Synthesis of Monocyanopolyynes and Dicyanopolyynes with the Submerged Electric Arc - Polyynes: Synthesis, Properties, and Applications - page 328

disappear from solution after some days storage at room temperature while
the latter remain unchanged. The same study was conducted on an aceto-
nitrile solution of dicyanopolyynes and hydrogen-terminated polyynes
prepared by arcing graphite electrodes in liquid nitrogen and trapping
the products in a CH 3 CN solution. HPLC-DAD analysis after only 1 day
storage of the solution at room temperature showed the complete disappear-
ance of all the dicyanopolyynes while the hydrogen-terminated polyynes
were still present at the same concentration as on the previous day. New
spectra of unknown products were detected. It appears that the dicyano-
polyynes, as the monocynopolyynes, rearrange into other products.
15.3.3 P OLYYNES FROM THE E LECTRIC A RC FROM G RAPHITE
E LECTRODES IN W ATER
In Section 15.3.2, the electric arc in liquid nitrogen produces cyanopoly-
ynes as expected but also produces hydrogen-terminated polyynes, which
are rather unexpected, unless we admit that traces of water play a role as a
supplier of hydrogen atoms during the plasmalysis caused by the electric arc.
In Section 15.2.3, we have clearly demonstrated that when the submerged
electric arc between graphite electrodes is struck under distilled water, it is
possible to observe the typical electronic spectrum of a mixture of polyynes
in water. Furthermore, liquid chromatographic analysis permits the sepa-
ration and identification of all the hydrogen-terminated polyyne series nor-
mally observed in organic solvents like methanol and n-hexane. Thus, it is
demonstrated that the hydrogen atoms in a system of graphite electrodes
and water are supplied by plasmalysis of water molecules. Consequently, it
is now clear that the hydrogen-terminated polyynes observed as secondary
products in the arcing experiment in liquid nitrogen derive from the
humidity and traces of water present in the liquid nitrogen itself and in the
reactor.
15.4 CONCLUSIONS
Arcing graphite electrodes in acetonitrile produces simultaneously two series
of polyynes: the ''ordinary'' hydrogen-terminated polyynes and a series of
monocyanopolyynes. Thus, acetonitrile is a solvent able to supply both H
and CN radicals which attach to the growing carbon chains. In our earlier
work (see also Chapter 8 ) [18-22] we have found that all the other solvents
tested, either hydrocarbons or alcohols, invariably were able to supply
hydrogen atoms as end capping and stabilizing moieties of the carbon
chains. This approach may lead to an easier route to monocyanopolyynes.
In acetonitrile solution the monocyanopolyynes are unstable and dis-
appear completely in some days if stored at room temperature. The hydrogen-
terminated polyynes present in the same solution appear much more stable