Hexanones mass spectra

In the mass spectrum of 2-hexanone (Figure 20.1), the molecular ion is apparent at m/z 100, which can be confirmed by preparing the methoxime derivative. The compound type is verified by the presence of m/z 43 and 58. 

Example The NR mass spectrum of acetone closely resembles its 70 eV El mass spectrum (Chap. 6.2.1), thereby demonstrating that the molecular ion basically retains the structure of the neutral (Fig. 2.29). [144] However, the isomeric CsHeO" ions formed by McLafferty rearrangement of 2-hexanone molecular ion are expected to have enol structure (Chap. 6.7.1), and thus the corresponding NR mass spectrum is easily distinguished from that of acetone. 

White EL, Bus JS, Heck HD. 1979. Simultaneous determination of n-hexane, 2-hexanone and 2,5-hexanedione in biological tissues by gas chromatography mass spectrometry. Biomed Mass Spectrum 6 169-172. 

Figure 19.19 Mass spectrum of 5-methyl-2-hexanone. The peak at m/z = 58 is due to McLafferty rearrangement. The abundant peak at m/z = 43 is due to cleavage at the more highly substituted side of the carbonyl group. Note that the peak due to the molecular ion is very small.

FIGURE 9.23 Electron ionization mass spectrum of 3-hexanone and the two a cleavages possible from the molecular ion. 

Figure 9.44 depicts the eliminafron of carbon monoxide from m/z 71 and m/z 57 ions issued from ionized 3-hexanone (see the Figure 9.23 spectrum). The resulting m/z 43 and m/z 29 ions are abundant in the corresponding mass spectrum. 

Because 3-hexanone does not contain any nitrogen atoms, the molecular ion has an even mass/charge ratio m/z 100. The fragment ions observed in its spectrum are m/z 27, 29,41,43, 57, and 71. Their odd m/z ratios indicate that they issued from simple cleavages (and subsequent fragmentations) none is the product of a rearrangement. 

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