Kinetic data and 13C-labeling have been used to elucidate the correct mechanism, and have led organic chemists to believe that one of the benzene ring contractions is the
most likely mechanism through which these isomerizations of aromatic hydrocarbons occur.
These reactions can be categorized in two major types: one that involves a complete and permanent skeletal reorganization (isomerization), and one in which the atoms are scrambled
but no net change in the aromatic ring occurs (automerization).
The diradical mechanism has been supported by kinetic studies performed on the reaction, which have revealed that the reaction is not truly unimolecular, as it is most likely
initiated by hydrogen addition from another gas-phase species.
 This mechanism would therefore involve an azulene intermediate and is depicted below: Subsequent work showed that the isomerization of azulene to naphthalene is not
readily reversible ( the free energy of a naphthalene to azulene isomerization was too high – approximately 90 kcal/mol).
This is currently the preferred mechanism and is as follows: Isomerizations The isomerization of unsubstituted azulene to naphthalene was the first reported thermal
transformation of an aromatic hydrocarbon, and has consequently been the most widely studied rearrangement.
The four described mechanisms would all result in the isomerization from azulene to naphthalene.
FVP has numerous limitations: • First, it requires a slow rate of sublimation to minimize bimolecular reactions in the gas phase, limiting the amount of material that can
be reacted in a given amount of time.
It was quickly determined that the reversible ring-closure mechanism was inaccurate, and it was later decided that there must be multiple reaction pathways occurring simultaneously.
 Four mechanisms for thermal isomerizations have been proposed: a dyotropic mechanism, a diradical mechanism, and two benzene ring contraction mechanisms; a 1,2-carbon
shift to a carbene preceding a 1,2-hydrogen shift, and a 1-2-hydrogen shift to a carbene followed by a 1,2-carbon shift.
Both mechanisms are shown as follows for the ring contraction of biphenylene: The first involves a 1,2-hydrogen shift to a carbene followed by a 1,2-carbon shift on the same
C-C bond but in opposite directions.
Benzene ring contractions are the last two mechanisms that have been suggested, and they are currently the preferred mechanisms.
 Possible applications Thermal rearrangements of aromatic hydrocarbons have been shown to be important in areas of chemical research and industry including fullerene synthesis,
materials applications, and the formation of soot in combustion.
 A new reaction mechanism was suggested that involved a carbene intermediate and consecutive 1,2-hydrogen and 1,2-carbon shifts across the same C-C bond but in opposite
 • Third, the high temperatures used in FVP do not allow for the presence of functional groups, thereby limiting possible products.
These reaction mechanisms proceed through the lowest free energy transition states compared to the diradical and dyotropic mechanisms.
[‘Scott, Lawrence T. (1982). “Thermal rearrangements of aromatic compounds”. Accounts of Chemical Research. 15 (2): 52–58. doi:10.1021/ar00074a004.
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4. ^ Scott, Lawrence T.; Hashemi, Mohammed M.; Schultz, Thomas H.; Wallace, Michael B. (1991). “Thermal rearrangements of aromatic compounds.
15. Automerization of naphthalene. New evidence consistent with the intermediacy of benzofulvene”. Journal of the American Chemical Society. 113 (25): 9692–9693. doi:10.1021/ja00025a055.
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Nguyen, Phuong T.; Santander, Mitchell V.; Castro, Claire; Karney, William L. (2013). “Hydrogen shifts and benzene ring contractions in phenylenes”. Journal of Physical Organic Chemistry. 26 (9): 750–754. doi:10.1002/poc.3126.
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Schimeczek, Michael; Piskorz, Pawel; Moncrieff, David (1999). “Thermal Rearrangement of Ethynylarenes to Cyclopentafused Polycyclic Aromatic Hydrocarbons: An Electronic Structure Study”. Journal of the American Chemical Society. 121 (15): 3773–3778.
7. ^ Scott, Lawrence T. (1984). “Thermal rearrangements of aromatic compounds, part 8. Azulene-to-naphthalene rearrangement. A comment on the kinetics”. The Journal of Organic Chemistry. 49 (16): 3021–3022. doi:10.1021/jo00190a030.
Jump up to:a b Scott, Lawrence T.; Roelofs, Nicolas H. (1987). “Thermal rearrangements of aromatic compounds. 11. Benzene ring contractions at high temperatures. Evidence from the thermal interconversions of aceanthrylene, acephenanthrylene, and fluoranthene”.
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L. A. Automerizations in Benzenoid Hydrocarbons. New Mechanistic Insights from the Thermal Rearrangement of Benz[a]anthracene-5-13C. Tetrahedron Letters 1984, 25 (16), 1661–1664. DOI: 10.1016/S0040-4039(01)81138-2
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Photo credit: https://www.flickr.com/photos/chriswaits/5996421299/’]