Huckel rule heterocyclic aromatic compounds

Section 11 23 Huckel s rule can be extended to heterocyclic aromatic compounds Unshared electron pairs of the heteroatom may be used as tt electrons as necessary to satisfy the 4n + 2 rule... 

Heterocyclic Aromatic Compounds 440 Heterocyclic Aromatic Compounds and Huckel s Rule 442 Summary 444 Problems 448... 

We know that aromatic compounds, polynuclear and heterocyclic compounds follow Huckel s rule in which they have a (4n + 2) system of n electrons and the protons attached to such systems are extremely deshielded due to the circulating sextet of n electrons. As a result, the signals of aromatic protons appear at a very low field than those observed even for benzene. The tau value of benzene is T = 2.63. From this the aromaticity of a compound can be verified. 

This chapter deals with heteromacrocycles rather than smaller-ring heterocycles. Traditionally, heterocycles are three- to eight- or nine-membered rings that contain one or more noncarbon heteroatoms. Many, if not most, of the known heterocycles are aromatic compounds in which the noncarbon atoms contribute electrons to meet Huckel rule requirements. The crown ether situation contrasts with that of traditional heterocycles. The bulk of crown ether compounds contain 12 or more members and are aliphatic rather than aromatic. Those heterocycles that have cation binding properties usually contribute one or at most two donors to a cation s primary solvation... 

Huckel s rule (1931) for planar species states that if the number of tt electrons is equal to 2 -L 4/t, where n equals zero or a whole number, the species is aromatic. The rule was first applied to carbon-containing monocyclics in which each C is capable of being -hybridized to provide a p orbital for extended n bonding it has been extended to unsaturated heterocyclic compounds and fiised-ring compounds. Note that benzene corresponds to n = 1. 

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