An Introduction to Spectroscopic Methods for the by F. Scheinmann (Eds.)

By F. Scheinmann (Eds.)

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Extra resources for An Introduction to Spectroscopic Methods for the Identification of Organic Compounds. Mass Spectrometry, Ultraviolet Spectroscopy, Electron Spin Resonance Spectroscopy, Nuclear Magnetic Resonance Spectroscopy (Recent Developments), Use of Various Spectral

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The most frequently encountered of these are shown in Table 4. L. A. ) 2 14 16 17 18 20 26 27 28, 42, . . ) 44 45 46 48 54 56 56 58 59 60 60 62 64 74 78 80 128 The scissions with and without rearrangement of ions in the mass spectrometer, such as have been described and as will be outlined for various classes of organic compounds below, often appear somewhat bewildering on first acquaintance with their courses difficult to rationalize. Perhaps the easier in this respect are the retro-Diels-Alder fragmentation and the McLafferty rearrangement, with their obvious parallels in classical organic chem­ istry.

Cori /Chu* V 'CH; -^CH2 2 C,hU CH 2 H H - CH2 = CH2 ( X L I ) m/e 120 (M) Ί1 m/ e 92 [-C3H7 C3Hp C3H3+ m/e m/e 91 (B) CH=CH m/e 39 77 - CH = CH a* m/e 65 m / e 51 SCHEME 1. Principal fragmentations of the molecular ion (XLI) of n-propylbenzene. (ii) Other aromatic hydrocarbons The fragmentation of polyalkylbenzenes proceeds as expected from the above obser­ vations. With polycyclic aromatic compounds very little fragmentation occurs unless substituted tropylium ions can be formed, that is, for naphthalene the molecular ion peak is the base peak and all other peaks are small, whereas with 2-n-butylnaphthalene the base peak is at mfe 141 (benzotropyhum ion); doubly charged ions are of notable occurrence in the spectra of polycyclic aromatic compounds.

Note that the process depicted as loss of H2 is the same as that mentioned above in the fragmentation of an ion from benzyl alcohol, and that, for example, the ion of m/e 91 undergoes the further fragmentation expected by considering it as the C7H7i" ion from benzyl compounds. It is impossible, using this single spectrum, to decide the orientation of the phenol as an unknown compound. 40 -co H2 C2H5 C7H7+ m/e 91 -C2H5 1* H. Ϋ M C2H5 -co \ / CH3 (XLIV) m / e m / e 108 136 (M) ♦ -H" OH C22H "5 i/e C2H5 135 C2H5 m/e 105 -H?

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