p - Block Elements

1) Nitrogen Group

Introduction

The elements in this group are nitrogen, phosphorous, arsenic, antimony and bismuth.

Nitrogen is the most important component of the earth’s atmosphere (78.1% by volume).

Both nitrogen and phosphorous are essential constituents of plant and animal tissues.

The last three elements had long been isolated and studied extensively by alchemists.

Physical Properties of Group 15 Elements

Nitrogen is a colourless diatomic gas having a triple bond, which confers unusual stability.

For more than a century the only isolable chemical species containing only nitrogen were N2 and the azide ion, N3-.

In 1999 the product N5+ was reported to be formed in the following reaction.

N2F+ [AsF6]- + HN3 → N5+ [AsF6]- + HF

The N5+ ion is stable below – 780C and has a V – shaped structure as shown

1.1) Allotropy

- Phosphorous has many allotropes, the most common being while phosphorous existing as discrete tetrahedral P4 molecules in the gaseous, liquid and solid states.

- It is soft, waxy and highly reactive and glows in moist air emitting a greenish – yellow light.

This phenomenon is called Phosphorescence and is the origin of the name of the element.

- It is stored under water.

- At very high temperature P4 molecules dissociates to P2:

Condensation of the vapours gives brown phosphorous, which probably contains P2 molecules.

If white phosphorous is heated in absence of air, red phosphorous is obtained.

It exists in a variety of polymeric modifications.

The most thermodynamically stable form is black phosphorous obtained by heating white phosphorous at high pressure.

It is inert having a layer structure examples of these structures are shown in fig :

Allotropes of Phosphorus

Arsenic, antimony and bismuth exist in several allotropic modifications.

Arsenic vapour contains tetrahedral As4 molecules.

In the solid state it exists in a yellow form comprising of As4 molecules and a stable grey (α) form having a rhombohedral structure.

Antimony exists in α form and another form having hexagonal close packed structure.

Bismuth exists in α – form and a form having body- centred cubic structure.

1.2) Oxidation States and Bond Type

The elements exhibit a maximum oxidation state of five towards oxygen by utilizing all valence electrons towards bond formation.

Trivalency is important for heavier members (inert pair effect).

They show oxidation states of three and five towards halogens and sulphur and three towards hydrogen.

Nitrogen exhibits a range of oxidation slates from -3 to +5

Example : NH3 (-3), N2H4 (-2), NH2OH (-1), N2 (O), N2O (+1), NO (+2), HNO2 (+3), NO2(+4) and HNO3 (+5).

It exists in three anionic forms N3- (nitride), N-3 (azide) and N2-2.

Nitrides are known for lithium, magnesium and aluminium.

Phosphides and arsenides (Na3P, Ca3As2) are also known.

1.3) Chemical Properties

Nitrogen is rather unreactive.

The other elements are fairly reactive and some reactions are shown in table:

General trends are not as apparent as in other groups as one encounters with a variety of elements – non-metals, metalloids and metals.

Ammonia is one of the most highly- produced inorganic chemicals.

The Haber- Bosch Process manufactures most of it synthetically from nitrogen and hydrogen.

A small amount is obtained during coal gas purification and during production of coke from coal.

The worldwide production in 2004 was 109,000,000 tons, the chief producer being China followed by India.

About 80% or more of the ammonia produced is used for fertilizing agricultural crops.

The main fertilizers manufactured from ammonia are urea, ammonium nitrate, ammonium phosphate and ammonium sulphate.

Ammonia is also used for the production of many inorganic and organic chemicals, plastics, fibres, explosives and intermediates for dyes and pharmaceuticals.

Synthetic ammonia is the key to the production of most nitrogen compounds as shown below.

Hydrazine is prepared by the action of sodium hypochlorite on ammonia in presence of a small amount of glue or gelatin, which suppresses the side reactions:

Advantage has been taken of this reaction in the major use of hydrazine and its methyl derivative, in rocket fuels.

Hydrazine is a convenient reducing agent.

It can also act as a coordinating ligand forming complexes with transition metals.

Phosphorous forms an unstable hydride diphosphine, P2H4, which has very little similarity with hydrazine.

Hydrogen azide (hydrazoic acid HN3) is an unstable compound, which decomposes on heating

2HN3 → H2 + 3N2

It is slightly more stable in aqueous solution and behaves as weak acid forming salts known as azides.

Sodium azide is obtained by reaction of sodamide with nitrous oxide.

2 NaNH2 + N2 O → NaN3 + NaOH + NH3

2NaN3 + H2SO4 → 2HN3 + Na2SO4

The acid and some its salts are explosive in nature.

Ionic azides are more stable than covalent azides as a greater number of resonating structures can be drawn for the former.

1.4) Halides

Two series of halides are known – the trihalides (MX3) and pentahildes (MX5).

The trihalides are known for all the elements and can be prepared by direct combination.

But if the halogen is in excess than the pentahalide is formed in addition.

The trihalides are all covalent apart from BiF3 that is ionic.

SbF3 and the other halides of bismuth have partial ionic character.

The central atom is sp3 hybridized and the shape is pyramidal (like NH3) .

Apart from NF3 that is very stable the trihalides hydrolyze but the products vary.

NCl3 + 3H2 O → NH3 + 3HOCl

PCl3 / As Cl3 + 3H2O → H3PO3 / H3AsO3 + 3HCl

SbCl3 / BiCl3 + H2O → SbOCl / BiOCl + 2HCl

The different products obtained in case of hydrolysis of NCl3 and PCl3 is due to a difference in mechanism, which results in formation of different intermediates.

This structure is retained in the solid state for PF5 but PCl5 dimerises in the solid state and exists as [PCl4]+[PCl6]-

Phosphorous pentachloride is the most important pentahalide and is obtained by treating phosphorous trichloride with chlorine.

PCl3 + Cl2 → PCl5

It hydrolyses readily.

PCl5 + H2O → POCl3 + 2 HCl

POCl3 + 3H2O → H3PO4 + 3HCl

It is widely used in organic synthesis as a halogenating agent.

ROH + PCl5 → RCl + POCl3 + HCl

RCOOH + PCl5 → RCOCl + POCl3 + HCl

1.5) Nitrogen oxides and oxo acids

The common oxides and oxoacids of nitrogen are summarized in Table and their structures are also depicted there.

It is an excellent oxidizing agent particularly when hot and concentrated.

A mixture of concentrated HNO3 and concentrated HCl in 1:3 ratios is called aqua regia and noble metals like gold and platinum dissolve in it.

The reactions of HNO3 with some metals and non-metals are summarized in table :

1.6) Oxoacids of Phosphorous, Arsenic & Antimony

The oxoacids of arsenic and antimony are not well – characterized but their salts are known.

Phosphorous forms two series of oxo acids – phosphoric acid series containing P (V) and phosphorous acid series containing P (III)

The following generalization can be made:

a) P is sp3 hybridized.

b) Acidic hydrogens are attached to oxygen

c) The bond between P and non-hydroxylic oxygen has appreciable double bond character.

d) The P-H bond confers reducing properties.

Some properties of the oxoacids of phosphorous are summarized in below table:

Phosphazenes

Phosphazenes are a group of P(V)/N(III) compounds featuring chain or cyclic structures, and are oligomers of the hypothetical N≡PR2.

The reaction of PCl5 with NH4Cl in a chlorinated solvent (e.g. C6H5Cl) gives a mixture of colourless solids of formula (NPCl2)n in which the predominant species have n = 3 or 4.

The compounds (NPCl2)3 and (NPCl2)4 are readily separated by distillation under reduced pressure.

Although equation summarizes the overall reaction, the mechanism is complicated; there is some evidence to support the scheme which illustrates the formation of the trimer.

nPCl5 + nNH4Cl ⟶ (NPCl2)n + 4nHCl

Reaction is the traditional method of preparing (NPCl2)3, but yields are typically 50%.

Improved yields can be obtained by using reaction.

Again, although this looks straight forward, the reaction pathway is complicated and the formation of (NPCl2)3 competes with that of Cl3P=NSiMe3.

Yields of (NPCl2)3 can be optimized by ensuring a slow rate of addition of PCl5 to N(SiMe3)3 in CH2Cl2.

Yields of Cl3P=NSiMe3 (a precursor for phosphazene polymers, see below) are optimized if N(SiMe3)3 is added rapidly to PCl5 in CH2Cl2, and this is followed by the addition of hexane.

3N(SiMe3)3 + 3PCl5 ⟶(NPCl2)3 + 9Me3SiCl

N(SiMe3)3 + PCl5 ⟶Cl3P=NSiMe3 + 2Me3SiCl

Above Reaction can be adapted to produce (NPBr2) n or (NPMe2) n by using PBr5 or Me2PCl3 (in place of PCl5) respectively.

The fluoro derivatives (NPF2)n (n = 3 or 4) are not made directly, but are prepared by treating (NPCl2) n with NaF suspended in MeCN or C6H5NO2.

The Cl atoms in (NPCl2)3, 14.68, and (NPCl2)4, readily undergo nucleophilic substitutions.

e.g. The following groups can be introduced:

. F using NaF (see above);

. NH2 using liquid NH3;

. NMe2 using Me2NH;

. N3 using LiN3;

. OH using H2O;

. Ph using LiPh.

Two substitution pathways are observed:

If the group that first enters decreases the electron density on the P centre (e.g. F replaces Cl), the second substitution occurs at the same P atom.

If the electron density increases (e.g. NMe2) substitutes for Cl), then the second substitution site is at a different P centre.

Small amounts of linear polymers, are also produced in reaction, and their yield can be increased by using excess PCl5.

Polymers of (NPCl2)3 with molecular masses in the range 106, but with a wide mass distribution, result from heating molten (NPCl2)3 at 480–520 K.

Room temperature cationic-polymerization can be achieved using Cl3P=NSiMe3 as a precursor (equation); this leads to polymers with molecular masses around 105 and with a relatively small mass distribution.

The Cl atoms in the polymers are readily replaced, and this is a route to some commercially important materials.

Treatment with sodium alkoxides, NaOR, yields linear polymers [NP(OR)2] n which have water-resistant properties, and when R=CH2CF3, the polymers are inert enough for use in the construction of artificial blood vessels and organs Many phosphazene polymers are used in fire-resistant materials.

The structures of (NPCl2)3, (NPCl2)4, (NPF2)3 and (NPF2)4 are shown in Figure.

Each of the 6-membered rings is planar, while the 8-membered rings are puckered.

In (NPF2)4, the ring adopts a saddle conformation, but two ring conformations exist for (NPCl2)4.

The metastable form has a saddle conformation, while the stable form of (NPCl2)4 adopts a chair conformation .

Although structure indicate double and single bonds in the rings, crystallographic data show that the P-N bond lengths in a given ring are equal.

Data for (NPCl2)3 and (NPF2)3 are given in Figure; in (NPF2)4, d(P-N)=154 pm, and in the saddle and chair conformers of (NPCl2)4, d(P-N)=157 and 156 pm respectively.

The P-N bond distances are significantly shorter than expected for a P-N single bond (e.g. 177pm in the anion in Na[H3NPO3]), indicating a degree of multiple bond character.

(a) Structural parameters for the phosphazenes (NPX2)3 (X=Cl or F);

(b) Schematic representations of the P4N4 ring conformations in (NPF2)4 (saddle conformation only) and (NPCl2)4(saddle and chair conformations).

Resonance structures could be used to describe the bonding in the planar 6-membered rings.

Traditional bonding descriptions for the 6-membered rings have involved N(2p)–P(3d ) overlap, both in and perpendicular to the plane of the P3N3-ring.

However, this model is not consistent with current opinion that phosphorus makes little or no use of its 3d orbitals.

Structure provides another resonance form for a 6-membered cyclophosphazene, and is consistent with the observed P-N bond equivalence, as well as the observation that the N and P atoms are subject to attack by electrophiles and nucleophiles, respectively.

Theoretical results support the highly polarized Pδ+ -Nδ- bonds and the absence of aromatic character in the P3N3-ring.