Phosphazenes

 PHOSPHAZENES OR PHOSPHONITRILIC HALIDES

Nitrogen and phosphorus catenate together, forming an interesting series of polymers containing P-N single bonds and P=N double bonds. In these compounds, the phosphorus atom is in a +5 oxidation state, and the nitrogen atom is in a +3 oxidation state. The compounds are formally unsaturated.

PREPARATION OF PHOSPHAZENES

When PCl₅ is refluxed with NH₄Cl in presence of C₂H₄Cl₂ or C₆H₅Cl, or heated with solid NH₄Cl at 145⁰C–160⁰C, ring compounds of the type [NPCl₂]_n where (n = 3-8) and chain compounds of the type Cl₄P(NPCl₂)_nNPCl₃ are produced. The newly synthesized compounds are called phosphazenes or phosphonitrilic halides.

Preparation of phosphazenes

The bromo derivative is obtained by heating PBr₅ with NH₄Br and the fluoro derivative is obtained from the chloride on treatment with NaF in acetonitrile.

[NPCl₂]_n + 2n NaF -----> [NPF₂] + 2n NaCl

Alkoxy groups can be introduced in the polymer by replacing Cl.

[NPCl₂]_n + 2n NaOR -----> [NP(OR)₂]_n + 2n NaCl

The iodide derivatives are not yet known. The rings compounds where n=3 and n=4 are stable and can be prepared by under controlled conditions.

STRUCTURE OF PHOSPHAZENES

The structure involves sp² hybridised trivalent nitrogen (N) and roughly sp³ hybridised pentavalent phosphorus (P). Alternate single and double bonds are present between N and P. All the P―N bond distances (167 pm) become same through resonance. This distance is shorter than P―N single bond distance (178 pm). The multiple bonding can be explained by pπ―dπ bonding between filled pπ orbital of N and empty dπ orbital of P. Electron delocalisation similar to benzene and graphite occurs here also, this gives a pseudo aromatic character to the ring.

Cyclophosphazene

INORGANIC RUBBER

When excess PCl₅ is refluxed with NH₄Cl in S-tetra chloro ethane linear polymers are obtained. They are rubbery in nature, elastic, thermally stable, and insoluble in petroleum ether. A long chain polymer [NPCl₂]_x of molecular weight of 20,000 or more is obtained by heating [NPCl₂]₃ to 250⁰C-350⁰C. The end group of such high molecular weight polymers are not known. These rubbery substances are called inorganic rubber. These are water proof and fire proof.

Inorganic rubber

PROPERTIES OF PHOSPHAZENES

The phosphazenes range from fluids and definite solids to polymeric elastomers. The lower polymers are insoluble in water but soluble in organic solvents and resemble vulcanized rubber in properties and are often called inorganic rubber.

REACTIONS OF PHOSPHAZENES

The chlorine atoms in chlorophosphazenes are reactive and may be substituted by a variety of reagents.

[NPCl₂]₃ + 6CH₃MgI ------> [NP(CH₃)₂]₃ + 3MgCl₂ + 3MgI₂

[NPCl₂]₃ + 6C₆H₅Li ------> [NP(C₆H₅)₂]₃ + 6LiCl

[NPCl₂]₃ + 6NaOR ------> [NP(OR)₂]₃ + 6NaCl

[NPCl₂]₃ + 6NaSCN ------> [NP(SCN)₂]₃ + 6NaCl

[NPCl₂]₃ + 6H₂O ------> [NP(OH)₂]₃ + 6HCl

[NPCl₂]₃ + 6NH₃ ------> [NP(NH₂)₂]₃ + 6HCl

The lone pair of electrons present on each N atom in [NPCl₂]₃ molecule makes it basic and hence it forms adduct with Lewis acids like HClO₄, SO₃, AlCl₃ etc.

[NPCl₂]₃ + 2AlCl₃ ------> [NPCl₂]₃•2AlCl₃

[NPCl₂]₃ + 3SO₃ -------> [NPCl₂]₃•3SO₃

[NPCl₂]₃ + 2HClO₄ ------> [NPCl₂]₃•2HClO₄

USES OF PHOSPHAZENES

Phosphazenes or phosphonitrilic halides find uses in flame-proofing of fabrics, as plasticisers and as catalysts in the manufacture of silicones.

About Lithium

 LITHIUM

Symbol --- Li

Abundance --- 0.0017% in earth's crust.

Physical state --- solid

Elemental Category --- Alkali metal

Colour --- silver

Discovery --- Johan August Arfwedson (1817)

Atomic no --- 3

Atomic weight --- 6.93

Period --- 2

Group --- 1

Block --- s-block

Known Isotopes--- ₃Li³ ₃Li⁴ ₃Li⁵ ₃Li⁶ ₃Li⁷ ₃Li⁸ ₃Li⁹ ₃Li¹⁰ ₃Li¹¹ ₃Li¹²

Stable Isotopes --- ₃Li⁶ ₃Li⁷ 

Isotopic abundance --- [₃Li⁶ (7.59%), ₃Li⁷ (92.41%)]

Melting Point --- 453.65 K (180.5⁰C)

Boiling Point --- 1603 K (1330⁰C)

Density --- 0.534 g/cc

Electron Configuration --- [He] 2s¹

Oxidation State --- +1

Valence --- 1

Electronegativity --- 0.98

Electron Affinity --- 59.6 KJ/mol

Ionisation Energy --- 520.2 KJ/mol (1st), 7298.1 KJ/mol (2nd), 11815 KJ/mol (3rd)

Covalent Radius --- 128 pm

Van der Waals radius --- 182 pm

Crystal Structure --- B.C.C (Body Centered Cubic) 

Heat of fusion --- 3 KJ/mol

Heat of vaporisation --- 136 KJ/mol

Critical Point --- 3220 K, 67 MPa (661.239 atm)

Molar heat capacity --- 24.86 J/mol-K

Specific heat --- 3570 J/(Kg-K)

Thermal conductivity --- 84.8 W/(m k)

Molar volume --- 0.00001297

Speed of sound --- 6000 m/s

Magnetic type --- paramagnetic

Mass magnetic susceptibility ---   +2.56x10^-8 m³/kg

Molar magnetic susceptibility --- + 1.78 x 10^-10 m³/mol 

Lattice angles --- π/2, π/2, π/2

Lattice constants --- 351 pm,351 pm,351 pm

Quantum numbers --- ²S_1/2

Neutron cross section --- 71

Electrical conductivity --- 1.1 x 10⁷ S/m

Electrical resistivity --- 92.8 nΩ m

Mohs hardness --- 0.6

Brinell hardness --- 5 MPa

Young's modulus --- 4.9 GPa

Bulk modulus --- 11 GPa

Shear modulus --- 4.2 GPa

ABOUT LITHIUM

 Fullerenes

Fullerene is the allotropes of carbon and commonly refers to a molecule with 60 carbon atoms, C₆₀, and with an icosahedral symmetry. Now a days there are several such hollow closed-cage (polyhedral) cluster molecules, C_n (n only even in the range 30-600), and all of them have structures based on polyhedra formed by fusing pentagons and hexagons. Larger molecular weight fullerenes are C₇₀, C₇₆, C₇₈, C₈₀ which possess different geometric structure, such as C₇₀ has a rugby ball-shaped symmetry. The structure of fullerenes very much comparable to geodesic domes used in architecture built by R. Buckminster Fuller. The most widely used fullerene C₆₀, was called buckminster fullerene or buckyball.

Preparation of Fullerenes

Synthesis of fullerenes is done by arc discharge between graphite electrodes in approximately 200 Torr of He gas. The formation of fullerenes occurs due to evaporation of carbon, because of heat generated at the contact point between the electrodes. This discharge produces a carbon soot that can contain upto 15% fullerenes, C₆₀ (13%) and C₇₀ (2%). The fullerenes are separated according to their mass, by use of liquid chromatography and using a solvent such as a toluene.

Properties of Fullerenes

The colour of thin films of C₆₀, are mustard-coloured, but in bulk it appears dark brown. Where as the colour of thin films of C₇₀, are red-brown coloured, but in bulk it appears grey-black. The colour of the solutions of C₆₀, is magenta colour, where as the colour of the solutions of C₇₀, are dull red. Fullerenes are dissolved slowly in organic solvents, indicating close packing in the crystal and have high melting points. The C-13 nmr spectrum of C₆₀, shows a single peak (142.68 pm), suggesting uniform environment for all 60 carbon atoms.

Structure and Bonding of Fullerene

In fullerene molecule, each carbon may be supposed to sp² hybridized, and forming three σ bonds to other three carbon atoms. Therefore, the remaining electron at each carbon is delocalized into a system of molecular orbitals that imparts some aromatic character to the whole molecule. One mechanism suggested that, the addition of carbon atoms occur, first in pentagonal and hexagonal patterns which gradually curl under appropriate conditions to form the cage-like structure. To close into a spheroid, a fullerene must have exactly 12 five membered faces, but the number of six membered faces can vary widely (1/2 n - 10).The number of five membered or pentagonal faces are 12 in C₆₀, whereas number of six membered or hexagonal faces are [1/2 (60) - 10] = 20. In case of C₇₀, the number of five membered or pentagonal faces are 12, and number of six membered or hexagonal faces are [1/2 (70) - 10] = 25. In the case of C₆₀, each carbon atom is at the junction of two six membered rings and one five membered ring. Each six membered rings containing three double bonds. Thus C₆₀ has the structure of a truncated icosahedron. Fullerenes have two different types of C―C bonds:

(a) Those at an edge shared between two fused hexagons, that is ( 6 : 6 rings), the bond length is 139.1 ± 1.8 pm. This is also the distance between pentagons.

(b) Those at an edge between a pentagon and a hexagon or the bonds within a given pentagon are somewhat longer, the bond length is 145.5 ± 1.2 pm.

Structural unit of fullerene

The unit cell of solid C₆₀ consists of a face centered cubic array of C₆₀ molecule in close packing. The molecules can rotate freely at their lattice sites by the thermal energy available at room temperature and can be considered to be spherical.

Structure of fullerene