Chemistry of group 15 elements

Chemistry of group 15 elements

Occurrence and extraction

Nitrogen makes up 78% (by volume) of air, whereas phosphorus can be found in several minerals and ores. Phosphorus is an essential constituent of plants and animals, being present in deoxyribonucleic acid (DNA), bones, teeth and other components of high biological importance. Phosphorus does not occur in its elemental state in nature, as it readily oxidises and therefore is deposited as phosphate rock. The remaining elements of group 15 are mostly obtained from minerals, but can also be found in their elemental form in the earth’s crust. Arsenic is mostly presented in nature as mispickel (FeAsS), realgar (As₄S₄) and orpiment (As₂S₃). Bismuth occurs as bismuthinite (Bi₂S₃) as well as in its elemental form.

Dinitrogen (N₂) is extracted by fractional distillation of liquid air. By-products such as dioxygen (O₂) are removed by addition of H₂ and the use of a Pt catalyst. Elemental phosphorus is extracted from phosphate rock by reacting with sand and coke in an electrically heated industrial oven. Phosphorus vapour is isolated and condensed under water – white phosphorus is extracted.

2Ca₃(PO₄)₂ + 6SiO₂ + 10C → P₄ + 6CaSiO₃ + 10CO

Elemental arsenic is extracted mainly from FeAsS by heating and subsequent condensation of arsenic in the absence of air.

FeAsS → FeS + As (in absence of air)

Antimony is obtained from stibnite (Sb₂S₃) after reduction with iron. Bismuth is extracted from its sulfide or oxide ores via a reduction with carbon.

Sb₂S₃ + 3Fe → 2Sb + 3FeS

Physical properties

The physical properties of group 15 elements vary widely, from nitrogen being a gas to the remaining elements being solids with increasing metallic character. Nitrogen exists as a diatomic molecule N₂ and is a colourless and odourless gas (condensation at 77 K). Nitrogen forms relatively strong and short bonds, resulting in the formation of a triple bond in the N₂ molecule. Furthermore, nitrogen has an anomalously small covalent radius and therefore can form multiple bonds with N, C and O atoms. Group 15 elements follow the general trend showing an increasing covalent radius when descending within the group.

Phosphorus has several allotropes, with white, red and black phosphorus being the main ones.

Allotropes are defined as the two or more physical forms of one element. Allotropes of carbon are graphite, carbon and diamond. These allotropes are all based on carbon atoms but exhibit different physical prop-erties, especially with regard to hardness.

White phosphorus is a solid consisting of tetrahedral P₄ molecules with single bonds. White phospho-rus is the standard state of the element, but it is metastable, potentially due to the strained 60^∘ bond angles (Figure 6.2).

Heating of white phosphorus in an inert gas atmosphere results in red phosphorus, which is an amorphous solid (several crystalline forms are known) with an extended covalent structure (Figure 6.3).

Black phosphorus is the most stable allotrope of phosphorus and can be obtained by heating white phospho-rus under high pressure. In contrast to white phosphorus, black phosphorus does not ignite spontaneously in air. The reactivity of red phosphorus lies between those of the white and black allotropes. White phosphorus is insoluble in water and is therefore stored under water to prevent oxidation.

Arsenic and antimony vapour consists of As₄ or Sb₄ molecules, respectively. In the solid state, arsenic, antimony and bismuth are grey solids with a lattice structure similar to that of black phosphorus.

Oxidation states and ionisation energy

The general electron configuration for group 15 elements is ns²np³ and all elements form the oxidation states of +3 and +5. Nitrogen is more versatile and shows a range of oxidation states ranging from −3 to +5.

The ionisation energies relating to the removal of the first five electrons (two s and three p electrons) are relatively low. There is a significant increase in the ionisation energy necessary for the removal of a sixth electron, as this will be removed from an inner complete quantum shell (Table 6.1).

Chemical properties

Nitrogen is relatively unreactive because the bond enthalpy of the nitrogen–nitrogen triple bond is very high (944 kJ/mol). N₂(g) is usually used as an inert atmosphere for reactions that cannot be carried out in oxy-gen. Only lithium reacts directly with nitrogen with the formation of Li₃N. Nitrogen fixation is an important mechanism developed by some microorganism in order to directly incorporate nitrogen gas into proteins. This process is an important step in the early food chain.

There are five oxides of nitrogen known – N₂O, NO, N₂O₃, NO₂ and N₂O₅ (oxidation numbers ranging from +1 to +5, respectively). Nitric(III) acid (nitrous acid) HONO and nitric(V) acid (nitric acid) HNO₃ are the most important oxoacids of nitrogen. HNO₃ is a highly reactive oxidising and nitrating agent.

In general, phosphorus is more reactive than nitrogen. White phosphorus ignites spontaneously in air and forms phosphorus(V) oxide. Phosphoric acid (H₃PO₄) is the most important oxoacid of phosphorus and its main use is in the manufacture of fertilisers (Figure 6.4).

Hydrogen-containing compounds of nitrogen and phosphorus, namely NH₃ and PH₃, both act as a Lewis base because of their lone pair. Phosphine (PH₃) is less water soluble than NH₃ as it does not form hydrogen bonds. NH₃ (ammonia) is produced in the so-called Haber Bosch process. This industrial process uses finely divided iron as catalyst and a reaction temperature of around 450 ^∘C at a pressure of 50 atm. Ammonia is used to produce fertilisers, nitric acid, nylon and many more products important to our modern life style.

N₂(g) + 3H₂(g) → 2NH₃(g)

For clinical applications, nitrogen and phosphorus compounds are mostly used as heteroatoms in organic compounds or counter-ions in inorganic salts with no specific therapeutic effect. Arsenic differs because it exhibits its own typical therapeutic and toxic properties, which has resulted in its long-standing use in clinical applications and in the invention of chemotherapy. Therefore, the following clinical discussion will concentrate on arsenic-based drugs.