2- Methylbutane fragmentation

The premise is to utilise a liquid film to provide a reaction environment which can be dynamically controlled in terms of heat and mass flux (influx/effiux) and to complement this with the on-line monitoring technique of Atmospheric Pressure chemical Ionisation (APcI)-Ion Trap Mass Spectrometry (ITMS). This technique allows the flux of protonated molecular ions (Mlf to be directly monitored (mass spectral dimension 1) and to fragment these species under tailored conditions within the ion trap (Collision Induced Dissociation (CID),mass spectral dimension 2), to produce fragment ions representative of the parent ion. This capability is central to allowing species with a common molecular weight to be quantified, for example butan-2,3-dione (MW=86 MH =87, glucose degradation product) and 3-methylbutanal (MW=86 MH =87, Strecker aldehyde from leucine). 

The effect of temperature on the formation of ion 74 (ms ) and the associated CID ms fragment ion 56 shows the onset of production at 130°C and a further accelerated but transient production at 170 C (Figure 5). The decay in production may be attributed to a number of conditions for example the consumption of precursors, further reaction or perhaps the masking by other species present. The latter two conditions may also account for the apparent absence of ion 87 (3-methylbutanal, Stecker aldehyde) in the spectrum at 170 C (Figure 3, lower spectra) however it is clearly observed at 130°C. 

Draw the disconnect fragments for disconnection of 2-(N,N-dimethylamino)-3-methylbutane at C2-C3 and label each as its most logical donor or acceptor. 

As you might surmise, molecules more complex than methane produce more complicated fragmentation patterns, as illustrated by the mass spectrum of 2-methylbutane (Fig. 8.60). A generalized representation of the fragmentation of the parent ion and daughter ions into combinations of smaller charged and neutral species for such molecules is shown in Equations 8.18 and 8.19. In these equations and the subsequent discussion, the radical cations are represented in a simplified form in which we omit the symbol for the unpaired electron. 

Application of these principles is important for interpreting the mass spectrum of 2-methylbutane (15) (Fig. 8.60). Several peaks reveal information in support of its structure. For example, the molecular ion, P, is seen at w/z 72, which is the molar mass you would calculate for 15 by using atomic masses of 12.000 for carbon and 1.000 for hydrogen. The small + 1 peak at m/z73 results from the presence of and in the compound. A P + 2 peak is not observed because it is statistically unlikely that a molecule of 2-methylbutane would contain two atoms of either or or one of each of these isotopes. As you can see, the most intense peak in the spectrum is not the molecular ion, but rather is a fragmentation peak having ml2 43. This peak is defined as the base peak its relative intensity is set as 100%, and the intensities of all other peaks in the spectrum are measured relative to it. 

Figure 11-26 Mass spectrum of 2-methylbutane. The peaks at m/z = 43 and 57 result from preferred fragmentation around C2 to give secondary carbocations.

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