Polyynes: Possible Bulk Synthesis and Chemical Properties - Polyynes: Synthesis, Properties, and Applications - page 426

solution in decalin is treated with a CuCl solution in aqueous ammonia
taken from (4, Chart 18.2 ) . After the complete precipitation, the polyynes
acetylides are recovered in the filter (5, Chart 18.2) while the purified decalin
is sent to a storage tank (6, Chart 18.2) in order to be recycled into the
process. If needed, decalin can be distilled at this stage without any problem
to separate the PAHs accumulated. The polyynes acetylides in (7, Chart
18.2) are hydrolized with diluted hydrochloric acid solution (or diluted
sulphuric acid) with the release of the free polyynes [7-10]. The hydrolysis is
conducted by stirring the polyynes acetylides with the acid solution in
presence of a solvent, for instance n-hexane, in order to collect the purified
polyynes, which can be used as a technical mixture or can be separated into
the components by preparative liquid chromatographic techniques.
18.3 HYDROGENATION OF POLYYNES TO ENE-YNES: AN EASY
ACCESS TO BIOLOGICALLY ACTIVE MOLECULES
The polyynes produced by the submerged electric arc in n-hexane show
the typical band pattern in the UV spectrum that we have discussed else-
where [14-18]. The spectrum of the polyynes in n-hexane is shown in Figure
18.7(A) . When the polyynes solution is shaken with a mixture of zinc dust
and hydrochloric acid, a reagent which is able to hydrogenate fullerene to
fulleranes [19], in a few minutes the UV spectrum changes radically as
shown in Figure 18.7(B). The changes in the band pattern are so radical that
they can be interpreted only in terms of formation of completely new
chemical species. The other aspect emerging from Figure 18.7(B-D) is that
the hydrogenation practically stops at this stage, although prolonged
shaking with Zn/HCl causes a certain reduction in the intensity of the bands
at about 217 nm, which appears less intense and shifted at 220 nm (compare
Figure 18.7(B) with 18.7(C)), the other bands are not affected at all. If the
hexane solution is left stirring overnight with the mixture of Zn/HCl, again
only the part of the spectrum around 220 nm is affected and appears with
reduced intensity (compare Figure 18.7(C) with Figure 18.7(D)). The curve
in Figure 18.7(E) is the difference spectrum between the ene-yne solution
and the pristine polyyne solution: the bands pointing upward are the new
bands formed as a consequence of the hydrogenation reaction while the
bands pointing downward are those disappeared because of the transforma-
tion of polyynes to ene-ynes.
Fortunately, the ene-ynes are easily recognizable because of their
peculiar electronic absorption spectra. In the past we have produced ene-
ynes by dehydrohalogenation reactions of chlorinated paraffins [20-22],
hence we have acquired a certain specific experience in the field.
Furthermore, the spectra of ene-ynes are well known in literature [23-25]
starting from the spectrum of mycomycin and its isomer isomycomycin.
In the context of the spectra of Figure 18.7(B-D), particularly interesting is