Polyynes: Simple Synthesis in Solution Through the Glaser Reaction - Polyynes: Synthesis, Properties, and Applications - page 487

and soot are formed mainly by the plasmalysis of the solvent caused by the
arc. In any case, the HPLC analysis has shown that the PAHs concentration
is two orders of magnitude lower than the polyynes concentration and for
some synthetic purposes this could be a sufficient condition. It is possible to
purify the polyynes, separating them from the PAHs and the soot by preci-
pitation as acetylides, but this operation is relatively complex and not com-

pletely satisfactory (see Chapter 18 ). The best purification way remains the

preparative HPLC [8].
Another drawback suffered by the polyynes solution prepared by the
submerged electric arc technique is the relatively low concentration achiev-
able. Usually, concentrations as high as 10 5 -10 6 M can be reached.
Higher concentrations can be reached by prolonging arcing for hours [8].
Alternatively, it is also possible to concentrate the solutions by distillation
under reduced pressure (see Chapter 18). Under these circumstances it is
also possible to increase the relative concentration of C 8 H 2 and higher
polyynes chains at the expenses of C 6 H 2 which, being more volatile, distils
away with the solvent.
The synthesis of polyynes by arcing graphite electrodes remains a very
simple synthetic approach and has also many implications in different fields
of science from astrochemistry to soot formation mechanism, as already
discussed in other chapters of this topic.
As an alternative to the submerged electric arc, the polyynes can be
produced simply by hydrolizing calcium carbide (CaC 2 ) into an NH 4 Cl
aqueous solution of CuCl/CuCl 2 , followed by acid hydrolysis of the
resulting oxidized acetylides. The hydrolysis of calcium carbide produces
in situ acetylene which, by reacting with Cu(I), forms dicopper acetylide
Cu 2 C 2 . The latter compound is then oxidized by the action of Cu 2 þ , causing
the coupling reaction. It has been observed that the presence of Cu 2 þ is not
essential, since the polyynes are formed even when only Cu þ ions are
present. This is due to the fact that even the atmospheric oxygen is sufficient
to cause the oxidative coupling reaction. It is also possible to start from
CuCl 2 solutions; the addition of CaC 2 and the resulting acetylene produced
causes the reduction of Cu 2 þ to Cu þ so that the formation of Cu 2 C 2 preci-
pitate is observed. In any of the above mentioned cases, the liberation of the
polyynes is caused by the addition of hydrochloric acid after the complete
hydrolysis of calcium carbide. Schematically, the reactions involved can be
represented as follows:

1 Þ CuCl 2 = CuCI

CaC 2 þ

H 2 O ! H ð C C Þ m H ð m ¼ 3,4,5,6,7 Þ

2 Þ Heptane þ HC1 aq

The polyynes liberated from their Cu þ salts by the acid hydrolysis are
trapped into hydrocarbon solvents by shaking the solution with n-hexane,
n-heptane or n-octane. The polyynes are easily detected by HPLC analysis