Polyynes: Synthesis with the Submerged Electric Arc - Polyynes: Synthesis, Properties, and Applications - page 152

reactions remained almost the only approach for the production of these
molecules and was adopted recently [24-27]. However, the formation of
polyyne chains has been detected by mass spectrometry both in the eva-
poration of elemental carbon in vacuum [28] and more recently by laser
ablation of graphite [29]. The latest two mentioned approaches for the
production of polyynes are not useful for preparative purposes since the
amount of these molecules obtained is so low that it is sufficient only to feed
a mass spectrometer. Very recently it has been found that polyynes in
solution can be produced by lased ablation of graphite particles or even
from fullerenes and other carbonaceous materials suspended in organic
solvents [30,31]. Alternatively, and more easily, polyyne solutions can be
prepared by an electric arc between graphite electrodes submerged in an
organic solvent [32-39]. The extremely easy access to polyynes will have an
enormous impact in the possibility of studying the chemical and physical
properties of these molecules and in using them as intermediates for organic
synthesis.
8.2 EXPERIMENTAL ASPECTS ON THE SYNTHESIS AND
ANALYSIS OF POLYYNES WITH THE ELECTRIC ARC
8.2.1 S YNTHESIS OF P OLYYNES IN A S UBMERGED E LECTRIC A RC
The synthesis of polyynes from a submerged electric arc is a simple and easy
technique, not expensive and accessible to everybody. It requires only two
graphite electrodes having 99.999% purity (but this high purity is not
strictly required) which can be acquired from Aldrich. The graphite elec-
trodes used in our study were graphite rods having a diameter of 0.6 cm and
length of 15 cm.
The graphite electrodes were mounted inside to rubber stopcocks and
inserted into a 100ml three-necked round-bottomed flask as illustrated in
Figure 8.1 . The two electrodes were put in contact each other with a ''V''
geometry and connected to a d.c. power supply using commercial copper
wires. The contacts between the graphite electrodes and the wires were
ensured by an insulating tape band (see Figure 8.1). The third neck of the
flask was fitted with a valve. A solvent (usually 50ml) was poured inside
the flask so that the ends of the electrodes are fully submerged into the
solvent. Typical solvents that can be used are hydrocarbons and alcohols.
With hydrocarbons we have obtained very good synthetic results by using
decahydronaphthalene (decalin). Additionally n-alkanes, especially those
having relatively high boiling points, give good results. Thus, dodecane,
decane, octane, heptane and even hexanes are all satisfactory. Among the
alcohols, methanol, ethanol, and n-propanol or i-propanol give satisfactory
results.