Trivalent carbon sp2 hybridization typically leads to a planar geometry. In fullerenes, the bonding arrangement at carbon is far from planar—the three bonds form a shallow pyramid.
In these diagrams, the pattern of 5-membered and 6-membered rings is easily seen. Only the pattern of a bonds is shown in Figs. There is an extensive and critically important set of p bonds that involves every carbon atom in the molecule. It is clear from the reactivity of C60 as well as from theory that these bonds are not localized between pairs of carbon atoms, but are instead delocalized over the surface of the fullerene cage.
The closed-cage carbon molecules known as fullerenes provide an entirely new branch of chemistry, materials science, and physics. Fullerene research is now. Initially envisaged as rather unreactive, aromatic-like molecules, the fullerenes instead undergo a wide variety of reactions characteristic of.
There is some debate over whether fullerenes can be properly classified as aromatic. Fullerenes display a degree of bond length alternation. The bonds that form the fusion of a 6-membered ring with a 5-membered ring are somewhat longer bond length of 1. Fullerenes are much more reactive toward addition reactions than are classical aromatic compounds such as benzene.
This likely reflects the relatively poor overlap of p-orbitals splayed apart as a result of the curvature of structure , as well as the tremendous release of strain that results from conversion of carbon atoms from highly strained sp2 geometry to a much less strained sp3 geometry. Ring currents in fullerenes are segregated into opposing diamagnetic and paramagnetic currents that sum to 0 in the case of C60 and to small values for the other fullerenes. It is clear that traditional notions of aromaticity and indeed some of the traditional hallmarks of aroma-ticity, which were originally developed for planar, monocyclic systems, are not easily applied in the case of these spherical, polycyclic structures.
C60 has three degenerate lowest unoccupied molecular orbitals LUMOs and can be reduced to the C6cT anions, where n is The reduction can be achieved electro-chemically, by electron transfer from various anions, or by direct reduction with metals. Reviews on electron-transfer reactions of fullerenes and of the preparation and properties of the anions fullerides and the less well-known cations fullerenium ions have been published.
These beautiful molecules were first detected in the plume above a laser-evaporated carbon target and have also been formed on a large scale in electric arcs between carbon electrodes and in particular types of flames.
There has been a total synthesis of C60 reported, but the ease, low cost, and scalability of arc and of flame methods clearly make these the preferred routes. In essentially all cases, the dominant fullerene formed is C60, and the amounts of the higher fullerenes C70 and higher diminish rapidly, to the point where miniscule amounts of fullerenes larger than C84 are formed. In , a large-scale facility dedicated to fullerene production was established, using the flame method. Purification of fullerenes is usually accomplished by chromatography, and numerous methods have proven effective.
On smaller scales, sublimation can be used to produce highly pure, solvent-free fullerenes. Fullerenes are typically dark, sometimes lustrous, solids. The liquid phase has not been observed, but the solids sublime well under high vacuum. The solubility of fullerenes is negligible in hydrocarbon solvents, but reasonable in aromatic and halogenated aromatic sol-vents. When macroscopic amounts of fullerenes became available for study around , it did not take long to discover that these molecules were quite reactive.
The chemistry of fullerenes has turned out to be very rich. Hydrogenation of fullerenes is easily accomplished under a variety of conditions. Interestingly, C60H2 is a remarkably acidic compound for a hydrocarbon, with a pKa below 5. While C60H18 is isolated as a single species, C60H36 is isolated as a mixture of isomers. Treatment of C60 with most oxidizing agents results in the formation of C60O. This compound is less soluble than C60 itself and less chromatographically mobile.
Multiple oxidations occur with excess oxidizing agent, and multiple isomers of the C60Ox product are formed. These oxides are increasingly insoluble and difficult to characterize. Oxidation of C70 is somewhat more complex, as multiple isomers of the monoadduct are possible. The two predominate isomers formed are shown in Fig. Fullerenes react readily in a variety of different cycload-ditions. In most cases, a single cycloaddition to C60 will produce a single isomer of monoadduct, but a second cycloaddition produces a mixture of isomeric diadducts.
Reactions on less-symmetrical higher fullerenes are even more problematic. This reaction is thermal, although there are analogous photochemical reactions between fullerenes themselves, for example, the photopolymerization of C Nitrile oxides add to C60 to form isoxazolines, azomethine ylids add to form pyrrolidines, and diazoalkanes add to form pyrazolines.
The latter case is one of the most important reactions in fullerene chemistry, and the Cpyrazoline adduct is thermally unstable and decomposes to new products. In the case of Ph2CN2, the primary product is the methanofullerene. The opened bond is typically the bond that formed the fusion of a 5-membered ring with a 6-membered ring Fig. Similar chemistry occurs with alkyl azides , leading to the formation of azafulleroids. Azafulleroids have served as the starting material for the formation of C59N-based species, a rare example of a fullerene cage with a hete-roatom replacing one of the carbon atoms Fig.
Reaction with dienes such as cyclopentadiene yields adducts, and, as is often the case, multiple additions can occur. Of the various 1,3-dipolar cycloaddition reactions that have been demonstrated on fullerenes, the addition of azomethyne ylids has proven to be particularly versatile. This reaction converts a fullerene into a fulleropyrrolidine Fig.
The nitrogen atom provides a convenient point of attachment for a host of different groups. Fullerenes react in a manner not unlike electron-deficient alkenes. Strong nucleophiles add readily and often add repeatedly if conditions are not carefully controlled. Examples of carbon nucleophiles that react with C60 include alkyl lithium reagents, alkyl magnesium Grignard reagents, acetylide anions, and cyanide. Addition of several equivalents of nucleophile is a common side reaction. Organocuprates also add readily, resulting in a symmetrical cyclopentadienyl pattern. Heteroatom nucleophiles, including amines, are also known to add to C One of the most useful reactions in all of fullerene chemistry is the addition of dialkyl bromomalonate anion.
This reaction, often called the Bingel reaction, results in formation of a cyclopropane Fig. Most likely, this reaction involves addition of the enolate nucleophile, followed by displacement of the bromide ion by the resulting fulleride anion. This reaction has produced Td-symmetrical species that display luminescence. Luminescence from fullerenes and fullerene derivatives is uncommon. Radicals add readily and repeatedly to fullerenes.
Alkyl radicals can add over 30 times to C Sulfur- and nitrogen-based radicals have been observed to add to fullerenes. The tendency for multiple additions and multiple isomers has limited the synthetic utility of this reaction. However, radical addition to fullerenes has been used as method for covalent incorporation of fullerenes into polymers. Halogenation of fullerenes is easily accomplished with a variety of reagents. The resulting compounds are reactive under a variety of conditions, establishing the halogenated compounds as useful synthetic intermediates.
Attempts to explain the remarkable stability of the C 60 cluster led the scientists to the conclusion that the cluster must be a spheroidal closed cage in the form of a truncated icosahedron—a polygon with 60 vertices and 32 faces, 12 of which are pentagons and 20 hexagons. They chose the imaginative name buckminsterfullerene for the cluster in honour of the designer-inventor of the geodesic dome s whose ideas had influenced their structure conjecture.
From to , a series of studies indicated that C 60 , and also C 70 , were indeed exceptionally stable and provided convincing evidence for the cage structure proposal. Experiments showed that the size of an encapsulated atom determined the size of the smallest surrounding possible cage. In physicists Donald R. The resulting condensed vapours, when dissolved in organic solvents, yielded crystals of C With fullerenes now available in workable amounts, research on these species expanded to a remarkable degree, and the field of fullerene chemistry was born.
The C 60 molecule undergoes a wide range of novel chemical reactions. It readily accepts and donates electron s, a behaviour that suggests possible applications in batteries and advanced electronic devices. The molecule readily adds atoms of hydrogen and of the halogen element s. The halogen atoms can be replaced by other groups, such as phenyl a ring-shaped hydrocarbon with the formula C 6 H 5 that is derived from benzene , thus opening useful routes to a wide range of novel fullerene derivatives. Some of these derivatives exhibit advanced materials behaviour.
Particularly important are crystalline compounds of C 60 with alkali metal s and alkaline earth metal s; these compounds are the only molecular systems to exhibit superconductivity at relatively high temperatures above 19 K. Particularly interesting in fullerene chemistry are the so-called endohedral species, in which a metal atom given the generic designation M is physically trapped inside a fullerene cage. The resulting compounds assigned the formulas M C 60 have been extensively studied. Alkali metals and alkaline earth metals as well as early lanthanoids may be trapped by vaporizing graphite disks or rods impregnated with the selected metal.
Helium He can also be trapped by heating C 60 in helium vapour under pressure. Minute samples of He C 60 with unusual isotope ratios have been found at some geologic sites, and samples also found in meteorite s may yield information on the origin of the bodies in which they were found. Article Media. Info Print Print.
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Introduction Buckminsterfullerenes Carbon nanotubes Potential applications of fullerenes. Written By: David R. Walton Harold W.