2- Methylbutane mass spectrum

The McLafferty rearrangement, as it is known, is useful for structure identification. For example, the mass spectrum of 2-methylbutanal shows a peak at m/e 58, while that of 3-methylbutanal shows a peak at m/e 44. 

The mass spectrum of 3-methylbutan-l-ol (Figure 12-21) shows a favorable loss of water. The even-numbered peak at m/z 70 that appears to be the molecular ion is actually the intense M-18 peak. The molecular ion (m/z 88) is not observed because it loses water very readily. The base peak at m/z 55 corresponds to loss of water and a methyl group. 

How could you distinguish the mass spectrum of 2,2-dimethylpropane from those of pentane and 2-methylbutane ... 

Chain branching increases the likelihood of cleavage at a branch point because a more stable carbocation can result. When we compare the mass spectrum of 2-methylbutane (Fig. 9.38) with the spectrum of hexane, we see a much more intense peak at — 15. 

PRACTICE PROBLEM 9.17 In contrast to 2-methylbutane and neopentane, the mass spectrum of 3-methylpentane... 

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. 

In contrast to 2-methylbutane and neopentane, the mass spectrum of 3-methylpentane (not given) has a peak of very low relative abundance at — 15. It has a peak of very high... 

Methylbutane has the same molecular formula as pentane. Thus, it too has a molecular ion with an m/z value of 72 (Figure 14.3). Its mass spectrum is similar to that of... 

The mass spectrum of 2-methylbutane. The peak at m/z = 57 corresponds to the loss of a methyl group and the formation of a relatively stable secondary carbocation. 

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.

The mass spectrum of 2-methylbutane (Figure 11-26) shows a pattern similar to that of pentane however, the relative intensities of the various peaks differ. Thus, the peak at m/z = 71 (M - 1) is larger, because loss of H from C2 gives a tertiary cation. The signals at m/z = 43 and 57 are more intense, because both arise from loss of an alkyl radical from C2 to form a secondary carbocation. 

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