JEE Chemistry The P Block Elements Notes Set 06

Back Bonding

1. Back bonding is the donation of a lone pair from a small atom (N, O, F, C) into a vacant orbital of an adjacent atom (B, Si, P, S, Cl), in addition to the normal sigma bond.
2. It needs a filled orbital on one atom and a vacant orbital of suitable energy and size on the other. The common types are pπ-pπ (for example in BF₃, where F lone pairs enter the empty 2p of B) and pπ-dπ (for example N(SiH₃)₃, O(SiH₃)₂).
3. Effects: BF₃ has partial double bond character and a shorter B–F bond; it is a weaker Lewis acid than BCl₃ and BBr₃ in the order BF₃ < BCl₃ < BBr₃. N(SiH₃)₃ is planar and a poor base, while Me₃N is pyramidal and basic.
4. In some cases the hybridisation of the central atom does not change on back bonding, for example the shape of the carbene species CCl₂ and CF₂.

Dimerisation and Polymerisation

1. Electron-deficient molecules complete their octet by dimerising or polymerising through bridge bonds. Examples are B₂H₆ (3c-2e bridge bonds), Al₂Cl₆ (bridging Cl atoms) and (BeCl₂)n chains in the solid.
2. In solid BeCl₂ each Be is bonded to four Cl atoms and has a complete octet, whereas in the BeCl₂ dimer Be is still electron deficient.
3. BCl₃ does not dimerise because B is too small to accommodate four large Cl atoms, but AlCl₃ does.

Inert Pair Effect

1. The two s electrons of the valence shell of heavier p-block elements are held tightly because d and f electrons shield the nucleus poorly. They resist taking part in bonding.
2. As a result the oxidation state two less than the group state becomes more stable down the group: Tl(+1), Pb(+2), Bi(+3) and Po(+4) are more stable than Tl(+3), Pb(+4), Bi(+5) and Po(+6).
3. Consequences: Pb(IV) and Bi(V) are strong oxidants, Sn(II) is a reductant, and Tl³⁺ oxidises I⁻, so TlI₃ is really Tl⁺I₃⁻.

Oxyacids and Their Strength

1. Acid strength of oxyacids of the same element rises with the number of non-hydroxyl oxygen atoms and with the oxidation state of the central atom: HClO < HClO₂ < HClO₃ < HClO₄ and H₂SO₃ < H₂SO₄.
2. Thermal stability rises in the same order, while oxidising power falls: HClO is the strongest oxidiser and HClO₄ the weakest on the basis of stability.
3. Basicity of an oxyacid equals the number of P–OH (or S–OH) groups, not the number of hydrogen atoms: H₃PO₄ is tribasic, H₃PO₃ dibasic and H₃PO₂ monobasic.
4. Reducing character depends on P–H bonds: H₃PO₂ > H₃PO₃ > H₃PO₄. H₃PO₂ and H₃PO₃ disproportionate on heating: 4H₃PO₃ → 3H₃PO₄ + PH₃.
5. Oxyacids of phosphorus all contain tetrahedral phosphorus; H₃PO₄ is used to make triple superphosphate fertiliser. H₄P₂O₇ (pyrophosphoric acid) is formed by heating H₃PO₄.
6. Oxyacids of sulphur: H₂SO₃, H₂SO₄, H₂S₂O₃ (thiosulphuric, S–S bond), H₂S₂O₄ (dithionous), H₂S₂O₅, H₂S₂O₆ (dithionic), H₂S₂O₇ (pyrosulphuric, S–O–S), H₂S₂O₈ (peroxodisulphuric, O–O bridge), H₂SO₅ (Caro's acid, peroxy link).
7. Oxyacids of halogens: only HOF is known for fluorine, while Cl, Br and I form HOX, HXO₂, HXO₃ and HXO₄ types.

Hydrolysis of Halides and Oxides

1. Covalent halides hydrolyse when the central atom has vacant d orbitals (or a vacant orbital) to accept the lone pair of water. SiCl₄, PCl₃, PCl₅, SbCl₃ and BCl₃ hydrolyse easily.
2. CCl₄ and NF₃ are not hydrolysed because C and N have no vacant d orbitals. SF₆ is inert for kinetic reasons because six F atoms surround the small S atom.
3. Examples: PCl₃ + 3H₂O → H₃PO₃ + 3HCl; PCl₅ + 4H₂O → H₃PO₄ + 5HCl; BCl₃ + 3H₂O → H₃BO₃ + 3HCl; NCl₃ + 3H₂O → NH₃ + 3HOCl.
4. Xenon fluorides are hydrolysed: 2XeF₂ + 2H₂O → 2Xe + 4HF + O₂ and 6XeF₄ + 12H₂O → 4Xe + 2XeO₃ + 24HF + 3O₂.

Allotropes

1. Carbon: diamond (sp³, hard, insulator), graphite (sp², layered, conducting) and fullerenes (cages such as C₆₀).
2. Phosphorus: white (discrete P₄ tetrahedra, very reactive, soluble in CS₂, stored under water), red (polymeric, stable, insoluble in CS₂, non-toxic) and black (layered, most stable).
3. Sulphur: rhombic (stable below 369 K) and monoclinic (stable above 369 K), both made of puckered S₈ rings; they dissolve in CS₂ but not in water.
4. Tin has grey (alpha) tin, which is a non-metallic form stable below 286 K, and white (beta) tin.

Silicates and Silicones

1. All silicates are built from SiO₄⁴⁻ tetrahedra. The structure depends on how many corner oxygens are shared per tetrahedron: 0 gives orthosilicates (for example zircon, ZrSiO₄), 1 gives pyrosilicates Si₂O₇⁶⁻, 2 gives chain (SiO₃²⁻)n or rings (beryl, Si₆O₁₈¹²⁻), 3 gives sheets (Si₂O₅²⁻)n as in talc and mica, and 4 gives three-dimensional frameworks (quartz, zeolites).
2. Chain silicates include pyroxenes (single chain) and amphiboles (double chain, asbestos).
3. Silicones are polymers (R₂SiO)n made by hydrolysis and condensation of R₂SiCl₂; R₃SiCl is a chain stopper and RSiCl₃ causes cross linking.
4. Pyrex is a man-made borosilicate glass.

Dehydrating and Reducing Behaviour of Common Reagents

1. Conc. H₂SO₄ removes water from sugar (charring), from formic acid (HCOOH → CO + H₂O) and from oxalic acid (giving CO and CO₂).
2. P₄O₁₀ is a stronger dehydrating agent and takes water even from acids: 4HNO₃ + P₄O₁₀ → 2N₂O₅ + 4HPO₃.
3. Conc. H₂SO₄ oxidises HBr and HI to Br₂ and I₂ and reduces to SO₂, but it does not oxidise HCl, so HCl and HF can be made from their salts and conc. sulphuric acid.