The Fragmentation Patterns of Functional Groups

Each functional group has characteristic fragmentation patterns that can help identify a compound. The patterns began to be recognized after the mass spectra of many compounds containing a particular functional group had been studied. We will look at the fragmentation patterns of alkyl halides, ethers, alcohols, and ketones as examples. 

The mass spectrum of 1-bromopropane. The M and M + 2 peaks are about equal 

The way a molecular ion fragments depends on the strength of Its bonds and the stability of the fragments. 

The weakest bond in the molecular ion is the one most apt to break. In this case, the C — Br bond is the weakest. When the C—Br bond breaks, it breaks heterolytically, with both electrons going to the more electronegative of the atoms that were joined by the bond, forming a propyl cation and a bromine atom. As a result, the base peak in the mass spectrum of 1-bromopropane is at miz = 43 (M - 79 - 43 or [M-H2] - 81 = 43). 

The C—Cl (85 kcal/mol) and C — C (89 kcal/mol) bonds have similar strengths, so both bonds can break. The C—Cl bond breaks heterolytically, giving a base peak at m/z = 43. The C—C bond breaks homolytically the peaks at mIz = 63 and 

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Mass spectrometry of organic molecules has become well established in recent years and both the recording of spectra and the fragmentation patterns of functional groups have been comprehensively discussed (14, 19, 33, 126, 142). In the last 2 3 years the study of the behavior of organometallic compounds in the mass spectrometer, previously a neglected... 

The fragmentation patterns of 4-hydroxycoumarins which possess a carbonyl function (as an ester or an acyl group) at C-3 have been studied in considerable detail (66JCS(C)1712). 

Interpretation of mass spectra depends on the type of mass spectrometer and ionisation technique used. Hard ionisation methods such as El produce molecular ion fragmentation, which can be used to identify diagnostic fragmentation patterns and functional groups. Softer ionisation techniques such as ESI and MALDI provide pseudomolecular ion formation, and rules in accordance with spectral information can be used to identify corresponding molecular structure and elemental composition. Table 13.3 lists some of the types of information that can be provided by mass spectrometry, and Table 13.4 gives dehnitions of molecular masses that are highly relevant in mass spectrometry. 

A search of the infrared spectrum for the familiar characteristic groups is now in order. Note in particular the unsaturated functional groups. Look also at the fragmentation pattern of the mass spectrum for recognizable fragments. 

From a mass spectrum, the molecular ion can be identified and compared with the empirical formula to establish the molecular formula (Chapter 1). The functional groups present can be established from the infrared spectrum. The NMR spectrum and the fragmentation pattern of the mass spectrum will establish or confirm the structural formula. 

As can be seen in Table XIII, the presence of supplementary functional groups often strongly influences the fragmentation pattern. For example, a hydroxyl group at C-6 or C-7 position generally strongly favors the cleavage of the ethylene derivative (Routes A and E) (53,63,166,171). The substituents... 

There are certain rules determining fragmentation of organic compounds in a mass spectrometer. That is why on the basis of the fragmentation pattern it is possible to define the molecular mass, elemental composition, presence of certain functional groups, and often the structure of an analyte. There are a lot of similarities in the mass spectrometric behavior of related compounds. This fact facilitates manual interpretation of a mass spectrum, although it requires some experience. It is also worth mentioning that mass... 

This summary is provided for rapid reference to the common fragmentation patterns of simple functional groups. Some of these functional groups are discussed in more detail in later chapters. 

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