Alternant hydrocarbon, definition

For the definition of alternant hydrocarbons, see Magnasco (Magnasco, 2007, 2009a). 

Alternant hydrocarbons form a special class of hydrocarbons in which the carbon atomis can be sorted into two groups so that neighboring carbons always belong to different groups. A chain obviously meets this criterion, and rings formed by an even number of atoms will also do. Clearly, alternant hydrocarbons cannot contain odd-numbered rings by definition. Some examples for alternant and non-alternant hydrocarbons are shown in Fig. 10.1. 

Definition An alternant hydrocarbon is any polycyclic conjugated hydrocarbon having only even-member rings. 

That branch of chemistry which deals with the compounds of the element carbon the simpler carbon-containing compounds (such as calcium carbonate) are usually classed with inorganic chemistry and an alternative definition of organic chemistry is the chemistry of the hydrocarbons and their compounds. 

In the end, while computations on these alternative pathways, oxidative addition and cr-bond metathesis, have provided some insight, the question by which way does a particular reaction proceed cannot yet be answered definitively. It is very interesting, however, that both mechanisms involve the same Pt(II) intermediate in which the hydrocarbon binds in the square-planar coordination sphere of the metal. 

If the hydrocarbon radical cation has a definitive structure, proton loss occurs from one particular, well-defined position and these transformations are more selective than the alternative C-H abstractions from alkanes with radical reagents (Eq. 2). For example, C-H substitutions of the adamantane cage with radical reagents always give mixtures of 1 and 2-substituted adamantanes [2], As the adamantane radical cation (4) has one single structure, proton transfer from the radical cation to the solvent occurs highly selectively. Scheme 2 shows the geometry of 4 and the structure of the complex of the adamantane radical cation with acetonitrile (S) where the tertiary C-H bond is already half-broken. 

As an alternative, Lindstrom et a. [16] used a Fourier transform infrared (FTIR) spectrometry with flame ionization detection to determine the sum of hydrocarbons in the gas phase as NMHCs (non-methane hydrocarbons). The best result for diesel slip indicates a background signal of 10 ppmv corresponding to 120-200 ppmv carbon. A higher accuracy of lOOppbv for each Cy fraction leads to the best case conversion of 99.9966%. TOC measurements in the range 0.5-2 mgl are definitely more accurate. Conversion higher than 99.99966% demands a TOC lower than 1.75 mg 1 or a residual amount of carbon in the gas phase of 0.77 ppmv. In principle, accuracies in the ppb range are possible with specialized GC techniques [21, 22]. 

The structures shown in Figure 63, which include kekulene as the last structure, which was prepared in 1978 by Staab, Diderich, and co-workers, have been excluded from both alternative definitions of benzenoids, although they are expected to show considerable similarities in their properties to ben-zenoid hydrocarbons. The reason for their exclusion from the class of benzenoids is that, although they can be viewed as derived by fusion of benzene rings, they also incorporate a larger (central) ring that does not represent benzene. Hence, such structures should not be taken as a standard for characterization of benzenoid hydrocarbons. Because of their considerable similarity to benzenoids, one may refer to these as pseudo-benzenoids. Such compounds have been of considerable theoretical interest, particularly in view of the intriguing notion of super-aromaticity that we will address later. 

The main reason to mention perfect Clar structures is to draw the attention of readers to different definitions used here by Fowler and Pisanski, differ ent classifications of Clar cages by Flocke, Schmalz, and Klein, and different definitions of Clar structures of fullerenes as we have outlined at the beginning of this section. Upon closer examination, we expect readers to agree at our approach is the only one that truly extends the spirit of the jt-aro-matic sextets of Clar from benzenoid hydrocarbons to fullerenes. That does not make the alternative approaches, which are equally legitimate, less worthy, as each of them serves its own purposes — but readers should be alerted in advance and should be spared possible confusion due to use and promotion of the name of Clar in several different conceptual connections. Observe that in our definition of the Clar structure of fullerene, only hexagonal faces can be the site of aromatic jr-sextets, but that is not the case in the approach of Fowler and Pisanski, where pentagonal faces play a role similar to that of hexagonal faces. 

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