Submerged electric arc between graphite electrodes: A one-pot tool for the synthesis of long-chain polyynes in solution

Article abstract of DOI:10.1016/j.tetlet.2003.10.100

Polyynes, a class of molecules described by the general formula H-(CC) _mH (where m is an integer) can be synthesized using an electric arc between graphite electrodes submerged in an organic solvent such as methanol, n-hexane, n-dodecane, decahydronaphthalene or acetonitrile. When the electric arc is used in acetonitrile at -40°C, polyyne chains of up to 18 carbon atoms (m=9) have been produced together with monocyanopolyyne as by-product. The polyynes can be reduced to ene-ynes by shaking a hexane solution of them with Zn/HCl.

Full text of DOI:10.1016/j.tetlet.2003.10.100

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 141–144
Submerged electric arc between graphite electrodes: a one-pot tool
for the synthesis of long-chain polyynes in solution
Franco Cataldo*
Soc. Lupi Chemical Research Institute, Via Casilina 1626/A, 00133 Rome, Italy
Received 21 August 2003; revised 10 October 2003; accepted 20 October 2003
Abstract—Polyynes, a class of molecules described by the general formula H–(CBC)mH (where m is an integer) can be synthesized
using an electric arc between graphite electrodes submerged in an organic solvent such as methanol, n-hexane, n-dodecane, deca-
hydronaphthalene or acetonitrile. When the electric arc is used in acetonitrile at )40 ꢀC, polyyne chains of up to 18 carbon atoms
(m ¼ 9) have been produced together with monocyanopolyyne as by-product. The polyynes can be reduced to ene–ynes by shaking a
hexane solution of themwith Zn/HCl.
ꢁ 2003 Elsevier Ltd. All rights reserved.
Polyynes are widely diffuse in nature and characterized
by the presence of several carbon–carbon triple bonds.1
More than one thousand naturally occurring polyynes
are known.1 Some are biologically active and some have
antibiotic and/or antitumoral activity.1 The simplest
polyynes, carbon chains connected by a sequence of
single and triple bonds, are described by the general
formula:
compounds of interest in the context of the mechanism
of fullerene,5 polycyclic aromatic and soot formation.6;7
Polyynes are also thought to be carriers of certain
spectral features of the interstellar medium, while
cyanopolyynes have been identified in molecular clouds
using radioastronomy.8
The arc discharge between carbon electrodes is one of
the preferred methods for the bulk production of fulle-
renes9 and also of carbon nanotubes.10 For their syn-
thesis, the AC or DC arc discharge is usually performed
in the gas phase under reduced pressure in the presence
of a quenching gas, for example, He.8;9 Usually very
high currents are employed ranging from 50 to 100 A.
For the synthesis of polyynes at roomtemperature two
graphite electrodes submerged into the solvent and
arranged in a ÔVÕ geometry and electric current at
5–10 A11 were used. The identification of the products
was made by HPLC analysis using a diode-array de-
tector and a C-8 column.11;12 It has been found11;12 that
the formation of polyynes depends exclusively upon the
vaporization of carbon fromthe graphite electrodes and
is independent of the nature of the solvent, although in
certain cases the solvent may interact and give by-
products (e.g., polyynes end capped with adventitious
groups and not with hydrogen). The polyynes produced
fromthe submerged electric arc are essentially hydrogen
terminated, because they react promptly with IlosvayÕs
reagent (CuI, aq ammonia and hydroxylamine) giving
copper acetylides, which are insoluble and brown col-
oured with metallic lustre in some cases. Furthermore,
H–ðCBCÞmH ðwhere m is an integerÞ:
Specific polyynes have been synthesized generally by
complex multi-step coupling reactions2 in low yields.
The difficulty of synthesis has hindered research on the
chemistry, properties and applications of these mole-
cules. Mixtures of polyynes can be produced by the acid
hydrolysis of oxidized dicopper acetylide or diacetylide3
or by a dehalogenation and dehydrohalogenation reac-
tion of chlorinated paraffins,4 the latter reaction tending
to give more ene–ynes than pure polyynes.
In this letter we present a one-pot method for the syn-
thesis of mixtures of polyynes and cyanopolyynes in
solution. Carbon chains of up to 18 carbon atoms are
detectable through this process, which could provide
Keywords: polyynes; ene–ynes; synthesis; electric arc; graphite elec-
trodes.
* Tel./fax: +39-062050800; e-mail: cdcata@flashnet.it
0040-4039/$ - see front matter ꢁ 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.10.100

142
F. Cataldo / Tetrahedron Letters 45 (2004) 141–144
the FT-IR spectra of the polyyne solutions show a sharp
peak at 3312 cmꢀ1, which is due to the terminal triple
bond –CBC–H stretch proving that the polyynes are
hydrogen capped.11;12 HPLC analysis of the polyynes
produced by the electric arc in CH3CN at roomtem-
perature has permitted the unequivocal identification of
all the polyynes series from m ¼ 4 to m ¼ 8, hence car-
11;12
bon chains from8 to 16 atoms
members of the series.12
and also the lower
The longest polyyne chain detected by arcing at room
temperature in acetonitrile (and also in other solvents)
consisted of 16 carbon atoms11;12 (m ¼ 8). In order to
produce polyynes with chains having m > 8, we have
arced acetonitrile at )40 ꢀC and detected13 a polyyne
having a chain of 18 carbon atoms (m ¼ 9). The results
of the HPLC analysis14 are summarized in Table 1. The
polyynes were identified fromtheir characteristic elec-
tronic absorption spectra and by comparison with
published spectra.2;15 Table 1 shows the band pattern of
each individual polyyne detected compared with the
pattern taken fromthe literature: the agreement is
excellent. FromTable 1, it can be observed that together
with the hydrogen-capped polyynes, another series of
molecular species was detected, which possessed inter-
mediate retention times with respect to normal polyynes.
For example, between the polyynes H–(CBC)5–H and
H–(CBC)6–H, which have retention times of 2.13 and
2.83 min, a smaller peak in the HPLC chromatogram
was observed at 2.41 min. A similar situation occurred
regularly in all the chromatographic analyses (see Table
1). The spectra of these species are essentially similar to
those of the ÔnormalÕ polyynes but regularly shifted in
the band position in an intermediate fashion between
the preceding and the following hydrogen-capped
polyyne. This fact can be appreciated in Table 1, where
the most intense absorption band of each polyyne
spectrumis reported in bold. Fromthe electronic
absorption spectra, the retention time and due to the
fact that these intermediate species have been detected
only by arcing acetonitrile (and not by arcing metha-
nol or n-hexane), these species have been assigned as
monocyanopolyynes. Thus, for instance the species
having the retention time of 2.41 min is H–(CBC)5–
CBN. The FT-IR of the crude reaction mixture13 shows
acetylenic stretching bands at 2201 and 2160 cmꢀ1 and
also a nitrile band at 2247 cmꢀ1. Table 1 also reports
both the retention time of H–(CBC)9–H, a polyyne with
18 carbon atoms, and its UV spectrum, which is in good
agreement with literature data. This polyyne with m ¼ 9
was never detected when acetonitrile was arced at room
temperature but was obtained at )40 ꢀC. Thus, it ap-
pears reasonable to assume that arcing graphite elec-
trodes at much lower temperatures, in opportune liquid
plasma conditions and liquid media should produce
even longer chains.
The polyynes can be reduced in solution by treatment
with Zn dust and HCl.16 The reduction stops with the
formation of ene–ynes. This conclusion can easily be
rationalized by examining the changes in the UV spec-
16
trumintroduced by the hydrogenation. The spectrum
of the crude reduced mixture16 resembles that of several

F. Cataldo / Tetrahedron Letters 45 (2004) 141–144
143
ene–ynes reported in literature15b and further arguments
in favor of the ene–yne nature of the products can be
inferred fromthe FT-IR spectra by the ethylenic
stretching band at 3119 cmꢀ1, by the shift of the residual
acetylenic triple bonds stretching at 2260 cmꢀ1 due to
the change in conjugation and by the development of an
5. Heath, J. R.; Zhang, Q.; OÕBrien, S. C.; Curl, R. F.;
Kroto, H. W.; Smalley, R. E. J. Am. Chem. Soc. 1987,
109, 359–363.
6. Krestinin, A. V. Combust. Flame 2000, 121, 513–524.
7. Richter, H.; Howard, J. B. Prog. Energy Combust. Sci.
2000, 26, 565–608.
8. Kroto, H. W. Angew. Chem., Int. Ed. Engl. 1992, 31, 111–
129.
allenic band at 1957 cmꢀ1
.
9. Holmes-Parker, D.; Chatterjee, K.; Wurz, P.; Lykke, K.
R.; Pellin, M. J.; Stock, L. M.; Hemminger, J. C. Carbon
1992, 30, 1167–1182.
10. Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical
Properties of Carbon Nanotubes; Imperial College Press:
London, 1999; pp 77–79.
The submerged electric arc technique not only permits
the one-pot and economic synthesis of long-chain
polyynes but also permits the use of these products as
cheap intermediates for the synthesis of other interesting
molecules. For instance, the simple reduction of poly-
ynes yields ene–ynes, which are another class of mole-
cule of extreme importance for their antibiotic and
antitumour activity.1
11. Cataldo, F. Carbon 2003, 41, 2671–2674.
12. (a) Cataldo, F. Fullerenes, Nanotubes and Carbon Nano-
structures. 2004, 12, issue 2, in press; (b) Cataldo, F.
Carbon, in press.
13. A 100 mL three-necked Pyrex round-bottomed flask was
filled with 70 mL of acetonitrile (HPLC grade, Fluka).
Two graphite electrodes (99.999% purity, Aldrich), having
a diameter of 3 mm and length of 15 mm, were inserted
into rubber stopcocks. The stopcocks with the electrodes
were inserted into two adjacent necks of the flask so that
the electrodes were submerged into the solvent and
arranged in a ÔVÕ geometry, in contact with each other.
The third neck of the flask was equipped with a
thermometer and used for sampling the solution. A DC
electric arc was ignited between the two electrodes at
about 15 V and a current of 10 A. For the generation of
the polyynes the electrodes were kept in contact under the
solvent and moved up and down slightly in order to
generate a very bright light. The power supply maintained
the current at 10 A while the tension was changing
15 10 V due to the movement of the electrodes. CAU-
TION: THE ELECTRODES MUST BE ELECTRI-
CALLY ISOLATED EXTERNALLY TO AVOID
ELECTRIC SHOCK. During arcing the flask was kept
suspended (but not immersed) inside a Dewar flask half
filled with a mixture of acetone and solid CO2. In this way
the temperature of acetonitrile inside the flask was
maintained at )40 ꢀC. After 1 min of arcing it was
possible to record the crude UV–vis spectrumof the
polyyne mixture formed in acetonitrile: 228, 236, 249, 259,
273, 294, 314 nm. The FT-IR of the crude mixture showed
m–CBC–H ¼ 3312 cmꢀ1 (hexane); m–CN ¼ 2247 cmꢀ1 (KBr,
sh); m–CBC– ¼ 2201 cmꢀ1 (KBr) and m–CBC– ¼ 2160 cmꢀ1
(KBr).
The easy availability of polyyne solutions has permitted
us to explore some properties of these molecules. Sur-
prisingly, diluted polyyne solutions in methanol, n-hex-
ane and decahydronaphthalene are stable for more than
a week if left in air or diffuse daylight.11;12 The polyynes
are easily photolysed by UV radiation in solution.12 We
have shown that, by using selected monochromatic
radiation, it is possible to run selective photolysis of
certain polyynes in the crude mixture whilst preserving
others.12 This may have some future practical applica-
tions.
Despite the polyyne solutions being air-stable, they are
reactive with ozone,12 as expected, and undergo easily
the addition of bromine.12
Acknowledgements
The financial support of ASI (contract I/R/070/02), the
Italian Space Agency is gratefully acknowledged.
References and Notes
14. The crude acetonitrile solution containing the polyynes
was filtered through a polyvinylidene fluoride (PVDF)
Acrodisc membrane having a pore size of 0.45 lm. From 5
to 10 lL of the filtered solution was injected into a
4.6 · 150 mm column (Zorbax Eclipse XDB-C8) con-
nected to an HPLC fromAgilent Technologies model
1100. A mobile phase of CH3CN/H2O 80/20 v/v was used
under isocratic conditions at a flow rate of 1.5 mL/min
and a pressure of 151 bar. The eluted polyynes were
detected by a diode-array detector and identified from
their characteristic UV spectra.
15. (a) Gillam, A. E.; Stern, E. S.; Jones, E. R. H. An
Introduction to Electronic Absorption Spectroscopy in
Organic Chemistry; Edward Arnold: London, 1954; pp
78–79; (b) Perkampus, H. H. UV–vis Atlas of Organic
Compounds. 2nd ed. VCH: Weinheim, 1992. Table A12T1.
16. Graphite electrodes were arced at roomtemperature and
10 A in 70 mL of n-hexane until the polyyne absorption
1. Steglich, W.; Fugmann, B.; Lang-Fugmann, S. Rompp
Encyclopedia of Natural Products; Georg Thieme: Stutt-
gart, 2000.
2. (a) Eastmond, R.; Johnson, T. R.; Walton, D. R. M.
Tetrahedron 1972, 28, 4601–4616; (b) Gibtner, Th.;
Hampel, F.; Gisselbrecht, J. P.; Hirsch, A. Chem. Eur.
J. 2002, 8, 408–432 (and references cited therein).
3. (a) Cataldo, F. Eur. J. Solid State Inorg. Chem. 1998, 34,
53–63; (b) Cataldo, F. Polym. Int. 1998, 44, 191–200; (c)
Cataldo, F. Eur. J. Solid State Inorg. Chem. 1999, 35,
281–291; (d) Cataldo, F. Eur. J. Solid State Inorg. Chem.
1999, 35, 293–304; (e) Cataldo, F. Polym. Int. 1999, 48,
15–22; (f) Cataldo, F. Carbon 1999, 37, 161–163; (g)
Cataldo, F.; Capitani, D. Mater. Chem. Phys. 1999, 59,
225–231; (h) Cataldo, F. Fullerene Sci. Technol. 2001, 9,
153–160.
4. (a) Cataldo, F. Angew. Makromol. Chem. 1999, 264, 65–
72; (b) Cataldo, F. J. Mater. Sci. 2000, 35, 2413–2419; (c)
Cataldo, F. J. Macromol. Sci. Pure Appl. Chem. 2000,
A37, 881–892.
band at 226 nmreached
a value of absorbance
A ¼ 2:5 units (about 3–4 min arcing). The crude polyyne
solution in hexane had a UV spectrumas follows: 207,

144
F. Cataldo / Tetrahedron Letters 45 (2004) 141–144
reduced polyynes. Further prolonged contact and shaking
217, 226, 239, 251, 262, 275, 296, 313 nm. FT-IR of the
crude mixture: m–CBC–H ¼ 3312 cmꢀ1 (hexane), m–CBC–
¼
with Zn/HCl did not induce any further changes in the
spectrum. Crude ene–yne mixture UV spectrum (hexane):
211(sh), 217, 225(sh), 236, 246, 255, 265, 277, 287, 301,
317, 333 nm. FT-IR of the crude ene–yne mixture:
m–HC@CH– ¼ 3119 cmꢀ1 (KBr), m–CBC– ¼ 2260 cmꢀ1 (KBr),
m–C@C@C– ¼ 1957 cmꢀ1 (KBr); Other bands: 1380, 684, 667,
2190 cmꢀ1 (KBr) and m–CBC– ¼ 2096 ꢀ1 (KBr). The hexane
solution was transferred to a conical flask and 16 g of Zn
dust (Fluka) was added. Distilled water (50 mL) and
concd HCl (150 mL) were then added and the mixture was
hand shaken vigorously for 5 min. A sample of the hexane
solution was taken to record the UV spectrumof the
653 and 616 cmꢀ1
.