Mass spectra resonance stabilized cations

In the other type of fragmentation, a bond is cleaved so that the positive charge remains with one fragment and the odd electron goes with the other. Only the positive fragment is detected and appears in the mass spectrum. The stability of the product cation and radical determine the favorableness of this type of cleavage. You are already quite familiar with the factors that affect the stability of cations, especially carbocations. Although radicals are inherently more stable than carbocations because they are less electron deficient, they are stabilized by the same factors that stabilize carbocations. Thus, tertiary radicals are more stable than secondary radicals, and secondary radicals are more stable than primary radicals. Resonance stabilization is also important. 

The mass spectrum of pentanoic acid is given in Figure 20-7. The base peak at m/z 60 corresponds to the fragment from loss of propene via the McLafferty rearrangement. The strong peak at m/z 73 corresponds to loss of an ethyl radical with rearrangement to give a resonance-stabilized cation. 

Chapter 22). In contrast, the mass spectrum at the right shows extensive fragmentation, as one frequently sees with alkynes (Section 13-3). Note, for example, the strong peak at m/z = 39 for the resonance-stabilized cation HC=CCH2+. 

In addition, fragmentation at the carbon once removed from the triple bond is observed, giving resonance-stabilized cations. For example, the mass spectrum of 3-heptyne (Figure 13-7) shows an intense molecular ion peak at m/z = 96 and loss of both methyl... 

Mass Spectrometry The most common mass spectral fragmentation of alkylben-zene derivatives is the cleavage of a benzylic bond to give a resonance-stabilized benzylic cation. For example, in the mass spectrum of -butylbenzene (Figure 16-18),... 

Alkenes characteristically show a strong molecular ion peak, most probably formed by removal of one tt electron from the double bond. Furthermore, they cleave readily to form resonance-stabilized allylic cations, such as the allyl cation seen at tniz 41 in the mass spectrum of 1-butene (Figure 14.8). 

The fragmentation patterns of alkenes also reflect a tendency to break weaker bonds and to form more stable cationic species. The bonds one atom removed from the alkene function—the so-called allylic bonds— are relatively easily broken, because the result is a resonance-stabilized carbocation. For example, the mass spectra of terminal straight-chain alkenes sudi as 1-butene reveal formation of the 2-propenyl (allyl) cation, at m/z = 41, the base peak in the spectrum (Figure 11-29 A). 

Branched and internal alkenes fragment similarly at allylic bonds. Figure 11-29B shows the mass spectrum of 2-hexene, in which the base peak at m/z = 55 corresponds to formation of the resonance-stabilized 2-butenyl cation. 

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