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230 Chapter 11 ALKENES; INFRARED SPECTROSCOPY AND MASS SPECTROMETRY Spectrum A shows a base peak at = 57 and other significant ions with = 56, 41, 29 (C₂H₅) and 27. The molecular ion at = 86 is weak. Spectrum B also shows a base peak at = 57 The peak at m/z = 43 (C₃H₇) is bigger than in spectrum A. while that at = 29 (C₂H₅) is smaller. The molecular ion is more intense. Spectrum C shows a base peak at = 43 (C₃H₇). The molecular ion is weak. but = 71 (C₅H₁₁) is prominent in this spectrum. Now consider the three structures and the bonds in the molecular ion most likely to fragment in each: CH₃CH₂⁺ (m/z = 29) Hexane: CH₃CH₂CH₂⁺ (m/z = 43) = 57) Cleavage is most favorable between CH₂ groups (avoiding formation of methyl cation), but since the best cations that you can get are only primary-and not very stable-fragmentation as a whole will be less likely. Spectrum B seems the best match because of the prominent molecular ion. CH₃ (m/z = 43) 2-Methylpentane: = 71) Cleavage will occur mainly about the CH to give secondary cations. The best match is spectrum C. CH₃ 3-Methylpentane: = 57) Fragmentation occurs at the indicated bond to form mainly sec-butyl cation to a lesser extent, ethyl cation = 29). Spectrum A fits best. As is typical in mass spectrometry, the large amount of energy imparted to molecules in the process induces rearrangements and other modes of fragmentation as well. Fortunately, ions arising from such processes do not usually dominate the spectrum. 59. Major peaks: 43 (CH₃CH₂CH₂)⁺ from M-Br m/= 41 from M-HBr-H Minor peaks: 109 107 } from M-CH₃ 42 from M-HBr m/z 29 from 28 from 27 (CH₂=CH)⁺ from M-Br-CH₃-H 60. Compound is saturated (see Section 11-9). Try to use the general guidelines that intense fragment peaks either result from the loss of relatively stable neutral species or are due to relatively stable cations. So. looking at the high intensity 73 peak for isomer C. it corresponds to (M or loss of CH₃. This is most likely if the remaining fragment is a very stable cation. for example. CH₃ + + CH₃ CH₃ 73 cation. stabilized by oxygen lone pair.