hexamethylbenzene#Dication

The complete IUPAC name for this compound is 1,2,3,4,5,6-hexamethylbenzene.{{Cite web |last=PubChem |title=Hexamethylbenzene |url=https://pubchem.ncbi.nlm.nih.gov/compound/6908 |access-date=2024-05-05 |website=pubchem.ncbi.nlm.nih.gov |language=en}} 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.{{cite book|last1 = Favre|first1 = Henri A.|last2 = Powell|first2 = Warren H.|title = Nomenclature of Organic Chemistry. IUPAC Recommendations and Preferred Name 2013|publisher = Royal Society of Chemistry|year = 2013|isbn = 9780854041824}} 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 sp² hybridised and displays trigonal planar geometry, while each methyl carbon is tetrahedral with sp³ hybridisation, consistent with the empirical description of its structure. Solid hexamethylbenzene occurs as colourless to white crystalline orthorhombic prisms or needles with a melting point of 165–166 °C, a boiling point of 268 °C, and a density of 1.0630 g cm⁻³. It is insoluble in water, but soluble in organic solvents including benzene and ethanol.{{cite book|url = https://books.google.com/books?id=c1rNBQAAQBAJ&pg=SA3-PA296|title = CRC Handbook of Chemistry and Physics|edition = 93rd|editor-first = William M.|editor-last = Haynes|publisher = CRC Press|year = 2016|at = p. 3-296|isbn = 9781439880500}}

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|footer = The mineral mellite (left) is composed of a hydrated aluminium salt of mellitic acid (right)

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Hexamethylbenzene is sometimes called mellitene, a name derived from mellite, a rare honey-coloured mineral ({{lang|grc|μέλι}} meli (GEN {{lang|grc|μέλιτος}} melitos) is the Greek word for honey.{{LSJ|me/li^|μέλι|longref}}.) Mellite is composed of a hydrated aluminium salt of benzenehexacarboxylic acid (mellitic acid), with formula {{chem|Al|2|[C|6|(CO|2|)|6|]•16H|2|O}}.{{cite book|last1 = Wenk|first1 = Hans-Rudolf|last2 = Bulakh|first2 = Andrey|publisher = Cambridge University Press|year = 2016|title = Minerals – Their Constitution and Origin|edition = 2nd|isbn = 9781316423684|chapter-url = https://books.google.com/books?id=TMaSDQAAQBAJ&pg=PT1361|chapter = Organic Minerals}} Mellitic acid itself can be derived from the mineral,{{cite journal |journal= The Lancet |year= 1844 |volume= 2 |issue= 1106 |pages= 190–192 |first= Justus |last= Liebig |title= Lectures on organic chemistry: delivered during the winter session, 1844, in the University of Giessen | doi = 10.1016/s0140-6736(02)64759-2 |url= https://zenodo.org/record/1762488 }} and subsequent reduction yields mellitene. Conversely, mellitene can be oxidised to form mellitic acid:{{cite journal|last1 = Wibaut|first1 = J. P.|last2 = Overhoff|first2 = J.|last3 = Jonker|first3 = E. W.|last4 = Gratama|first4 = K.|year = 1941|title = On the preparation of mellitic acid from hexa-methylbenzene and on the hexachloride of mellitic acid|journal = Recl. Trav. Chim. Pays-Bas|volume = 60|issue = 10|pages = 742–746|doi = 10.1002/recl.19410601005}}

File:Mellitic redox.png

Treatment of hexamethylbenzene with a superelectrophilic mixture of methyl chloride and aluminum trichloride (a source of Me^δ⊕Cl---^δ⊖AlCl₃) gives heptamethylbenzenium cation, one of the first carbocations to be directly observed.

File:Heptamethylbenzenium2.png

In 1927 Kathleen Lonsdale determined the solid structure of hexamethylbenzene from crystals provided by Christopher Kelk Ingold. Her X-ray diffraction analysis was published in Nature{{cite journal|first = Kathleen|last = Lonsdale|author-link = Kathleen Lonsdale|journal = Nature|volume = 122|issue = 810|pages = 810|year = 1928|doi = 10.1038/122810c0|title = The Structure of the Benzene Ring| bibcode=1928Natur.122..810L |s2cid = 4105837|doi-access = free}} and was subsequently described as "remarkable ... for that early date".{{cite journal|pages = 34–35|first = John|last = Lydon|title = Letters|url = https://www.iop.org/activity/groups/subject/hp/newsletter/archive/file_66191.pdf|journal = Newsletter of the History of Physics Group|date = July 2006|issue = 20}} Lonsdale described the work in her book Crystals and X-Rays,{{cite book|title = Crystals and X-Rays|first = Kathleen|last = Lonsdale|author-link = Kathleen Lonsdale|year = 1948|publisher = George Bell & Sons}} 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. This work definitively showed that hexamethylbenzene is flat and that the carbon-to-carbon distances within the ring are the same,{{cite journal|url = https://www.iop.org/activity/groups/subject/hp/newsletter/archive/file_66399.pdf|pages = 8–11|first = John|last = Lydon|title = A Welcome to Leeds|journal = Newsletter of the History of Physics Group|date = January 2006|issue = 19}} providing crucial evidence in understanding the nature of aromaticity.

The compound can be prepared by reacting phenol with methanol at elevated temperatures over a suitable solid catalyst such as alumina. The mechanism of the process has been studied extensively, with several intermediates having been identified. Alkyne trimerisation of dimethylacetylene also yields hexamethylbenzene in the presence of a suitable catalyst.

In 1880, Joseph Achille Le Bel and William H. Greene reported{{cite journal |journal= American Chemical Journal |volume= 2 |year= 1880 |title= On the decomposition of alcohols, etc., by zinc chloride at high temperatures | first1= Joseph Achille |last1= Le Bel |author-link1= Joseph Achille Le Bel |first2= William H. |last2= Greene |pages= 20–26 }} what has been described as an "extraordinary" zinc chloride-catalysed one-pot synthesis of hexamethylbenzene from methanol. At the catalyst's melting point (283 °C), the reaction has a Gibbs free energy (ΔG) of −1090 kJ mol⁻¹ and can be idealised as:{{cite journal|title = Hydrocarbons from Methanol|first = Clarence D.|last = Chang|pages = 1–118|doi = 10.1080/01614948308078874|journal = Catal. Rev. - Sci. Eng.|volume = 25|issue = 1|year = 1983}}

:15 {{chem|CH|3|OH}}   →   {{chem|C|6|(CH|3|)|6}}   +   3 {{chem|CH|4}}   +   15 {{chem|H|2|O}}

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. The major products were a mixture of saturated hydrocarbons, with hexamethylbenzene as a minor product.{{cite journal|title = Onium Ylide chemistry. 1. Bifunctional acid-base-catalyzed conversion of heterosubstituted methanes into ethylene and derived hydrocarbons. The onium ylide mechanism of the C₁→C₂ conversion|first1 = George A.|last1 = Olah|first2 = Hans|last2 = Doggweiler|first3 = Jeff D.|last3 = Felberg|first4 = Stephan|last4 = Frohlich|first5 = Mary Jo|last5 = Grdina|first6 = Richard|last6 = Karpeles|first7 = Takashi|last7 = Keumi|first8 = Shin-ichi|last8 = Inaba|first9 = Wai M.|last9 = Ip|first10 = Koop|last10 = Lammertsma|first11 = George|last11 = Salem|first12 = Derrick|last12 = Tabor|journal = J. Am. Chem. Soc.|year = 1984|volume = 106|issue = 7|pages = 2143–2149|doi = 10.1021/ja00319a039}} 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.{{OrgSynth|year = 1930|volume = 10|pages = 32|doi = 10.15227/orgsyn.010.0032|title = Durene|first1 = Lee Irvin|last = Smith|prep = cv2p0248|collvol = 2|collvolpages = 248}}

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.{{cite journal|title = Über eine einfache Darstellung des Hexamethyl-benzols|trans-title = A simple representation of hexamethylbenzene|first1 = Hans|last1 = Reckleben|first2 = Johannes|last2 = Scheiber|journal = Ber. Dtsch. Chem. Ges.|volume = 46|issue = 2|pages = 2363–2365|year = 1913|language = de|doi = 10.1002/cber.191304602168|url = https://zenodo.org/record/1426519}} Combining phenols with methanol over alumina in a dry carbon dioxide atmosphere at 410–440 °C also produces hexamethylbenzene,{{cite journal|title = Recherches sur la déshydration catalytique des systèmes phénols-alcools|language = fr|trans-title = Research on the catalytic dehydration of phenol-alcohol systems|first1 = E.|last1 = Briner|first2 = W.|last2 = Plüss|first3 = H.|last3 = Paillard|year = 1924|doi = 10.1002/hlca.192400701132|journal = Helv. Chim. Acta|volume = 7|issue = 1|pages = 1046–1056}} though as part of a complex mixture of anisole (methoxybenzene), cresols (methylphenols), and other methylated phenols.{{cite journal|title = 215. The action of methanol on phenol in the presence of alumina. Formation of anisole, methylated phenols, and hexamethylbenzene|first1 = N. M.|last1 = Cullinane|first2 = S. J.|last2 = Chard|journal = J. Chem. Soc.|year = 1945|pages = 821–823|doi = 10.1039/JR9450000821|pmid = 21008356}} An Organic Syntheses preparation, using methanol and phenol with an alumina catalyst at 530 °C, gives approximately a 66% yield,{{OrgSynth|year = 1955|volume = 35|pages = 73|doi = 10.15227/orgsyn.035.0073|title = Hexamethylbenzene|first1 = N. M.|last1 = Cullinane|first2 = S. J.|last2 = Chard|first3 = C. W. C.|last3 = Dawkins|prep = cv4p0520|collvol = 4|collvolpages = 520}} though synthesis under different conditions has also been reported.{{cite journal|doi = 10.1021/jo01037a517|year = 1963|last1 = Landis|first1 = Phillip S.|last2 = Haag|first2 = Werner O.|journal = J. Org. Chem.|volume = 28|issue = 2|pages = 585|title = Formation of Hexamethylbenzene from Phenol and Methanol}}

File:Hexamethylbenzene synthesis.png

The mechanisms of such surface-mediated reactions have been investigated, with an eye to achieving greater control over the outcome of the reaction,{{cite journal|last1 = Ipatiew|first1 = W.|last2 = Petrow|first2 = A. D.|year = 1926|title = Über die katalytische Kondensation von Aceton bei hohen Temperaturen und Drucken. (I. Mitteilung)|journal = Ber. Dtsch. Chem. Ges. A/B|volume = 59|issue = 8|pages = 2035–2038|doi = 10.1002/cber.19260590859|language = de|trans-title = On the catalytic condensation of acetone at high temperatures and pressures. (I. Communication)}}{{cite journal|last1 = Ipatiew|first1 = W. N.|last2 = Petrow|first2 = A. D.|year = 1927|title = Über die katalytische Kondensation des Acetons bei hohen Temperaturen und Drucken (II. Mitteilung)|journal = Ber. Dtsch. Chem. Ges. A/B|volume = 60|issue = 3|pages = 753–755|doi = 10.1002/cber.19270600328|language = de|trans-title = On the catalytic condensation of acetone at high temperatures and pressures (II. Communication)}} especially in search of selective and controlled ortho-methylation.{{cite journal|year = 1971|volume = 44|issue = 7|pages = 1961–1964|title = Methylation of Phenol over Metallic Oxides|journal = Bulletin of the Chemical Society of Japan|first1 = Takeshi|last1 = Kotanigawa|first2 = Mitsuyoshi|last2 = Yamamoto|first3 = Katsuyoshi|last3 = Shimokawa|first4 = Yuji|last4 = Yoshida|doi = 10.1246/bcsj.44.1961|doi-access = free}}{{cite journal|title = Mechanisms for the Reaction of Phenol with Methanol over the ZnO–Fe₂O₃ Catalyst|first = Takeshi|last = Kotanigawa|journal = Bull. Chem. Soc. Jpn.|year = 1974|doi = 10.1246/bcsj.47.950|volume = 47|issue = 4|pages = 950–953|doi-access = free}}{{cite journal|title = The Alkylation of Phenol over the ZnO–Fe₂O₃ Catalyst|first1 = Takeshi|last1 = Kotanigawa|first2 = Katsuyoshi|last2 = Shimokawa|year = 1974|doi = 10.1246/bcsj.47.1535|volume = 47|issue = 6|pages = 1535–1536|journal = Bull. Chem. Soc. Jpn.|doi-access = free}}{{cite journal|title = The Methylation of Phenol and the Decomposition of Methanol on ZnO–Fe₂O₃ Catalyst|journal = Bull. Chem. Soc. Jpn.|first = Takeshi|last = Kotanigawa|doi = 10.1246/bcsj.47.2466|year = 1974|volume = 47|issue = 10|pages = 2466–2468|doi-access = free}} Both anisole and pentamethylbenzene 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.{{cite journal|journal = Russ. Chem. Bull.|year = 1969|volume = 18|issue = 7|pages = 1479–1482|title = Reaction of phenols with alcohols on aluminum oxide II. The mechanism of hexamethylbenzene formation from phenol and methyl alcohol|first1 = A. P.|last1 = Krysin|first2 = V. A.|last2 = Koptyug|author-link2 = Valentin Koptyug|doi = 10.1007/BF00908756}}{{cite journal|journal = Russ. Chem. Bull.|year = 1970|volume = 19|issue = 8|pages = 1643–1648|title = UV, IR, AND PMR spectra of hydroxyhexamethylbenzenonium ions|first1 = V. G.|last1 = Shubin|first2 = V. P.|last2 = Chzhu|first3 = I. K.|last3 = Korobeinicheva|first4 = A. I.|last4 = Rezvukhin|first5 = V. A.|last5 = Koptyug|author-link5 = Valentin Koptyug|doi = 10.1007/BF00996497}}

Trimerisation of three 2-butyne (dimethylacetylene) molecules yields hexamethylbenzene.{{cite journal|journal = J. Organomet. Chem.|volume = 127|issue = 1|year = 1977|pages = 45–54|title = Reactions of Bis(hexamethylbenzene)iron(0) with Carbon Monoxide and with Unsaturated Hydrocarbons|first1 = S. R.|last1 = Weber|first2 = H. H.|last2 = Brintzinger|doi = 10.1016/S0022-328X(00)84196-0|url = http://nbn-resolving.de/urn:nbn:de:bsz:352-238659|hdl = 2027.42/22975|hdl-access = free}} The reaction is catalyzed by triphenylchromium tri-tetrahydrofuranate{{cite journal|title = Acetylenic π-complexes of chromium in organic synthesis|first1 = H. H.|last1 = Zeiss|first2 = W.|last2 = Herwig|journal = J. Am. Chem. Soc.|volume = 80|year = 1958|issue = 11|page = 2913|doi = 10.1021/ja01544a091| bibcode=1958JAChS..80.2913Z }} or by a complex of triisobutylaluminium and titanium tetrachloride.{{cite journal|title = Titanium tetrachloride–trialkylaluminum complex—A cyclizing catalyst for acetylenic compounds|first1 = B.|last1 = Franzus|first2 = P. J.|last2 = Canterino|first3 = R. A.|last3 = Wickliffe|journal = J. Am. Chem. Soc.|volume = 81|year = 1959|issue = 6|page = 1514|doi = 10.1021/ja01515a061| bibcode=1959JAChS..81.1514F }}

File:2-Butyne cyclotrimerization.png

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.{{Cite journal|last1 = Saunders|first1 = Martin|last2 = Jiménez-Vázquez |first2 = Hugo A.|last3 = Khong|first3 = Anthony|journal = J. Phys. Chem.|year = 1996|volume = 100|issue = 39|pages = 15968–15971|doi = 10.1021/jp9617783|title = NMR of ³He Dissolved in Organic Solids}}

Oxidation with trifluoroperacetic acid or hydrogen peroxide gives 2,3,4,5,6,6-hexamethyl-2,4-cyclohexadienone:{{OrgSynth|first1 = Harold|last1 = Hart|first2 = Richard M.|last2 = Lange|first3 = Peter M.|last3 = Collins|year = 1968|volume = 48|page = 87|doi = 10.15227/orgsyn.048.0087|title = 2,3,4,5,6,6-Hexamethyl-2,4-cyclohexadien-1-one|prep = cv5p0598|collvol = 5|collvolpages = 598}}

File:Hexamethylbenzene oxidation.png

As with benzene itself, the electron-rich aromatic system in hexamethylbenzene allows it to act as a ligand in organometallic chemistry.{{cite journal|journal = Coord. Chem. Rev.|volume = 254|issue = 5–6|year = 2010|pages = 402–419|title = Aromatic hydrocarbons as ligands. Recent advances in the synthesis, the reactivity and the applications of bis(η⁶-arene) complexes|first = Guido|last = Pampaloni|doi = 10.1016/j.ccr.2009.05.014}} Such complexes have been reported for a variety of metal centres, including cobalt, chromium, iron, rhenium,{{cite journal|author-link1 = Ernst Otto Fischer|last1 = Fischer|first1 = Ernst Otto|last2 = Schmidt|first2 = Manfred W.|year = 1966|title = Über Aromatenkomplexe von Metallen, XCI. Über monomeres und dimeres Bis-hexamethylbenzol-rhenium|journal = Chem. Ber.|volume = 99|issue = 7|pages = 2206–2212|doi = 10.1002/cber.19660990719}} rhodium, ruthenium, and titanium. It also π-stacks to form an orange-yellow 1:1 charge-transfer adduct with picryl chloride.{{cite journal|title = Molecular Compounds. I. Picryl Chloride-Hexamethylbenzene in Chloroform Solution|last1 = Ross|first1 = Sidney D.|last2 = Bassin|first2 = Morton|last3 = Finkelstein|first3 = Manuel|last4 = Leach|first4 = William A.|journal = J. Am. Chem. Soc.|year = 1954|volume = 76|issue = 1|pages = 69–74|doi = 10.1021/ja01630a018| bibcode=1954JAChS..76...69R }}

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.{{Kirk-Othmer|last1 = Earhart|first1 = H. W.|first2 = Andrew P.|last2 = Komin|title = Polymethylbenzenes|year = 2000|doi = 10.1002/0471238961.1615122505011808.a01|mode = cs1}}

Known cations of sandwich complexes of cobalt and rhodium with hexamethylbenzene take the form {{chem|[M(C|6|(CH|3|)|6|)|2|]}}{{sup|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:

:{{chem|CoBr|2}}   +   2 {{chem|AlBr|3}}   →   {{chem|[Co(C|6|(CH|3|)|6|)|2|]|2+}}   +   2 {{chem|AlBr|4|-}}

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:{{cite journal|journal = J. Organomet. Chem.|volume = 1|issue = 4|year = 1964|pages = 307–317|title = Über Aromatenkomplexe von Metallen. LXXVI. Di-hexamethylbenzol-metall-π-komplexe des ein- und zweiwertigen Kobalts und Rhodiums|language = de|trans-title = About Aromatic Complexes of Metals. LXXVI. Di-hexamethylbenzene metal-π-complexes of mono- and bivalent cobalt and rhodium|author-link1 = Ernst Otto Fischer|last1 = Fischer|first1 = Ernst Otto|first2 = Hans Hasso|last2 = Lindner|doi = 10.1016/S0022-328X(00)80056-X}}

:3 {{chem|[Co(C|6|(CH|3|)|6|)|2|]|2+}}   +   Al   →   3 {{chem|[Co(C|6|(CH|3|)|6|)|2|]|+}}   +   {{chem|Al|3+}}

In the field of organoruthenium chemistry, the redox interconversion of the analogous two-electron reduction of the dication and its neutral product occurs at −1.02 V in acetonitrile and is accompanied by a structural change.{{cite journal|last1 = Huttner|first1 = Gottfried|last2 = Lange|first2 = Siegfried|last3 = Fischer|first3 = Ernst O.|author-link3 = Ernst Otto Fischer|year = 1971|title = Molecular Structure of Bis(Hexamethylbenzene)Ruthenium(0)|journal = Angew. Chem. Int. Ed. Engl.|volume = 10|issue = 8|pages = 556–557|doi = 10.1002/anie.197105561}}{{cite book|first1 = Martin A.|last1 = Bennett|author-link1 = Martin A. Bennett|first2 = T.-N.|last2 = Huang|first3 = T. W.|last3 = Matheson|first4 = A. K.|last4 = Smith|title = 16. (η⁶-Hexamethylbenzene)Ruthenium Complexes|journal = Inorg. Synth.|year = 1982|volume = 21|pages = 74–78|doi = 10.1002/9780470132524.ch16|isbn = 9780470132524}} 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)], with the structural change allowing each complex to comply with the 18-electron rule and maximise stability.

File:Ru(C6H6)2redox pi-highlight.png changes with the oxidation state of the metal centre
Left: n = 2, [Ru^II(η⁶-C₆(CH₃)₆)₂]²⁺
Right: n = 0, [Ru⁰(η⁴-C₆(CH₃)₆)(η⁶-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 −0.48 V in aqueous ethanol), but the two-electron reduction (at −1.46 V) is irreversible,{{cite book|chapter = The Electrochemistry of Transition Metal Organometallic Compounds|last = Kotz|first = John C.|pages = 83–176|title = Topics in Organic Electrochemistry|editor2-first = Wayne E.|editor2-last = Britton|editor1-first = Albert J.|editor1-last = Fry|publisher = Springer Science & Business Media|year = 1986|isbn = 9781489920348|chapter-url = https://books.google.com/books?id=zgUHCAAAQBAJ&pg=PA110}} suggesting a change in structure different from that found in the ruthenium system.

{{Main|Hexamethyl Dewar benzene#Dication}}

As discovered in the 1960s and '70s, two-electron oxidation of hexamethyl Dewar benzene gives pyramid-shaped ions with composition {{chem|C|6|(CH|3|)|6|H|+}}{{cite journal|last1 = Schäfer|first1 = W.|last2 = Hellmann|first2 = H.|year = 1967|title = Hexamethyl(Dewar Benzene) (Hexamethylbicyclo[2.2.0]hexa-2,5-diene)|journal = Angew. Chem. Int. Ed. Engl.|volume = 6|issue = 6|pages = 518–525|doi = 10.1002/anie.196705181}} and {{chem|C|6|(CH|3|)|6|2+}}:{{cite journal|last1 = Hogeveen|first1 = Hepke|last2 = Kwant|first2 = Peter W.|year = 1973|title = Direct observation of a remarkably stable dication of unusual structure: (CCH₃)₆^2⊕|journal = Tetrahedron Lett.|volume = 14|issue = 19|pages = 1665–1670|doi = 10.1016/S0040-4039(01)96023-X}}{{cite journal|last1 = Hogeveen|first1 = Hepke|last2 = Kwant|first2 = Peter W.|last3 = Postma|first3 = J.|last4 = van Duynen|first4 = P. Th.|year = 1974|title = Electronic spectra of pyramidal dications, (CCH₃)₆²⁺ and (CCH)₆²⁺|journal = Tetrahedron Lett.|volume = 15|issue = 49–50|pages = 4351–4354|doi = 10.1016/S0040-4039(01)92161-6}}{{cite journal|last1 = Hogeveen|first1 = Hepke|last2 = Kwant|first2 = Peter W.|year = 1974|title = Chemistry and spectroscopy in strongly acidic solutions. XL. (CCH₃)₆²⁺, an unusual dication|journal = J. Am. Chem. Soc.|volume = 96|issue = 7|pages = 2208–2214|doi = 10.1021/ja00814a034| bibcode=1974JAChS..96.2208H }}

:File:C6(CH3)6(2+) 3D skeletal.png with composition {{chem|C|6|(CH|3|)|6|2+}}|alt=Gray pentagonal-pyramid skeleton]]

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 4 K) shows that a hexagonal dication forms and then rapidly rearranges into the same pyramidal structure:{{cite journal|title = Probing Isomers of the Benzene Dication in a Low-Temperature Trap|first1 = Juraj|last1 = Jašík|first2 = Dieter|last2 = Gerlich|first3 = Jana|last3 = Roithová|journal = J. Am. Chem. Soc.|year = 2014|volume = 136|issue = 8|pages = 2960–2962|doi = 10.1021/ja412109h|pmid = 24528384| bibcode=2014JAChS.136.2960J }}

File:Oxidation of benzene to its dication.jpg

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.

{{reflist|30em|refs=

{{cite journal|title = Crystal Structure Determination of the Pentagonal-Pyramidal Hexamethylbenzene Dication C₆(CH₃)₆²⁺|first1 = Moritz|last1 = Malischewski|first2 = Konrad|last2 = Seppelt|author-link2 = Konrad Seppelt|journal = Angew. Chem. Int. Ed.|volume = 56|issue = 1|pages = 368–370|year = 2017|doi = 10.1002/anie.201608795|pmid = 27885766}}

}}

{{Hydrocarbons}}

Category:Alkylbenzenes

Category:Ligands