Cubane () is a synthetic
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
compound that consists of eight
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...
atoms arranged at the corners of a
cube
In geometry, a cube is a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex. Viewed from a corner it is a hexagon and its net is usually depicted as a cross.
The cube is the only ...
, with one
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
atom attached to each carbon atom. A solid
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...
line substance, cubane is one of the
Platonic hydrocarbon
In organic chemistry, a Platonic hydrocarbon is a hydrocarbon (molecule) whose structure matches one of the five Platonic solids, with carbon atoms replacing its vertices, carbon–carbon bonds replacing its edges, and hydrogen atoms as needed.
...
s and a member of the
prismanes. It was first synthesized in 1964 by
Philip Eaton
Philip E. Eaton (born 1936) is a Professor Emeritus of Chemistry at the University of Chicago. He and his fellow researchers were the first to synthesize the "impossible" cubane molecule in 1964.P. Eaton and T. W. Cole, The Cubane System, J. Am. ...
and Thomas Cole.
Before this work, Eaton believed that cubane would be impossible to synthesize due to the "required 90 degree
bond angles
Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that deter ...
". The cubic shape requires the carbon atoms to adopt an unusually sharp 90° bonding angle, which would be highly
strained as compared to the 109.45° angle of a
tetrahedral
In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ...
carbon. Once formed, cubane is quite
kinetically stable
In chemistry and physics, metastability denotes an intermediate energetic state within a dynamical system other than the system's state of least energy.
A ball resting in a hollow on a slope is a simple example of metastability. If the ball ...
, due to a lack of readily available decomposition paths. It is the simplest hydrocarbon with
octahedral symmetry
A regular octahedron has 24 rotational (or orientation-preserving) symmetries, and 48 symmetries altogether. These include transformations that combine a reflection and a rotation. A cube has the same set of symmetries, since it is the polyhedr ...
.
Having high potential energy but kinetic stability makes cubane and its derivative compounds useful for controlled energy storage. For example,
octanitrocubane
Octanitrocubane (molecular formula: C8(NO2)8) is a high explosive that, like TNT, is shock-insensitive (not readily detonated by shock). The octanitrocubane molecule has the same chemical structure as cubane (C8H8) except that each of the eight ...
and
heptanitrocubane
Heptanitrocubane is an experimental high explosive based on the cubic eight-carbon cubane molecule and closely related to octanitrocubane. Seven of the eight hydrogen atoms at the corners of the cubane molecule are replaced by nitro groups, g ...
have been studied as high-performance explosives.
These compounds also typically have a very high
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
for hydrocarbon molecules. The resulting high
energy density
In physics, energy density is the amount of energy stored in a given system or region of space per unit volume. It is sometimes confused with energy per unit mass which is properly called specific energy or .
Often only the ''useful'' or extrac ...
means a large amount of energy can be stored in a comparably small amount of space, an important consideration for applications in fuel storage and energy transport.
Synthesis
The classic 1964 synthesis starts with the conversion of
2-cyclopentenone to 2-bromo
cyclopentadienone:
:
Allylic
In organic chemistry, an allyl group is a substituent with the structural formula , where R is the rest of the molecule. It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, . ...
bromination
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymer ...
with
''N''-bromosuccinimide in
carbon tetrachloride
Carbon tetrachloride, also known by many other names (such as tetrachloromethane, also recognised by the IUPAC, carbon tet in the cleaning industry, Halon-104 in firefighting, and Refrigerant-10 in HVACR) is an organic compound with the chemi ...
followed by addition of molecular bromine to the
alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic ...
gives a 2,3,4-tribromocyclopentanone. Treating this compound with
diethylamine
Diethylamine is an organic compound with the formula (CH3CH2)2NH. It is a secondary amine. It is a flammable, weakly alkaline liquid that is miscible with most solvents. It is a colorless liquid, but commercial samples often appear brown due to im ...
in
diethyl ether
Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable li ...
causes
elimination of two equivalents of
hydrogen bromide
Hydrogen bromide is the inorganic compound with the formula . It is a hydrogen halide consisting of hydrogen and bromine. A colorless gas, it dissolves in water, forming hydrobromic acid, which is saturated at 68.85% HBr by weight at room tem ...
to give the diene product.
:
The construction of the eight-carbon cubane framework begins when 2-bromocyclopentadienone undergoes a spontaneous
Diels-Alder dimerization. One ketal of the
''endo'' isomer is subsequently selectively deprotected with aqueous
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dige ...
to 3.
In the next step, the ''endo'' isomer 3 (with both
alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic ...
groups in close proximity) forms the cage-like isomer 4 in a
photochemical +2cycloaddition
In organic chemistry, a cycloaddition is a chemical reaction in which "two or more unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity" ...
. The
bromoketone group is converted to ring-contracted
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxyli ...
5 in a
Favorskii rearrangement with
potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash.
Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...
. Next, the thermal
decarboxylation
Decarboxylation is a chemical reaction that removes a carboxyl group and releases carbon dioxide (CO2). Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is ...
takes place through the
acid chloride
In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group . Their formula is usually written , where R is a side chain. They are reactive derivatives of carboxylic acids (). A specific example o ...
(with
thionyl chloride
Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a chlorinating reagent, with approximately per year bein ...
) and the
''tert''-butyl perester
In organic chemistry, organic peroxides are organic compounds containing the peroxide functional group (). If the R′ is hydrogen, the compounds are called hydroperoxides, which are discussed in that article. The O−O bond of peroxides easily b ...
6 (with
''tert''-butyl hydroperoxide and
pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid w ...
) to 7; afterward, the acetal is once more removed in 8. A second Favorskii rearrangement gives 9, and finally another decarboxylation gives, via 10, cubane (11).
A more approachable laboratory synthesis of disubstituted cubane involves bromination of the ethylene ketal of
cyclopentanone
Cyclopentanone is the organic compound with the formula (CH2)4CO. This cyclic ketone is a colorless volatile liquid.
Preparation
Upon treatment with barium hydroxide at elevated temperatures, adipic acid undergoes ketonization to give cyclope ...
to give a tribromocyclopentanone derivative. Subsequent steps involve dehydrobromination, Diels-Alder dimerization, etc.
:
The resulting cubane-1,4-dicarboxylic acid is used to synthesize other substituted cubanes. Cubane itself can be obtained nearly quantitatively by photochemical decarboxylation of the thiohydroxamate ester (the
Barton decarboxylation
The Barton decarboxylation is a radical reaction in which a carboxylic acid is converted to a thiohydroxamate ester (commonly referred to as a Barton ester). The product is then heated in the presence of a radical initiator and a suitable hydroge ...
).
Derivatives
The synthesis of the octaphenyl
derivative
In mathematics, the derivative of a function of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool of calculus. ...
from tetraphenylcyclobutadiene nickel bromide by Freedman in 1962 pre-dates that of the parent compound. It is a sparingly soluble colourless compound that melts at 425–427 °C.
A
hypercubane, with a
hypercube
In geometry, a hypercube is an ''n''-dimensional analogue of a square () and a cube (). It is a closed, compact, convex figure whose 1-skeleton consists of groups of opposite parallel line segments aligned in each of the space's dimensions, p ...
-like structure, was predicted to exist in a 2014 publication. Two different isomers of
cubene have been synthesized, and a third analyzed
computationally. The alkene in ''ortho''-cubene is exceptionally reactive due to its
pyramidalized geometry. At the time of its synthesis, this was the most pyramidalized alkene to have been successfully made. The ''meta''-cubene isomer is even less stable, and the ''para''-cubene isomer probably only exists as a
diradical
In chemistry, a diradical is a molecular species with two electrons occupying molecular orbitals (MOs) which are degenerate. The term "diradical" is mainly used to describe organic compounds, where most diradicals are extremely reactive and i ...
rather than an actual diagonal bond.
Cubylcubanes and oligocubanes
Cubene (1,2-dehydrocubane) and 1,4-cubanediyl(1,4-dehydrocubane) are enormously strained compounds which both undergo
nucleophilic addition
In organic chemistry, a nucleophilic addition reaction is an addition reaction where a chemical compound with an electrophilic double or triple bond reacts with a nucleophile, such that the double or triple bond is broken. Nucleophilic additions d ...
very rapidly, and this has enabled chemists to synthesize cubylcubane. X-ray diffraction structure solution has shown that the central cubylcubane bond is exceedingly short (1.458 Å), much shorter than the typical C-C single bond (1.578 Å). This is attributed to the fact that the exocyclic orbitals of cubane are s-rich and close to the nucleus. Chemists at the
University of Chicago
The University of Chicago (UChicago, Chicago, U of C, or UChi) is a private research university in Chicago, Illinois. Its main campus is located in Chicago's Hyde Park neighborhood. The University of Chicago is consistently ranked among the b ...
extended and modified the sequence in a way that permits the preparation of a host of
ubylcubane oligomers. The
ubylcubanes are rigid molecular rods with the particular promise at the time of making
liquid crystals
Liquid crystal (LC) is a state of matter whose properties are between those of conventional liquids and those of solid crystals. For example, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way. The ...
with exceptional UV transparency. As the number of linked cubane units increases, the solubility of
ubylcubane plunges; as a result, only limited chain length (up to 40 units) have been successfully synthesized in solutions. The skeleton of
ubylcubanes is still composed of enormously strained carbon cubes, which therefore limit its stability. In contrast, researchers at
Penn State University
The Pennsylvania State University (Penn State or PSU) is a Public university, public Commonwealth System of Higher Education, state-related Land-grant university, land-grant research university with campuses and facilities throughout Pennsylvan ...
showed that poly-cubane synthesized by solid-state reaction is 100% sp
3 carbon bonded with a tetrahedral angle (109.5°) and exhibits exceptional optical properties (high
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, ...
).
Reactions
Cuneane
Cuneane (C8H8, pentacyclo .3.0.02,4.03,7.06,8ctane) is a saturated hydrocarbon. Its name is derived from the Latin ''cuneus'', meaning a wedge. Cuneane may be produced from cubane by metal-ion-catalyzed σ-bond rearrangement. Similar reactions ar ...
may be produced from cubane by a
metal-ion-catalyzed σ-bond rearrangement.
:
With a rhodium catalyst, it first forms ''syn''-tricyclooctadiene, which can thermally decompose to cyclooctatetraene at 50–60 °C.
:
See also
*
Basketane
Basketane is a polycyclic alkane with the chemical formula C10H12. The name is taken from its structural similarity to a basket shape. Basketane was first synthesized in 1966, independently by Masamune and Dauben and Whalen. A patent application ...
*
Perfluorocubane
References
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[{{cite journal, title=The Cubane System, first1=Philip E., last1=Eaton, first2=Thomas W., last2=Cole, journal=]J. Am. Chem. Soc.
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, date=1964, volume=86, issue=5, pages=962–964, doi=10.1021/ja01059a072
[{{cite book, first1=Michael B., last1=Smith, first2=Jerry, last2=March, title=March's Advanced Organic Chemistry, url=https://archive.org/details/organicchemistry00mich_115, url-access=limited, edition=5th, publisher=John Wiley & Sons, date=2001, pag]
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[{{cite journal, title=Tetraphenylcyclobutadiene Derivatives. II.1 Chemical Evidence for the Triplet State, first=H. H., last=Freedman, journal=]J. Am. Chem. Soc.
The ''Journal of the American Chemical Society'' is a weekly peer-reviewed scientific journal that was established in 1879 by the American Chemical Society. The journal has absorbed two other publications in its history, the ''Journal of Analytical ...
, date=1961, volume=83, issue=9, pages=2195–2196, doi=10.1021/ja01470a037
[{{cite journal, title=Tetraphenylcyclobutadiene Derivatives. IV.1 "Octaphenylcubane"; A Dimer of Tetraphenylcyclobutadiene, first1=H. H., last1=Freedman, first2=D. R., last2=Petersen, journal=]J. Am. Chem. Soc.
The ''Journal of the American Chemical Society'' is a weekly peer-reviewed scientific journal that was established in 1879 by the American Chemical Society. The journal has absorbed two other publications in its history, the ''Journal of Analytical ...
, date=1962, volume=84, issue=14, pages=2837–2838, doi=10.1021/ja00873a046
[{{cite journal, title=Structure of the Dimer of tetraphenylcyclobutadiene, first1=G. S., last1=Pawley, first2=W. N., last2=Lipscomb, first3=H. H., last3=Freedman, journal=]J. Am. Chem. Soc.
The ''Journal of the American Chemical Society'' is a weekly peer-reviewed scientific journal that was established in 1879 by the American Chemical Society. The journal has absorbed two other publications in its history, the ''Journal of Analytical ...
, date=1964, volume=86, issue=21, pages=4725–4726, doi=10.1021/ja01075a042
External links
Eaton's cubane synthesis at SynArchive.comTsanaktsidis's cubane synthesis at SynArchive.com
Polycyclic nonaromatic hydrocarbons
Molecular geometry
Theoretical chemistry
Cyclobutanes
Substances discovered in the 1960s
Pentacyclic compounds