Of tellurophene and congeners

In this section the discussion is confined to a quantitative comparison of some ground state properties of tellurophene and its congener systems the chemical aromatic properties are discussed in Section III.C. 

It has been suggested that the ground-state aromaticity of furan and its heavier congeners follows the trend below (see Ref 36 and references therein) thiophene > selenophene > tellurophene > furan. 

Tellurophene-2-carboxylic acid is the weakest in the congener series (Table XX). Nevertheless it is still stronger than benzoic acid, in spite of the fact that tellurium atom is less electronegative than carbon and is electron-releasing by resonance. However, it must be recalled that the positive end of the dipole moment is on the hydrocarbon part of the ring (Section II,B,2). A linear dependence between the pKa values and C=0 stretching frequencies of monomer and dimer form has been observed. 2... 

The mass spectrum of tellurophene has been recorded and compared with those of other congeners.104 The tellurophene spectrum shows the molecular ion as the base peak, the M-HTe and M-acetylene peaks, and an important ion of mass 52 corresponding to the loss of tellurium. 

The difference between the dipole moments of the aromatic compound and the corresponding tetrahydro derivative is called the mesomeric moment and represents a measure of the -electron delocalization. The mesomeric dipole moment is directed, in all the congener systems, from the heteroatom toward the ring. The values obtained (furan 1.03 D, thiophene 1.35 D, selenophene 1.29 D, tellurophene 1.17 D) are in excellent quantitative agreement with other aromaticity indices based on structural and magnetic properties12 (see discussion in Section II,C). 

It has been observed33 that the differences (A) between I(7T2) and I(n,) parallel the ground-state aromaticities of the congener rings, as estimated by several different approaches,12 in the sense that a higher aromaticity corresponds to a smaller energy difference thiophene (A = 2.61 eV) > selenophone (A = 2.85 eV) > tellurophene (A = 3.40 eV) > furan (A = 4.08 eV). 

There are two different basic methods for obtaining the five-membered heterocycles. In the first approach, the ring is constructed directly from aliphatic compounds the other and less common approach makes use of congener rings or other heterocyclic systems as starting materials. All the syntheses reported for the tellurophene system have been based on the first principle. 

Although the chemistry of congener benzol A Iheterocycles containing oxygen,13 sulfur,74 and selenium75-78 has been the subject of many investigations, the chemistry of the last member of the series, benzo[6]-tellurophene (27), has received less attention. 

The photoelectron spectrum of benzo[6]tellurophene has been recorded108 with He(I) and He(II) radiation and compared with those of other benzo[6]congeners. 

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