Dewar benzene (also spelled dewarbenzene) or bicyclo[2.2.0]hexa-2,5-diene is a
bicyclicisomer of
benzene with the molecular formula C6H6. The compound is named after
James Dewar who included this structure in a list of possible C6H6 structures in 1869.[1] However, he did not propose it as the structure of benzene, and in fact he supported the correct structure previously proposed by
August Kekulé in 1865.[2]
Structure and properties
Unlike benzene, Dewar benzene is not flat because the carbons where the rings join are bonded to four atoms rather than three. These carbons tend toward
tetrahedral geometry, and the two cyclobutene rings make an angle where they are cis-
fused to each other. The compound has nevertheless considerable
strain energy and reverts to benzene with a
chemical half-life of two days. This thermal conversion is relatively slow because it is
symmetry forbidden based on orbital symmetry arguments.[3]
It is sometimes incorrectly claimed that Dewar proposed his structure as the true structure of benzene. In fact, Dewar merely wrote the structure as one of seven possible isomers[1] and believed that his experiments on benzene supported the (correct) structure that had been proposed by
Kekulé.[2]
After the development of
valence bond theory in 1928, benzene was described primarily using its two major
resonance contributors, the two Kekulé structures. The three possible Dewar structures were considered as minor resonance contributors in the overall description of benzene, alongside other classic structures such as the isomers
prismane,
benzvalene and
Claus' benzene. Prismane and benzvalene were synthesized in the 1970s; Claus' benzene is impossible to synthesize.[7]
Using DMDO gives the epoxide as a stable product—the byproduct of the epoxidation is neutral
acetone. By varying the amount of DMDO, either the mono- or diepoxide can be formed, with the oxygen atoms exo on the bicyclic carbon framework.[19]
In 1973, the
dication of hexamethylbenzene, C 6(CH 3)2+ 6, was produced by Hepke Hogeveen and Peter Kwant.[20] This can be done by dissolving the hexamethyl Dewar benzene monoepoxide in
magic acid, which removes the oxygen as an anion.[21] NMR had previously hinted at a pentagonal pyramidal structure in a related cation[22] as had
spectral data on the Hogeveen and Kwant
dication.[23][24] The pyramidal structure having an apex carbon bonding to six other carbon atoms was confirmed by X-ray crystallographic analysis of the
hexafluoroantimonate salt published in 2016.[21]
Left: Structure of C 6(CH 3)2+ 6, as drawn by
Steven Bachrach[25] Right: Three-dimensional representation of the
dication's rearranged pentagonal-pyramid framework, from the crystal structure[21]
Computational organic chemist
Steven Bachrach discussed the dication, noting that the weak bonds forming the upright edges of the pyramid, shown as dashed lines in the structure he drew, have a Wiberg
bond order of about 0.54; it follows that the total bond order for the apical carbon is 5 × 0.54 + 1 = 3.7 < 4, and thus the species is not
hypervalent, but it is hypercoordinate.[25] From the perspective of organometallic chemistry, the species can be viewed as having a carbon(IV) centre (C4+ ) bound to an aromatic η5–
pentamethylcyclopentadienyl anion (six-electron donor) and a methyl anion (two-electron donor), thereby satisfying the
octet rule[26] and being analogous to the gas-phase
organozinc monomer [(η5 –C 5(CH 3) 5)Zn(CH 3)], which has the same
ligands bound to a zinc(II) centre (Zn2+ ) and satisfies the
18 electron rule on the metal.[27][28] Thus, while unprecedented,[21] and having attracted comment in Chemical & Engineering News,[29]New Scientist,[30]Science News,[31] and ZME Science,[32] the structure is consistent with the usual bonding rules of chemistry. Moritz Malischewski, who carried out the work with
Konrad Seppelt,[21] commented that one the motivations for undertaking the work was to illustrate "the possibility to astonish chemists about what can be possible."[30]
^Junker, Hans-Nikolaus; Schäfer, Wolfgang; Niedenbrück, Hans (1967). "Oxydationsreaktionen mit Hexamethyl-bicyclo[2.2.0]-hexadien-(2.5) (= Hexamethyl-Dewar-Benzol)" [Oxidation reactions with hexamethylbicyclo[2.2.0]-hexa-2,5-diene (= Hexamethyl Dewar Benzene)]. Chem. Ber. (in German). 100 (8): 2508–2514.
doi:
10.1002/cber.19671000807.
^
abAsouti, Amalia; Hadjiarapoglou, Lazaros P. (2000). "Regioselective and diastereoselective dimethyldioxirane epoxidation of substituted norbornenes and hexamethyl Dewar benzene". Tetrahedron Lett.41 (4): 539–542.
doi:
10.1016/S0040-4039(99)02113-9.
^Hogeveen, Hepke; Kwant, Peter W. (1973). "Direct observation of a remarkably stable dication of unusual structure: (CCH3)62⊕". Tetrahedron Lett.14 (19): 1665–1670.
doi:
10.1016/S0040-4039(01)96023-X.
^Paquette, Leo A.; Krow, Grant R.; Bollinger, J. Martin; Olah, George A. (1968). "Protonation of hexamethyl Dewar benzene and hexamethylprismane in fluorosulfuric acid – antimony pentafluoride – sulfur dioxide". J. Am. Chem. Soc.90 (25): 7147–7149.
doi:
10.1021/ja01027a060.
^Hogeveen, Hepke; Kwant, Peter W.; Postma, J.; van Duynen, P. Th. (1974). "Electronic spectra of pyramidal dications, (CCH3)62+ and (CCH)62+". Tetrahedron Lett.15 (49–50): 4351–4354.
doi:
10.1016/S0040-4039(01)92161-6.
^Hogeveen, Hepke; Kwant, Peter W. (1974). "Chemistry and spectroscopy in strongly acidic solutions. XL. (CCH3)62+, an unusual dication". J. Am. Chem. Soc.96 (7): 2208–2214.
doi:
10.1021/ja00814a034.
^Hogeveen, Hepke; Kwant, Peter W. (1975). "Pyramidal mono- and dications. Bridge between organic and organometallic chemistry". Acc. Chem. Res.8 (12): 413–420.
doi:
10.1021/ar50096a004.