The complete IUPAC name for this compound is 1,2,3,4,5,6-hexamethylbenzene.[9] The locants (the numbers in front of the name) are superfluous, however, as the name hexamethylbenzene uniquely identifies a single substance and thus is the formal IUPAC name for the compound.[10] It is an aromatic compound, with six π electrons (satisfying Hückel's rule) delocalised over a cyclic planar system; each of the six ring carbon atoms is sp2hybridised and displays trigonal planargeometry, while each methyl carbon is tetrahedral with sp3 hybridisation, consistent with the empirical description of its structure.[1] Solid hexamethylbenzene occurs as colourless to white crystalline orthorhombic prisms or needles[11] with a melting point of 165–166°C,[12] a boiling point of 268°C, and a density of 1.0630gcm−3.[11] It is insoluble in water, but soluble in organic solvents including benzene and ethanol.[11]
The mineral mellite (left) is composed of a hydrated aluminium salt of mellitic acid (right)
Hexamethylbenzene is sometimes called mellitene,[11] a name derived from mellite, a rare honey-coloured mineral (μέλιmeli ( GENμέλιτοςmelitos) is the Greek word for honey.[13]) Mellite is composed of a hydrated aluminium salt of benzenehexacarboxylic acid (mellitic acid), with formula Al 2[C 6(CO 2) 6]•16H 2O.[5] Mellitic acid itself can be derived from the mineral,[14] and subsequent reduction yields mellitene. Conversely, mellitene can be oxidised to form mellitic acid:[4]
Treatment of hexamethylbenzene with a superelectrophilic mixture of methyl chloride and aluminum trichloride (a source of Meδ⊕Cl---δ⊖AlCl3) gives heptamethylbenzenium cation, one of the first carbocations to be directly observed.
Structure
In 1927 Kathleen Lonsdale determined the solid structure of hexamethylbenzene from crystals provided by Christopher Kelk Ingold.[3] Her X-ray diffraction analysis was published in Nature[15] and was subsequently described as "remarkable ... for that early date".[3] Lonsdale described the work in her book Crystals and X-Rays,[16] explaining that she recognised that, though the unit cell was triclinic, the diffraction pattern had pseudo-hexagonal symmetry that allowed the structural possibilities to be restricted sufficiently for a trial-and-error approach to produce a model.[3] This work definitively showed that hexamethylbenzene is flat and that the carbon-to-carbon distances within the ring are the same,[2] providing crucial evidence in understanding the nature of aromaticity.
Le Bel and Greene rationalised the process as involving aromatisation by condensation of methylene units, formed by dehydration of methanol molecules, followed by complete Friedel–Crafts methylation of the resulting benzene ring with chloromethane generated in situ.[29] The major products were a mixture of saturated hydrocarbons, with hexamethylbenzene as a minor product.[30] Hexamethylbenzene is also produced as a minor product in the Friedel–Crafts alkylation synthesis of durene from p-xylene, and can be produced by alkylation in good yield from durene or pentamethylbenzene.[31]
Hexamethylbenzene is typically prepared in the gas phase at elevated temperatures over solid catalysts. An early approach to preparing hexamethylbenzene involved reacting a mixture of acetone and methanol vapours over an alumina catalyst at 400°C.[32] Combining phenols with methanol over alumina in a dry carbon dioxide atmosphere at 410–440°C also produces hexamethylbenzene,[17] though as part of a complex mixture of anisole (methoxybenzene), cresols (methylphenols), and other methylated phenols.[23] An Organic Syntheses preparation, using methanol and phenol with an alumina catalyst at 530°C, gives approximately a 66% yield,[12] though synthesis under different conditions has also been reported.[18]
The mechanisms of such surface-mediated reactions have been investigated, with an eye to achieving greater control over the outcome of the reaction,[20][33] especially in search of selective and controlled ortho-methylation.[21][22][34][35] Both anisole[23] and pentamethylbenzene[18] have been reported as intermediates in the process. Valentin Koptyug and co-workers found that both hexamethylcyclohexadienone isomers (2,3,4,4,5,6- and 2,3,4,5,6,6-) are intermediates in the process, undergoing methyl migration to form the 1,2,3,4,5,6-hexamethylbenzene carbon skeleton.[19][24]
Hexamethylbenzene has no commercial or widespread uses. It is exclusively of interest for chemical research. Most applications of hexamethylbenzene are as a chemical feedstock, although it has also been used as a solvent for 3He-NMR spectroscopy.[36]
The electron-donating nature of the methyl groups—both that there are six of them individually and that there are six meta pairs among them—enhance the basicity of the central ring by six to seven orders of magnitude relative to benzene.[41]
Known cations of sandwich complexes of cobalt and rhodium with hexamethylbenzene take the form [M(C 6(CH 3) 6) 2]n+ (M=Co, Fe, Rh, Ru; n=1,2), where the metal centre is bound by the π electrons of the two arene moieties, and can easily be synthesised from appropriate metal salts by ligand exchange, for example:[38]
The complexes can undergo redox reactions. The rhodium and cobalt dications undergo a reversible one-electron reduction with a suitable active metal (aluminium for the cobalt system, zinc for the rhodium), as follows:[38]
In the field of organoruthenium chemistry, the redox interconversion of the analogous two-electron reduction of the dication and its neutral product occurs at −1.02V in acetonitrile[7] and is accompanied by a structural change.[8][42] The hapticity of one of the hexamethylbenzene ligands changes with the oxidation state of the ruthenium centre, the dication [Ru(η6-C6(CH3)6)2]2+ being reduced to [Ru(η4-C6(CH3)6)(η6-C6(CH3)6)],[8] with the structural change allowing each complex to comply with the 18-electron rule and maximise stability.
The structure of the [Ru(C6(CH3)6)2] moiety changes with the oxidation state of the metal centre Left: n=2, [Ru (η -C6(CH3)6)2] Right: n=0, [Ru (η -C6(CH3)6)(η -C6(CH3)6)] Methyl groups omitted for clarity. The electron-pairs involved with carbon–ruthenium bonding are in red.
The equivalent iron(II) complex undergoes a reversible one-electron reduction (at −0.48V in aqueous ethanol), but the two-electron reduction (at −1.46V) is irreversible,[7] suggesting a change in structure different from that found in the ruthenium system.
Three-dimensional representation of the pyramidal carbocation with composition C 6(CH 3) 6
Synthesis from hexamethylbenzene would offer a cheaper feedstock to the same end.
Spectroscopic investigation of the two-electron oxidation of benzene at very low temperatures (below 4K) shows that a hexagonal dication forms and then rapidly rearranges into the same pyramidal structure:[47]
Two-electron oxidation of hexamethylbenzene could therefore result in a near-identical rearrangement to a pyramidal carbocation. However, this method has not successfully produced the dication in bulk.[48]
1 2 Lydon, John (January 2006). "A Welcome to Leeds"(PDF). Newsletter of the History of Physics Group (19): 8–11.
1 2 3 4 Lydon, John (July 2006). "Letters"(PDF). Newsletter of the History of Physics Group (20): 34–35.
1 2 Wibaut, J. P.; Overhoff, J.; Jonker, E. W.; Gratama, K. (1941). "On the preparation of mellitic acid from hexa-methylbenzene and on the hexachloride of mellitic acid". Recl. Trav. Chim. Pays-Bas. 60 (10): 742–746. doi:10.1002/recl.19410601005.
1 2 Pampaloni, Guido (2010). "Aromatic hydrocarbons as ligands. Recent advances in the synthesis, the reactivity and the applications of bis(η6-arene) complexes". Coord. Chem. Rev.254 (5–6): 402–419. doi:10.1016/j.ccr.2009.05.014.
1 2 Briner, E.; Plüss, W.; Paillard, H. (1924). "Recherches sur la déshydration catalytique des systèmes phénols-alcools" [Research on the catalytic dehydration of phenol-alcohol systems]. Helv. Chim. Acta (in French). 7 (1): 1046–1056. doi:10.1002/hlca.192400701132.
1 2 3 Krysin, A. P.; Koptyug, V. A. (1969). "Reaction of phenols with alcohols on aluminum oxide II. The mechanism of hexamethylbenzene formation from phenol and methyl alcohol". Russ. Chem. Bull.18 (7): 1479–1482. doi:10.1007/BF00908756.
1 2 Ipatiew, W.; Petrow, A. D. (1926). "Über die katalytische Kondensation von Aceton bei hohen Temperaturen und Drucken. (I. Mitteilung)" [On the catalytic condensation of acetone at high temperatures and pressures. (I. Communication)]. Ber. Dtsch. Chem. Ges. A/B (in German). 59 (8): 2035–2038. doi:10.1002/cber.19260590859.
1 2 3 Cullinane, N. M.; Chard, S. J. (1945). "215. The action of methanol on phenol in the presence of alumina. Formation of anisole, methylated phenols, and hexamethylbenzene". J. Chem. Soc.: 821–823. doi:10.1039/JR9450000821. PMID21008356.
↑ Le Bel, Joseph Achille; Greene, William H. (1880). "On the decomposition of alcohols, etc., by zinc chloride at high temperatures". American Chemical Journal. 2: 20–26.
↑ Ipatiew, W. N.; Petrow, A. D. (1927). "Über die katalytische Kondensation des Acetons bei hohen Temperaturen und Drucken (II. Mitteilung)" [On the catalytic condensation of acetone at high temperatures and pressures (II. Communication)]. Ber. Dtsch. Chem. Ges. A/B (in German). 60 (3): 753–755. doi:10.1002/cber.19270600328.
↑ Saunders, Martin; Jiménez-Vázquez, Hugo A.; Khong, Anthony (1996). "NMR of 3He Dissolved in Organic Solids". J. Phys. Chem.100 (39): 15968–15971. doi:10.1021/jp9617783.
1 2 3 4 Fischer, Ernst Otto; Lindner, Hans Hasso (1964). "Über Aromatenkomplexe von Metallen. LXXVI. Di-hexamethylbenzol-metall-π-komplexe des ein- und zweiwertigen Kobalts und Rhodiums" [About Aromatic Complexes of Metals. LXXVI. Di-hexamethylbenzene metal-π-complexes of mono- and bivalent cobalt and rhodium]. J. Organomet. Chem. (in German). 1 (4): 307–317. doi:10.1016/S0022-328X(00)80056-X.
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