Trapping aryl radicals with acetylene: Evidence for C2-accretion as a mechanism for polycyclic aromatic hydrocarbon growth [5]

Full text of DOI:10.1021/ja992115c

1548
J. Am. Chem. Soc. 2000, 122, 1548-1549
Scheme 1
Trapping Aryl Radicals with Acetylene: Evidence
for C2-Accretion as a Mechanism for Polycyclic
Aromatic Hydrocarbon Growth1
Atena Necula and Lawrence T. Scott*
Department of Chemistry
Merkert Chemistry Center, Boston College
Chestnut Hill, Massachusetts 02467-3860
ReceiVed June 22, 1999
ReVised Manuscript ReceiVed December 20, 1999
The formation of polycyclic aromatic hydrocarbons (PAH),2
fullerenes,3 soot,2,4 and other carbonaceous materials during the
combustion or pyrolysis of low-molecular weight hydrocarbons
requires, at a minimum, that small molecules and/or reactive
intermediates somehow become joined to make larger ones. Most
likely, more than one type of intermolecular C-C bond-forming
reaction plays a role. The accretion of C2-units has long been
considered a probable pathway for the stepwise growth of PAH
in flames,5 but evidence also points to the operation of bimolecular
processes in which both partners can be relatively large.6 The
experiments reported here address the former paradigm and
provide clear support for a specific C2-accretion pathway in which
the key C-C bond-forming step involves the simple trapping of
aryl radicals by acetylene (C2H2), both of which are abundant
species in flames.4,5
Scheme 2
Co-pyrolysis of 1-bromonaphthalene (C10H7Br) and maleic
anhydride in a flow system at 1100 °C/0.05 mmHg9 gives
acenaphthylene (AN, C12H8) as the dominant product, ac-
companied by minor amounts of naphthalene (Nap, C10H8) and
2-ethynylnaphthalene (2-EN, C12H8), plus traces of pyracylene
(Pyr, C14H8), in a ratio of approximately 100:19:12:210 (Scheme
1). A material balance of 75-80% was achieved, and no starting
material survived. In a control experiment, co-pyrolysis of
unsubstituted naphthalene (C10H8) and maleic anhydride under
the same conditions gave almost entirely recovered naphthalene
and only insignificant amounts of the products shown in Scheme
1.
Flash vacuum pyrolysis (FVP) of aryl bromides is known to
generate transient aryl radicals in the gas phase (eq 1).7 Under
the same conditions, maleic anhydride decomposes very cleanly
to acetylene, carbon monoxide, and carbon dioxide (eq 2).8 Thus,
co-pyrolysis of aryl bromides with a large molar excess of maleic
anhydride provides a convenient and relatively safe means of
producing aryl radicals in an acetylene-rich environment.
The major product (AN) obtained from the reaction in Scheme
1 results from the efficient trapping of the 1-naphthyl radical (1-
NR) by acetylene. Two distinct routes to AN can be envisaged
from the vinyl radical formed in the intermolecular C-C bond-
forming step: (1) direct radical cyclization followed by re-
aromatization and (2) â-scission followed by cyclization of the
resulting 1-ethynylnaphthalene (1-EN) (Scheme 2). Isotopic-
labeling studies (discussed below) argue against the former
mechanism but are compatible with the latter. Thermal cyclization
of 1-EN to AN has been known for many years11 and is believed
to occur by way of a vinylidene intermediate (Vin),12 although a
concerted alternative has been proposed.13 The great exothermicity
of the 1-EN f AN isomerization13 accounts for the absence of
any 1-EN in the product mixture.14
(1) First presented by Necula, A.; Scott, L. T. Abstracts of Papers, National
Meeting of the American Chemical Society, Boston, Massachusetts, August,
1998; American Chemical Society: Washington, DC, 1998; ORGN 55.
(2) (a) Wiersum, U. E. Janssen Chim. Acta 1992, 10, 3-13. (b) Lafleur,
A. L.; Howard, J. B.; Plummer, E.; Taghizadeh, K.; Necula, A.; Scott, L. T.;
Swallow, K. C. Polycyclic Aromat. Compd. 1998, 12, 223-237 and references
therein.
(3) (a) Pope, C. J.; Marr, J. A.; Howard, J. B. J. Phys. Chem. 1993, 97,
11001-13. (b) Lafleur, A. L.; Howard, J. B.; Marr, J. A.; Yadav, T. J. Phys.
Chem. 1993, 97, 13539-43. (c) Ahrens, J.; Bachmann, M.; Baum, T.;
Griesheimer, J.; Kovacs, R.; Weilmuenster, P.; Homann, K. H. Int. J. Mass
Spectrom. Ion Processes 1994, 138, 133-48. (d) Bachmann, M.; Griesheimer,
J.; Homann, K. H. Chem. Phys. Lett. 1994, 223, 506-10. (e) Lafleur, A. L.;
Howard, J. B.; Taghizadeh, K.; Plummer, E. F.; Scott, L. T.; Necula, A.;
Swallow, K. C. J. Phys. Chem. 1996, 100, 17421-17428.
(9) For a detailed description of the apparatus and the procedure, see: Scott,
L. T.; Bratcher, M. S.; Hagen, S. J. Am. Chem. Soc. 1996, 118, 8743-8744.
(10) The product ratios, normalized to a relative value of 100 for the
dominant product, were determined by integration of the 1H NMR spectra of
the crude pyrolysates; errors are estimated to be (5%.
(11) Brown, R. F. C.; Eastwood, F. W.; Jackman, G. P. Aust. J. Chem.
1977, 30, 1757-67.
(12) Brown, R. F. C.; Eastwood, F. W. Synlett 1993, 9-19.
(13) Cioslowski, J.; Schimeczek, M.; Piskorz, P.; Moncrieff, D. J. Am.
Chem. Soc. 1999, 121, 3773-3778.
(4) (a) Benish, T. G.; Lafeur, A. L.; Taghizadeh, K.; Howard, J. B. 26th
Symp. (Int.) Combust. 1996, The Combustion Institute, 2319-2326. (b)
Macadam, S.; Beer, J. M.; Sarofim, A. F.; Hoffmann, A. B. 26th Symp. (Int.)
Combust. 1996, The Combustion Institute, 2295-2302.
(5) Bittner, J. D.; Howard, J. B. 18th Symp. (Int.) Combust. 1981, The
Combustion Institute, 1105-1116.
(6) (a) Badger, G. M. Progr. Phys. Org. Chem. 1965, 3, 1. (b) Marr, J. A.;
Giovane, L. M.; Longwell, J. P.; Howard, J. B.; Lafleur, A. L. Combust. Sci.
Technol. 1994, 101, 301-9.
(7) (a) Ladaki, M.; Szwarc, M. J. Chem. Phys. 1952, 20, 1814. (b) Ladaki,
M.; Szwarc, M. Proc. R. Soc. London, A 1953, A219, 341. (c) Benson, S. W.;
O’Neal, H. E. Kinetic Data on Gas-Phase Unimolecular Reactions; NBS-
NSRDS 21, U.S. Government Printing Office: Washington, DC, 1970.
(8) Brown, A. L.; Ritchie, P. D. J. Chem. Soc. (C) 1968, 2007-2013.
(14) In cases where no further isomerization is possible, the arylacetylene
survives, e.g., copyrolysis of bromobenzene and maleic anhydride at 1100
°C gives phenyl acetylene and benzene as the only significant products. The
volatility of these two hydrocarbons precluded an accurate determination of
their relative abundances in our apparatus.
10.1021/ja992115c CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/02/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 7, 2000 1549
Scheme 3
Scheme 5
Scheme 4
anhydride in a flow system at 1100 °C/0.6 mmHg9 gives
acenaphthylene (AN, C12H8) again as the major product, ac-
companied by comparable amounts of 2-ethynylnaphthalene (2-
EN, C12H8), lesser quantities of naphthalene (Nap, C10H8), and
traces of pyracylene (Pyr, C14H8), in a ratio of approximately
100:87:16:210 (material balance 70-80%) (Scheme 5). At 975
°C, the product ratio changed to approximately 100:69:0:0, and
at 850 °C, only starting material was recovered.
The large amount of AN formed in this pyrolysis indicates that
the 1,2-shift of hydrogen depicted in Scheme 3 must be reversible,
although a portion of the “rearrangement product” (AN) in this
case probably does come from ethynyl group migration.18 It is
also apparent that complete equilibration was not achieved under
our conditions prior to trapping of the aryl radicals with acetylene,
since the ratio of the trapping products from the 1- and 2-naphthyl
radicals (AN:2-EN) varies in the expected way as a function of
which radical is formed first (compare Schemes 1 and 5).
To shed light on the relative importance of the two competing
mechanistic pathways illustrated in Scheme 2, we repeated the
co-pyrolysis of 1-bromonaphthalene with maleic anhydride using
maleic anhydride-d2. The direct cyclization pathway should lead
to retention of both deuterium atoms in the acenaphthylene
formed, whereas the â-scission pathway should give acenaph-
thylene in which only one deuterium atom from the D-CtC-D
is retained (see Scheme 2). To suppress the known thermal
automerization of acenaphthylene,20 the experiment with maleic
anhydride-d2 was conducted at 1000 °C (material balance 80-
85%).
1H NMR analysis of the product mixture obtained from co-
pyrolysis of 1-bromonaphthalene with anhydride-d2 reveals that
the signal for the hydrogens on the bridge of acenaphthylene does
not disappear completely, as would be required by the direct
cyclization mechanism (Scheme 2), but is reduced by only 50%,
as expected from the â-scission pathway (Scheme 2).
From these experiments, we conclude that direct cyclization
of a vinyl radical must be too slow to compete with â-scission to
any significant extent under our conditions. The main steps in
the C2-growth mechanism, then, are (1) capture of acetylene by
an aryl radical, (2) rapid â-scission of the vinyl radical to an
ethynyl-PAH, and (3) subsequent cyclization12,13 or migration of
the ethynyl group17 (e.g., Scheme 2). This pathway for C2-
accretion is far more favorable under our conditions than any
direct reaction of an intact PAH with acetylene.
Naphthalene (Nap), the second most abundant product obtained
from the 1-naphthyl radical (1-NR), can be readily explained by
the abstraction of hydrogen atoms from other organic species in
the gas phase or on the walls of the pyrolysis tube. Hydrogen
atom abstraction represents a common fate of aryl radicals at high
temperatures in the gas phase.15,16
The 2-ethynylnaphthalene (2-EN) obtained from co-pyrolysis
of 1-bromonaphthalene and maleic anhydride (Scheme 1) presum-
ably arises by a 1,2-shift of hydrogen and trapping of the
2-naphthyl radical (2-NR) with acetylene, followed by â-scission
(Scheme 3). The alternative pathway involving migration of the
ethynyl group17 (1-EN f 2-EN) can be ruled out, since FVP of
1-ethynylnaphthalene (1-EN) at 1100 °C gives only acenaph-
thylene (AN) and does not lead to 2-ethynylnaphthalene (2-EN).18
The 1,2-shift of hydrogen atoms in aryl radicals, on the other
hand, although observed only recently for the first time,16 is
probably a universal reaction of aryl radicals in the gas phase at
high temperatures. Evidence for the same 1,2-shift of hydrogen
in the reverse direction is described below (2-NR f 1-NR).
Pyracylene (Pyr), though only a minor product from the co-
pyrolysis of 1-bromonaphthalene and maleic anhydride (Scheme
1), is noteworthy in that it reveals a process whereby two
molecules of acetylene are captured. We are reluctant to speculate
about the details of such a minor pathway (1-NR f Pyr) but
can report that the co-pyrolysis of 5-bromoacenaphthylene (C12H7-
Br) and maleic anhydride in a flow system at 1100 °C/0.2 mmHg9
gives acenaphthylene (AN, C12H8) and pyracylene (Pyr, C14H8)
in a ratio of approximately 100:5010 (Scheme 4). The pyracylene
(Pyr) in this reaction is probably formed by way of 5-ethynyl-
acenaphthylene.19 Co-pyrolysis of underivatized acenaphthylene
(AN) with maleic anhydride under the same conditions gives no
more than traces of pyracylene.
Co-pyrolysis of 2-bromonaphthalene (C10H7Br) and maleic
(15) (a) Fahr, A.; Stein, S. E. J. Phys. Chem. 1988, 92, 4951-4955. (b)
Chen, R. H.; Kafafi, S. A.; Stein, S. E. J. Am. Chem. Soc. 1989, 111, 1418-
1423. (c) Scott, L. T.; Hashemi, M. M.; Bratcher, M. S. J. Am. Chem. Soc.
1992, 114, 1920-1.
(16) Brooks, M. A.; Scott, L. T. J. Am. Chem. Soc. 1999, 121, 5444-
5449.
(17) Sarobe M.; Jenneskens L. W.; Steggink R. G. B.; Visser T. J. Org.
Chem. 1999, 64, 3861-3866.
(18) In the absence of more facile competing reactions, ethynyl group
migration does occur on the naphthalene ring system: FVP of 2-ethynyl-
naphthalene (1100 °C, 0.2 mmHg) gives a 2:1 mixture of acenaphthylene
and starting material, presumably by the sequence 2-EN f 1-EN f AN.
(19) (a) Scott, L. T. Pure Appl. Chem. 1996, 68, 291-300. (b) Sarobe,
M.; Flink, S.; Jenneskens, L. W.; Zwikker, J. W.; Wesseling, J. J. Chem.
Soc., Perkin Trans. 2 1996, 2125-2131.
Acknowledgment. Financial support of this work by the U. S.
Department of Energy is gratefully acknowledged.
(20) Scott, L. T.; Roelofs, N. H. Tetrahedron Lett. 1988, 29, 6857-60.
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