Allotropes of carbon and its structure, properties and uses

Some of the allotropes of carbon are given below

• Diamond
• Graphite [ Graphene]
• Amorphous carbon
• Buckminsterfullerene [ Carbon nanotube, Carbon nanobuds]
• Glassy carbon
• Carbon nanofoam
• Lonsdaleite (hexagonal diamond)
• Linear acetylene carbon (LAC)
• Hyperenes – have penta coordinate

♦Diamond

Structures:

Each carbon atom in a diamond is covalently bonded to four other carbons in a tetrahedron. These tetrahedra together form a 3-dimentional network of six-membered carbon rings (similar to cyclohexane) in the chair conformation. There is zero-angle strain

The stable network of covalent bonds and hexagonal rings is the reason for the great strength of carbon.

Properties:

• Diamond is the hardest known natural mineral. It holds polish and lustier extremely well. No known naturally occurring substances can cut (or even scratch ) a diamond (10 on mohs scale)
• Diamond is a non-conductor of heat and electricity.
• However, the thermodynamic stability of diamond is less than that of graphite.

Uses:

• Used as a gem in jewelry
• Embedded in cutting tools and polish papers.
• Used in containment for high pressure experiment (diamond avail)
• Possible use in microchips

 

♦Graphite:

Structure:

Each carbon atom uses only three of its valance electron to bond covalently with three other carbon atoms in a planner fashion. Each carbon atom contributes one electron to a delocalized system of electron that is also part of the chemical bonding

The delocalized electrons are free to move throughout the plane.  For this reason, graphite conducts electricity along the planes of carbon atoms, but it does not conduct electricity in a direction at right angles to the plane.

Properties:

• It is a good conductor of electricity.
• It is thermodynamically more stable than diamond.
• Graphite is soft  (2 on mohs scale)
• Graphite lubricity is due to adsorbed air and water between the layers.
• Its specific gravity is 2.3 which make it lighter than diamond.
• Chromic acid oxidizes graphite to CO2. Burns without producing smokes.

Uses:

• In electrodes.
• As a dry lubricant
• In lead pencils
• As pyro lytic carbon (graphite layers bound by larger number of crystal defects) in blood-contacting implants such a prosthetic heart valves
• In pure glassy (isotopic) synthetic forms pyrolytic graphite and carbon fibre graphite are highly heat resistance (up to 3000°C). Used as reentry shields for missile nose cones.
• Under high temperature and pressure (2000°C and 5 G Pa) it is transformed to diamond.

Graphene:

• A single layer of graphite is called graphene
• It is produce by-

i) Epitaxy (layering on surface) on an insulating or conducting substrate.

ii) Exfoliation – repeated peeling

Also read:

• VSEPR, Valance shell electron pair repulsion theory

•  about Hybridization and hybrid orbital

 

♦Amorphous carbon:

It is carbon that does not have any crystalline structure. There is no long range pattern of atomic position. coal and soot or carbon black are informally called amorphous carbon.

♦Fullerenes, Buckminsterfullerene or Buckyballs:

A fullerene is a allotropes of carbon  which have the form of a hollow sphere, tube, ellipsoid and many other shapes. The spherical fullerene are also called as Buckminsterfullerene or buckyballs, it is a type of fullerene with the formula C60. Buckminsterfullerene has a cage like fused ring structure, resemble the balls used in association football. It is made of twelve pentagons and twenty hexagons with a carbon atom at each vertex of each polygon and a bond along each polygon edge.

Carbon nanotubes:

Are cylindrical fullerenes, It have unusual properties, which are valuable for electronics,  nanotechnology, optics and other fields of materials science and technology.

 

♦Glassy carbon or vitreous carbon:

A non graphitizing carbon

Preparation:

 

♦Carbon nano-form:

It has long density cluster-assembly of carbon atoms strong together in a loose three dimensional web.

Each cluster is about 6nm wide and consists of~4000 carbon atoms linked in graphite like sheets

These are given negative curvature by the inclusion  of heptagons amongst pentagons and hexagons. Fullerene has positive curvature as it contains only pentagons and hexagons. It is lighter than an aerogel.

 

♦Lonsdaleite (hexagonal diamond):

It is the hexagonal allotropes of carbon. It s the carbon allotrope diamond. It is believed to form from graphite present in meteorites upon their impact to earth

Synthesis:

 

♦Linear acetylene carbon (LAC):

A one dimensional carbon polymer with the structure

$–(\mathrm{C}\equiv \mathrm{C}{)}_{\mathrm{n}}–$

 

♦Hyperene:

These are berocarbon compound compounds with planner penta coordinate carbon units.

$–{\mathrm{C}}_3{\mathrm{B}}_3–, –{\mathrm{C}}_2{\mathrm{B}}_4–, –{\mathrm{CB}}_5–$