Allotropes of Carbon for IIT JEE

Carbon can form many allotropes. The physical characteristics of carbon vary considerably with the allotropic form. Diamond, for example, is highly transparent, but graphite is opaque and black. Diamond is the most difficult material known to occur naturally, while graphite is soft enough to form a strike on paper. Diamond's electrical conductivity is very low, while graphite is a very good conductor of electricity. Diamond, carbon nanotubes and graphene are carbon allotropes with the highest thermal conductivity of all known materials under normal conditions. Under normal conditions, all carbon allotropes are solids, with graphite being its most thermodynamically stable form. They are chemically resistant and require a high temperatures in order to react with oxygen.

Diamond

Carbon has an electronic configuration of 2, 4. Each carbon diamond shares electrons with four other atoms of carbon - forming four single bonds.

Only two bonds (or even one bond) seem to form some carbon atoms in the diagram, but that isn't really the case. We're just showing the whole structure a little bit. This is a giant structure of covalence - it begins in three dimensions. It's not a molecule because, depending on size of the crystal, the number of atoms joined in a real diamond is completely variable.

Physical Properties of Diamond

• The melting point is very high (nearly 4000 ° C). Before melting, very strong carbon - carbon covalent bonds must be broken throughout the structure.

• It's very difficult. This is again due to the need to break very powerful 3-dimensional covalent bonds.

• It does not supply electricity. All electrons are tightly held between the atoms and cannot move freely.

• Water and organic solvents are insoluble. There are no potential attractions between solvent molecules and carbon atoms that could outweigh the attractions between the covalently bound carbon atoms.

Graphite

Graphite is another carbon allotrope. It is an electrical conductor and a semi - metal, unlike diamond. Graphite is among the most stable forms of carbon allotropes under standard conditions and is used as the standard state in thermochemistry to define the heat of carbon compounds. There are three types of natural graphite:

1. Crystalline flake graphite: isolated, flat, plate-like particles with hexagonal edges

2. Amorphous graphite: fine particles, the result of thermal metamorphism of coal; sometimes called meta-anthracite

3. Lump or vein graphite: It occurs in fractures or fissure veins, appears as growths of fibrous or acicular crystalline aggregates

Graphite has a planar structure that is layered. The carbon atoms are arranged in a crystalline lattice with a separation of 0.142 nm in each layer, with a distance of 0.335 nm between planes (layers). The two known graphite forms, alpha (hexagonal) and beta (rhombohedral), have very similar facial properties (unless the layers stack slightly differently). It can be either flat or buckled hexagonal graphite. By mechanical treatment, the alpha form can be transformed to the beta form, and when heated above 1300 ° C, the beta form returns to the alpha form. Due to the vast electron delocalization within the carbon layers, graphite can conduct electricity. As the electrons are able to move, electricity moves through the layer plane. Graphite also has properties for self - lubricating and dry lubricating. Graphite is used in blood - comprising prosthetic materials and heat - resistant materials because it can withstand temperatures of up to 3000 ° C.

Graphene is called a single layer of graphite. This material exhibits exceptional electrical, thermal and physical properties. It is a carbon allotrope whose structure is a single planar sheet of carbon bonded sp2 atoms densely packed in a crystal honeycomb lattice. The length of the carbon-carbon bond in graphene is ~0.142 nm, and these sheets stack at an interplanar spacing of 0.335 nm to form graphite. Graphene is the structural element of allotropic carbon such as graphite, charcoal, nanotubes of carbon and fullerenes. Graphene is a semi-metal or zero-gap semiconductor that enables high mobility of electrons at room temperature. Graphene is an amazing new material class whose unique characteristics make it the topic of current research in many labs.

Amorphous Carbon

Amorphous carbon is carbon without a crystalline structure. Although amorphous carbon can be produced, some graphite-like or diamond-like carbon microscopic crystals still exist. Amorphous carbon properties depend on the ratio of hybridized sp2 to sp3 bonds in the material. Graphite is made up purely of hybridized sp2 bonds, while diamond is made up purely of hybridized sp3 bonds. Materials high in sp3 hybridized bonds are referred to as amorphous carbon tetrahedral or carbon diamond - like.

Fullerenes and Nanotubes

Carbon nanomaterials constitute another allotropic carbon class. Fullerenes (also known as buckyballs) are carbon - composed molecules of varying sizes that take the form of hollow spheres, ellipsoids or tubes. Buckyballs and buckytubes are the topic of great research, as both have unique chemistry and technological applications, particularly in the fields of materials science, electronics and nanotech. Carbon nanotubes are cylindrical carbon molecules with extraordinary strength and unique electrical properties. Carbon nanobuds are recently discovered allotropes where fullerene - like "buds" are covalently attached to a carbon nanotube's outer side walls. Consequently, nanobuds have both nanotubes and fullerenes.

Glassy Carbon

Glassy carbon is a non-graphitizing carbon class that is widely used in electrochemistry as an electrode material, as well as high temperature crucibles and as a component of some prothesis devices. This was first created at the General Electric Company's laboratories in the early 1960s using cellulose as the starting material. Shortly afterwards, Japanese workers found similar phenolic resin material.

This was first generated by Bernard Redfern at the Carborundum Company, Trafford Park, Manchester, UK laboratories in the mid-1950s. He set out to create a polymer matrix to frame a framework of diamonds and found a resole (phenolic) resin that would have been set without a catalyst with special preparation. The first glassy carbon was produced with this resin. Some of the patents were filed in the interests of national security. Original resin and product research samples are still available.

Glassy carbon preparation involves submitting a series of heat treatments to the organic precursors at temperatures up to3000°C. They are impermeable to gases and chemically extraordinarily inert, particularly those fully prepared at very high temperatures, unlike many non-graphitizing carbons. It has been shown that the oxidation rates in oxygen, carbon dioxide or water vapor of certain glassy carbons are lower than those of any other carbon. They are also very resistant to acid attacks. While normal graphite is reduced to powder by a combination of highly concentrated sulfuric and nitric acids at room temperature, glassy carbon is not affected by this treatment even after several months.

Carbon nanofoam

Carbon nanofoam is the fifth known carbon allotrope discovered at the Australian National University in Canberra in 1997 by Andrei V. Rode and colleagues. It consists of a low-density cluster of carbon atoms in a loose three - dimensional web.

Each cluster is about 6 nanometers large and comprises of about 4000 carbon atoms related in graphite-like sheets which are given highly negative curvature by heptagonal inclusion in the regular hexagonal pattern. This is the inverse of what occurs in the particular case of buckminsterfullerenes where the inclusion of pentagons gives carbon sheets positive curvature.

Carbon nanofoam's large-scale structure is similar to that of an aerogel, but with 1% of the density of carbon aerogels previously produced - only a few times the density of air at sea level. Carbon nanofoam is a poor electrical conductor, unlike carbon aerogels.

Lonsdaleite (hexagonal diamond)

Lonsdaleite is a hexagonal allotrope of allotropic carbon diamond, thought to have been formed when meteoric graphite fell to Earth. The high heat and stress of the impact turned the graphite into a diamond, but maintained the hexagonal crystallattice of graphite.

The Barringer Crater (also known as Meteor Crater) in Arizona, Lonsdaleite was first recognized from the Canyon Diablo meteorite. It was first found in 1967. Lonsdaleite occurs in the Canyon Diablo meteorite as microscopic diamond - related crystals; Kenna meteorite, New Mexico; and Allan Hills 77283, Victoria Land, Antarctica meteorite. It was also confirmed from the impact point of Tunguska, Russia.

Linear Acetylenic Carbon (LAC)

Chemists in the United States recently reported a carbon allotrope consisting of long chains of carbon atoms with different lengths of alternative carbon bonds; and consisting of C - C bonds and C ≡ C bonds.

The same polymer was synthesized by a group of Soviet chemists in the early 1960s and was referred to as carbyne (Russian: carbon). It seemed to be a very light - sensitive semiconductor, so it was suggested that it be used in photodiodes and similar devices.

Carbyne is another name for Linear Acetylenic Carbon (LAC), a carbon allotrope with a chemical structure -(C::C)n-. Carbon is linear with orbital hybridization in this modification and is a polymer with alternating single and triple bonds. This type of carbyne is of great interest to nanotechnology as the Young module is forty times the diamond module.