Monocyclic hetero systems

Some common monocyclic hetero systems have trivial names, for example, pyridine, furan (see Table 4.1). 

An example of oxidative cleavage of a monocyclic hetero system is the conversion of furfuraldehyde into fumaric acid, a reaction that has been widely used 153... 

Monocyclic hetero systems not having a retained trivial name can basically be named in two different ways. It should be kept in mind, however, that the principles according to which indexing and abstracting organisations opt for one or the other possibility are sometimes anything but obvious. 

For such purposes as locating substituents, monocyclic heterocycles having only one non-carbon atom are numbered starting with the hetero atom as 1. If there is more than one hetero atom of the same kind, the more saturated hetero atom takes precedence (cf. imidazole in Table 1). In isoxazole and isothiazole, the divalent atom takes precedence, in conformity with the pnaciples of the Hantzsch-Widman system (Section 1.02.2.2). 

Because these compounds are monocyclic aromatic compounds, they must obey Hiickel s Rule. Hiickel s Rule requires An + 2n electrons, so the simplest aromatic compound should contain 6 n electrons (n = 1). Pyrrole, furan, and thiophene appear, however, to have only 4 n electrons (2 n bonds). In systems such as these, the extra electrons needed to produce an aromatic condition come from the unshared electron pairs in sp hybrid orbitals around the hetero atom. 

The three MO-indices discussed in this chapter—charge, bond order and free valence—are calculable solely from a given electronic configuration and the Hiickel LCAO-MO-coefficients of an arbitrary, planar, conjugated system (hydrocarbon or hetero-molecule). As was emphasised at the end of Chapter Three ( 3.5), these quantities are therefore independent of the actual numerical values assumed for the parameters a and / , no mention of which whatsoever, it will be noted, has been made in the current chapter. In the next chapter we shall see what information (likewise not dependent on a and P) can be obtained from the LCAO- MO-energy levels of, specifically, monocyclic, planar, conjugated systems. 

Some known monocyclic conjugated hetero jr systems containing heteroatoms like phosphorous, sulphur or selenium, which may take partin the ir system by means of either pv or outer dn orbitals, are shown in Fig. 1. 

Fig. l. Monocyclic conjugated hetero n systems in which outer d orbitals may be involved in cyclic conjugation... 

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