aromatic species, benzene, is being questioned, but that is another story [ 56 ]). A
H€uckel number of cyclically delocalized electrons confers aromaticity on a mole-
cule (Section 4.3.5). The idea behind homoaromaticity (homologous aromaticity) is
that if a system is aromatic, then if we interpose one or more atoms between
adjacentporbitals of thepsystem, provided overlap is not lost the aromaticity
may persist (Fig.9.7). While there is little doubt about the reality of homoaroma-
ticity in ions, neutral homoaromaticity has been elusive [ 57 ].
One molecule that might be expected to be homoaromatic, if the phenomenon
can exist in neutral species, is triquinacene (Fig.9.7): the three double bonds are
held rigidly in an orientation which appears favorable for continuous overlap
with concomitant cyclic delocalization of sixpelectrons. Indeed, its potential
aromaticity was one of the reasons cited for the synthesis of this compound [ 58 ].
A measurement of the heat of hydrogenation of triquinacene found a value 18.8 kJ
mol"^1 lower than that for each of the next two steps (leading to hexahydrotriqui-
nacene) [ 59 ]. This was taken as proof of homoaromaticity in the triene, i.e. that the
compound was 18.8 kJ mol"^1 (4.5 kcal mol"^1 ) stabler than expected for an
interposed CH 2 group
benzene homobenzenei.e. i.e.?triquinaceneFig. 9.7 Homoaromaticity. Interposing a CH 2 group between one pair of formal double bonds of
benzene gives monohomobenzene. Is this delocalized like benzene, or is it just cycloheptatriene?
Is triquinacene, with a CH group interposed between each pair of formal double bonds, a
trishomobenzene?
570 9 Selected Literature Highlights, Books, Websites, Software and Hardware