Chemistry · Ch 9 — Elements of Group 13, 14 and 15
Allotropes of Carbon
Allotropes of Carbon
Diamond, graphite, fullerenes, carbon nanotubes and graphene are all allotropes of carbon, differing only in how their carbon atoms are bonded to one another. In diamond, every carbon atom is sp3 hybridised and linked tetrahedrally to four neighbouring carbon atoms by strong single covalent bonds 154 pm long (Fig 9.1), and these linked tetrahedra extend outward into a rigid three-dimensional network. This network structure makes diamond the hardest known natural substance, gives it an abnormally high melting point (about 3930°C) and makes it a poor conductor of electricity, since all four valence electrons of each carbon atom are tied up in localised sigma bonds — its hardness makes diamond useful for cutting glass, for dies that draw thin metal wire, and for jewellery. Graphite (Fig 9.2), by contrast, is built from two-dimensional sheets: within each sheet, every carbon atom is sp2 hybridised and forms three sigma bonds to its neighbours in a hexagonal net (C-C bond length 141.5 pm), while its fourth electron sits in an unhybridised p-orbital that overlaps sideways with the p-orbitals of every other carbon in the sheet to form a delocalised pi system spanning the whole layer. The sheets are held together only by weak van der Waals forces, spaced 335 pm apart, which is why graphite is soft and slippery, and why, despite being a nonmetal, it conducts electricity well (through the delocalised pi electrons) — graphite is in fact the thermodynamically most stable allotrope of carbon. Fullerenes are discrete carbon-cage molecules formed by linking a fixed number of carbon atoms; C60 (Fig 9.3) is the best known example, produced when an electric arc is struck between graphite electrodes in an inert atmosphere of argon or helium, giving a soot that contains substantial C60 along with smaller amounts of C32, C50, C70 and C84. C60 is shaped like a soccer ball (hence its name, Buckminsterfullerene or 'bucky ball'), built from twenty fused hexagonal rings and twelve fused pentagonal rings of carbon, and unlike diamond or graphite it shows two distinct C-C distances (143.5 pm and 138.3 pm). Fullerenes are true covalent molecules, soluble in organic solvents, and C60 can react with group-1 metals to give solids such as K3C60, which becomes a superconductor below 18 K. Carbon nanotubes (Fig 9.4) are cylindrical rolled-up sheets of graphite, either single-walled (under 1 nm in diameter) or multi-walled (over 100 nm in diameter), with lengths from micrometres to millimetres; they are mechanically robust — they can be bent and will spring back to their origi …
What this figure shows. Diamond's structure shows each carbon atom at the centre of a tetrahedron, covalently bonded to four other carbon atoms via sp3 hybrid orbitals; these tetrahedra repeat and link edge-to-edge throughout the crystal, building up a rigid three-dimensional covalent network with a C-C bond distance of 154 pm. The diagram distinguishes the carbon atoms (nodes of the network) from the covalent bonds linking them (the connecting lines) and is drawn to emphasise that every carbon is identically four-coordinate and tetrahedrally surrounded, with no layering and no delocalised electrons — …
What this figure shows. Graphite's structure is drawn as stacked, parallel two-dimensional sheets, each sheet a hexagonal honeycomb net of sp2-hybridised carbon atoms, each carbon bonded to three neighbours within its own sheet by sigma bonds (C-C bond length 141.5 pm, as labelled on the diagram). The diagram labels the perpendicular spacing between adjacent sheets as 335 pm, much larger than the 141.5 pm in-sheet bond length, visually showing why the weak, van der Waals interlayer forces let the sheets slide past each other easily (giving graphite its softness and slippery feel) while the strong in-plane covalent bonding keeps each sheet rigid. The delocalised pi-electron system running across each hexagonal sheet, from the unhybridised p- …
What this figure shows. The C60 fullerene is drawn as a hollow, closed, roughly spherical cage of sixty carbon atoms arranged like the seams of a soccer ball: twenty fused hexagonal rings and twelve fused pentagonal rings of carbon atoms tile the whole surface, with every carbon three-coordinate. The diagram is meant to show two slightly different C-C bond lengths at the ring fusions — 143.5 pm and 138.3 pm — which is why C60's bonding, unlike diamond's or graphite's single characteristic bond …
What this figure shows. Carbon nanotubes are drawn as long, hollow cylinders formed by conceptually rolling up a graphite sheet (or, for multi-walled tubes, several concentric rolled sheets) along its hexagonal carbon lattice, so the tube walls show the same repeating hexagonal net of sp2 carbons seen in graphite, just curved into a seamless tube. The diagram distinguishes single-walled nanotubes (one rolled layer, under 1 nm diameter) from multi-walled nanotubes (several concentric rolled layers, up to 100+ nm diameter), and depicts the tube as very long relative to its diameter (lengths from micrometres to millimetres), consistent wit …
What this figure shows. Graphene is drawn as a single, isolated hexagonal sheet of sp2-hybridised carbon atoms — essentially one layer peeled out of graphite's stacked structure — shown as a flat, two-dimensional honeycomb lattice with no second layer stacked above or below it, the structural feature that gives graphene its unique, genuinely two-dimensional elect …