Alkanes fragmentation patterns

Nepeta (Lamiaceae) is a genus of perennial or annual herbs found in Asia, Europe and North Africa. About 250 species of Nepeta are reported of which, 67 species are present in Iran. Some species of this genus are important medicinal plants and their extracts have been used for medicinal purposes. Aerial parts of Nepeta sintenisii Bornm. was subjected to hydrodistillation and the chemical composition of isolated essential oil has been analyzed by GC/MS method for first time. Identification of components of the volatile oil was based on retention indices relative to n-alkanes and computer matching with the Wiley275.L library, as well as by comparison of the fragmentation patterns of the mass spectra with those reported in the literature. 

Figure 8.6 Mass spectrum of C34 w-alkane (C34H70). The complete molecule appears at M = 478 and various fragment ions (m/z = 57, 71, etc.) at lower masses. The fragmentation pattern is shown on the molecular structure.

Figure 5-1 shows the GC of fraction 4. It shows alkanes ranging from to C-Q. It is quite possible that the fraction may contain hlgher alkanes which are not detected due to the GC-oven temperature limit. The peaks are identified from the MS fragmentation pattern. Fraction 5 is collected after a 0.5 minute... 

For branched alkanes, the fragmentation patterns could be very complex [3, 111]. The prompt yield of the -H radicals is always minor the highest yields are of the radicals formed by scission of skeletal C-C bonds next to the branches. For example, in radiolysis of isooctane only 15% of the radicals are of the -H type, the rest bemg tert-butyl and 2-propyl radicals [111]. In radiolysis of 2,3-dimethylbutane, 70% of radicals are 2-propyl and 30% are -H radicals (2,3-dimethyl-2-butyl). It is not known what species dissociate (singlets triplets excited holes excited radicals ) and what controls these fragmentation patterns. 

The fragmentation patterns of cycloalkanes may show mass clusters arranged in a homologous series, as in the alkanes. However, the most significant mode of cleavage of the cycloalkanes involves the loss of a molecule of ethene, either from the parent molecule or from intermediate radical-ions. The peak at mie = 42 in cyclopentane and the peak at mie = 56 in methylcyclopentane result from the loss of ethene from the parent molecule. Each of these fragment peaks is the most intense in the mass spectrum. 

In the case of MS study of n-alkanes, very little use has been made of other modes of ionization such as chemical ionization (Cl) or photoionization (PI), field ionization (FI) and others. This is because the sensitivity of these methods and the quantity of the produced ions are very low and the molecular information is minimal. The MS of branched alkanes is more structure-dependent, and the fragmentation pattern follows the position of branching. The abundance of C H2 +i fragments can be related to the... 

The cracking pattern for C Flo when considered in conjunction with nC5Fi2 and the CF3 removal process compares reasonably well in terms of the order of the most abundant fragments. The fit is certainly not a perfect one but in the light of fragmentation patterns quoted in the literature for perf1uorinated alkanes and alkanes C1-C5 and higher this explanation seems the only one at all feasible. 

In Summary Fragmentation patterns can be interpreted for structural elucidation. For example, the radical cations of alkanes cleave to form the most stable positively charged fragments, haloalkanes fragment by rupture of the carbon-halogen bond, alcohols readily dehydrate and undergo a cleavage, and alkenes break an allylic bond to form a resonance-stabilized carbocation. 

Some classes of compounds are so prone to fragmentation that the molecular ion peak IS very weak The base peak m most unbranched alkanes for example is m/z 43 which IS followed by peaks of decreasing intensity at m/z values of 57 71 85 and so on These peaks correspond to cleavage of each possible carbon-carbon bond m the mol ecule This pattern is evident m the mass spectrum of decane depicted m Figure 13 42 The points of cleavage are indicated m the following diagram... 

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