Hexamethylbenzene

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Hexamethylbenzene
Structural formula of hexamethylbenzene
Ball-and-stick model of the hexamethylbenzene molecule
Names
Preferred IUPAC name
Hexamethylbenzene
Other names
1,2,3,4,5,6-Hexamethylbenzene
Mellitene
Identifiers
3D model (
JSmol
)
ChEBI
ChemSpider
ECHA InfoCard
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UNII
  • InChI=1S/C12H18/c1-7-8(2)10(4)12(6)11(5)9(7)3/h1-6H3 ☒N
    Key: YUWFEBAXEOLKSG-UHFFFAOYSA-N ☒N
  • InChI=1/C12H18/c1-7-8(2)10(4)12(6)11(5)9(7)3/h1-6H3
    Key: YUWFEBAXEOLKSG-UHFFFAOYAF
  • c1(c(c(c(c(c1C)C)C)C)C)C
Properties
C12H18
Molar mass 162.276 g·mol−1
Appearance White crystalline powder
Density 1.0630 g cm−3
Melting point 165.6 ± 0.7 °C
Boiling point 265.2 °C (509.4 °F; 538.3 K)
insoluble
Solubility acetic acid, acetone, benzene, chloroform, diethyl ether, ethanol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Hexamethylbenzene, also known as mellitene, is a

molecular formula C12H18 and the condensed structural formula C6(CH3)6. It is an aromatic compound and a derivative of benzene, where benzene's six hydrogen atoms have each been replaced by a methyl group. In 1929, Kathleen Lonsdale reported the crystal structure of hexamethylbenzene, demonstrating that the central ring is hexagonal and flat[1] and thereby ending an ongoing debate about the physical parameters of the benzene system. This was a historically significant result, both for the field of X-ray crystallography and for understanding aromaticity.[2][3]

Hexamethylbenzene can be oxidised to

analogous organoiron system.[7]

In 2016 the crystal structure of the hexamethylbenzene

hypervalent, and also explained its aromaticity.[13] The idea of describing the chemical bonding in compounds and chemical species in this way through the lens of organometallic chemistry was proposed in 1975,[14] soon after the dication C
6(CH
3)2+
6 was first observed.[15][16][17]

Nomenclature and properties

According to the

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.[1] Solid hexamethylbenzene occurs as colourless to white crystalline orthorhombic prisms or needles[19] with a melting point of 165–166 °C,[20] a boiling point of 268 °C, and a density of 1.0630 g cm−3.[19] It is insoluble in water, but soluble in organic solvents including benzene and ethanol.[19]

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

Hexamethylbenzene is sometimes called mellitene,

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

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
.

Preparation

The compound can be prepared by reacting

dimethylacetylene also yields hexamethylbenzene[33] in the presence of a suitable catalyst.[34][35]

In 1880, Joseph Achille Le Bel and William H. Greene reported[36] what has been described as an "extraordinary" zinc chloride-catalysed one-pot synthesis of hexamethylbenzene from methanol.[37] 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:[37]

15 CH
3OH   →   C
6(CH
3)
6   +   3 CH
4   +   15 H
2O

Le Bel and Greene rationalised the process as involving

p-xylene, and can be produced by alkylation in good yield from durene or pentamethylbenzene.[39]

Hexamethylbenzene is typically prepared in the

alumina catalyst at 400 °C.[40] Combining phenols with methanol over alumina in a dry carbon dioxide atmosphere at 410–440 °C also produces hexamethylbenzene,[25] though as part of a complex mixture of anisole (methoxybenzene), cresols (methylphenols), and other methylated phenols.[31] An Organic Syntheses preparation, using methanol and phenol with an alumina catalyst at 530 °C, gives approximately a 66% yield,[20] though synthesis under different conditions has also been reported.[26]

The mechanisms of such surface-mediated reactions have been investigated, with an eye to achieving greater control over the outcome of the reaction,

methyl migration to form the 1,2,3,4,5,6-hexamethylbenzene carbon skeleton.[27][32]

complex of triisobutylaluminium and titanium tetrachloride.[35]

Uses

Synthetic uses

Hexamethylbenzene can be used as a

organometallic compounds
.

Other uses

Hexamethylbenzene has no commercial or widespread uses. It is exclusively of interest for chemical research.

Reactions

It forms orange-yellow 1:1 adduct with

π-stacking
of the aromatic systems.

Oxidation with trifluoroperacetic acid or hydrogen peroxide gives 2,3,4,5,6,6-hexamethyl-2,4-cyclohexadienone:[45][27][32])

It has also been used as a solvent for 3He-NMR spectroscopy.[46]

Just as with benzene itself, the electron-rich aromatic system in hexamethylbenzene allows it to act as a

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:[48]

CoBr
2   +   2 AlBr
3   →   [Co(C
6(CH
3)
6)
2]2+
   +   2 AlBr−
4

The complexes can undergo redox reactions. The rhodium and cobalt dications undergo a one-electron reduction with a suitable active metal (aluminium for the cobalt system, zinc for the rhodium), and the equations describing the reactions in the cobalt system are as follows:[48]

3 [Co(C
6(CH
3)
6)
2]2+
   +   Al   →   3 [Co(C
6(CH
3)
6)
2]+
   +   Al3+
The structure of the [Ru(C6(CH3)6)2]n+ moiety changes with the oxidation state of the metal centre[8]
Left: n = 2, [RuII(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.

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[7] and is accompanied by a structural change.[8][50] 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 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,[7] suggesting a change in structure different from that found in the ruthenium system.

Dication

Main article: Pyramidal carbocation
Pyramidal carbocation with composition C
6(CH
3)2+
6

The isolation of an ion with composition C
6(CH
3)
6H+
 was first reported from investigations of

disordered[9] crystal structure data.[53] In the early 1970s theoretical work led by Hepke Hogeveen predicted the existence of a pyramidal dication C
6(CH
3)2+
6, and the suggestion was soon supported by experimental evidence.[15][16][17] 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 a pyramidal structure:[54]

Three-dimensional representation of C
6(CH
3)2+
6 having a rearranged pentagonal-pyramid framework

Two-electron oxidation of hexamethylbenzene would be expected to result in a near-identical rearrangement to a

inverted tetrahedral geometry for the carbons of the base of the pyramid. The preparation method involved treating the epoxide of hexamethyl Dewar benzene with magic acid, which formally abstracts an oxide anion (O2−
) to form the dication:[9]

Though indirect spectroscopic evidence and theoretical calculations previously pointed to their existence, the isolation and structural determination of a species with a hexacoordinate carbon bound only to other carbon atoms is unprecedented,[9] and has attracted comment in Chemical & Engineering News,[11] New Scientist,[10] Science News,[12] and ZME Science.[55] The carbon atom at the top of the pyramid is bonding with six other atoms, an unusual arrangement as the usual maximum valence for this element is four.[11] The molecule is aromatic and avoids exceeding the octet on carbon by having only a total of six electrons in the five bonds between the base of the pyramid and its apex. That is, each of the vertical edges of the pyramid is only a partial bond rather than a normal covalent bond that would have two electrons shared between two atoms. Although the top carbon does bond to six others, it does so using a total of no more than eight electrons.[14]

The dication, noting the weak bonds forming the upright edges of the pyramid, shown as dashed lines in the structure, have a Wiberg

18 electron rule on the metal.[56][57]

Left: Structure of C
6(CH
3)2+
6, as drawn by Steven Bachrach[13]
Right: The analogous organometallic complex [(η5
–C
5(CH
3)
5)Zn(CH
3)][56]

It has been commented that "[i]t's super important that people realize that, although we're taught carbon can only have four friends, carbon can be associated with more than four atoms" and added that the "carbon isn't making six bonds in the sense that we usually think of a carbon-carbon bond as a two-electron bond."[12] "It is all about the challenge and the possibility to astonish chemists about what can be possible."[10]

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