Branched alkanes fragmentation

Branched alkanes fragment more readily than straight chain alkanes, so branched alkanes are less likely to show a molecular ion peak than n-alkanes. Cycloalkanes show a strong molecular ion peak and also show the characteristic peaks separated by 14 Da. Figure 10.14 is the mass spectmm of hexane, CeHi4, MW = 86, and Fig. 10.15 is the mass spectmm of cyclohexane, CeHi2, MW = 84. Cyclohexane has a stronger molecular ion... 

The molecular ion intensity decreases with increased branching, therefore the molecular ion peak may be nonexistent. The loss of 15 Daltons from the molecular ion indicates a methyl side chain. The mass spectra of branched alkanes are dominated by the tendency for fragmentation at the branch points, and hence are difficult to interpret. 

Some of the cations may fragment or undergo ion-molecule reactions before neutralization. As shown in the older literature, the fragmentation has higher yield in the radiolysis of the smaller and/or highly branched alkanes such as neopentane or isooctane. Ion-molecule reactions, such as the H transfer, may also reduce the Si yield ... 

In branched chain alkanes, fragmentation often occurs next to the branch site, allowing formation of the more stable secondary, or even tertiary, carbocation ... 

Spectra of branched saturated hydrocarbons are grossly similar to those of straight-chain compounds, but the smooth curve of decreasing intensities is broken by preferred fragmentation at each branch. The smooth curve for the -alkane in Figure 1.14 (top) is in contrast to the discontinuity at C)2 for the branched alkane (Figure 1.14, bottom). This discontinuity indicates that the longest branch of 5-methylpentadecane has 10 carbon atoms. 

Cation and radical stabilities help to explain the mass spectra of branched alkanes as well. Figure 12-19 shows the mass spectrum of 2-methylpentane. Fragmentation of a branched alkane commonly occurs at a branch carbon atom to give the most highly substituted cation and radical. Fragmentation of 2-methylpentane at the branched carbon atom can give a secondary carbocation in either of two ways ... 

Figure 5.1 Major reactions in catalytic reforming illustrated with specific examples (a) dehydrogenation of cyclohexanes to aromatic hydrocarbons (b) dehydroisomerization of alkylcyclopentanes to aromatic hydrocarbons (c) dehydrocyclization of alkanes to aromatic hydrocarbons (d) isomerization of n -alkanes to branched alkanes (e) fragmentation reactions (hydrocracking and hydrogenolysis) yielding low carbon number alkanes.

In cyclohexane and decalins, reaction (1) is endothermic by 0.1-0.4 eV [60] and it seems reasonable that the excitation of the hole may facilitate the proton transfer. Fragmentation of matrix-isolated hydrocarbon radical cations upon excitation with 2-4 eV photons was observed by EPR (see review [61]). For cycloalkanes, the main photoreaction is reaction (3). For radical cations of methyl-branched alkanes, the loss of CH4 was also observed, while the radical... 

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. 

This stability of the non-classical carbonium ion or its open C3H7 fragment makes the m/z 43 fragment the base peak even for higher normal or branched alkanes. The assumption that alkanes, which are the simplest of all organic compounds, will hence produce easy-to-interpret MS was proved erroneous . 

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... 

A very detailed discussion of branched alkanes MS appears in the literature reviews of biomarkers Most of the compounds were analysed by combined GC/MS. A more detailed examination of the use of fragmentation profiles will be given in our discussion of separation methods and petroleum analysis (Section VII. A). 

The fragmentation of an alkane can be understood by assuming that a C—C bond is broken, sometimes accompanied by a hydrogen shift in order to minimize the energy requirements this is, to a first approximation, a rather satisfactory model. In the case of a branched alkane the bond to a tertiary or quaternary carbon atom is more easily broken, a fact that allows the determination of the branching point(s) with a good level of confidence. Except for small alkanes, the cleavage of C—H bonds seems to be a rare event. 

So far we have made alkane molecules having extended chains (Scheme 4.2) by using H3C and H2C fragments. We can, however, easily imagine how we could have formed branched alkanes by using also the triple-connector HC fragment and the quadruple-connector fragment, C. Let us provide exemples of this in Scheme 4.3. 

Alkanes that have carbon groups attached to the longest unbranched chain (called substituents) are known as branched chain alkanes. When that branch is an alkane fragment, it is known as an alkyl group or an alkyl substituent. 

The most abundant peak in the spectrum of this compound is expected to be at M—43, corresponding with formation of a tertiary carbocation, via loss of a propyl radical. A tertiary carbocation can also be produced via loss of a methyl radical (from the left side of the molecular ion above) however, a methyl radical is less stable than a primary radical. Certainly, all possible fragmentations are observed under the high-energy conditions employed, but the most abundant peak will generally result from formation of the most stable carbocation via expulsion of the most stable possible radical. Therefore, it is generally possible to predict the location of the most abundant peak that is expected in the mass spectrum of a simple alkane and branched alkane. 

The cracking (rupture of C-C bonds) of hydrocarbons can occur by either a thermal or a catalytic mechanism, but catalytic cracking requires an acidic catalyst. The first step in acidic cracking is carbocation formation, followed by fragmentation, disproportionation, and isomerization. Simple olefins are protonated in mineral acid solutions, but protonation of aromatics is more difficult and requires a strong acid. Either Lewis or Bronsted acids can protonate branched alkanes, but severe conditions are necessary to protonate n-alkanes. 

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