one ring atom.
R
R
+
·
+
+
6. In alkyl-substituted ring compounds, cleavage is most probable at the bond beta
to the ring if the ring has a double bond next to the side chain.
CH
2
R
CH
2
+
R
+
·
+
7. A hetero-atom will induce cleavage at the bond beta to it.
[R-CH
2
-O-R']
+
·
→
R· + [CH
2
=O
+
-R']
8. Compounds containing a keto-group tend to break at this group, with the
positive charge remaining with the carbonyl portion.
[R-C(=O)-R']
+
·
→
R· + [R'-C≡O
+
]
9. Loss or neutral species is common (H
2
O from alcohols, HCN, CO)
