There are three allotropic forms for phosphorus - white, red, and black. White phosphorus is by far the most reactive and should not be exposed to air. It is also soluble in hydrocarbons such as benzene. So it is usually kept in water. The red and black phosphorus are stable in air.
The metallic arsenic, antimony and bismuth have crystal structures similar to black phosphorus. They burn in air to give the oxides. They also react readily with halogens and oxidising agents, but not with dilute mineral acids.
Phosphorus like nitrogen is essentially covalent in its chemistry. They follow the trend shown in Group IVB;
| NITROGEN | PHOSPHORUS |
| Very strong pπ-pπ bonds | No pπ-pπ bond |
| No pπ-dπ bond | important dπ-pπ bonds |
(We will deal with dπ-pπ bonding when we discuss the transition elements)
Arsenic, antimony and bismuth show increasing tendencies to cationic behaviour. Of course it is difficult for the elements to give away all their valence electrons, so show multiple oxidation states, with (+III) and (+V) being the most popular.
HYDRIDES
All the elements form gaseous hydrides (MH3), however SbH3 and BiH3 are unstable. It is important to remember that phosphine (PH3) and arsine (AsH3) are extremely poisonous.
The hydrides can react with silver and copper solutions to give the phosphides, arsenides and stibnides.
Phosphorus alone will form the diphosphine, P2H4. It differs from hydrazine (N2H4) in having no basic properties.
HALIDES
(The fluorides are exceptions).
The halides can be prepared by halogenation, that is reacting the metal with the halogen.
The trichlorides react violently with water to give a mixture of acids. For example, phosphorus trichloride will react with water to give the hydrochloric acid and phosphorus acid.
In the case of phosphorus when the trihalide is reacted further with the halogen the pentahalide, MX5, is obtained.
Beside the phosphorus pentahalide, PX5, only antimony pentachloride, SbCl5, is known.
In aqueous solution the larger elements can form complex halide ions. For example, [SbCl6]‾�.
OXIDES
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The best-known oxide is phosphorus pentoxide. It is a white crystalline material which sublimes at 360�C. Although the correct formula is P4O10 the empirical formula normally used is P2O5, since for all intent and purposes it does represent the chemistry correctly. This is because like most of the phosphorus compounds the basic unit tends to link to each other forming dimers, trimers, etc. So P4O10 is actually (P2O5)2.
Phosphorus pentoxide is a very effective drying agent. Its strong affinity for water allows it to even react with hydrogen sulphate to release the sulphur trioxide,
And with hydrogen nitrate to release a mixture of nitric oxide and nitrogen dioxide,
Of course it would react with water to form orthophosphoric acid, OP(OH)3.
The orthophosphoric acid is more commonly known as phosphoric acid and the molecule formula written as H3PO4.
It is also to be expected that P2O5 will also react with organic compound having hydroxyl group, -OH.
The products are written for simplicity, actually they are a mixture of various combinations of OH and OR groups, like OP(OH)(OR)2 and so on.
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Among the other lesser-known oxides, phosphorus trioxide (P2O3) is the most important. Of course the actual molecular formula is P4O6. This is prepared by burning phosphorus in a limited amount of oxygen. It is a volatile liquid and is stable at ambient condition. It can dissolve in water to give phosphorous acid, H2[HPO3].
The chemistry of arsenic trioxide and pentoxide are similar to that of phosphorus in most aspects. Antimony trioxide is insoluble in water. Antimony pentoxide decomposes readily to the trioxide.
The molecular formula for bismuth trioxide is well defined, Bi2O3. It is a yellow powder soluble in acids but insoluble in alkalies. The pentoxide is too unstable to be prepared.
All the oxides are prepared by burning the elements in oxygen. When the amount of oxygen is limited the lower oxides are obtained. The oxide clearly show two trends.
- The stability of higher oxidation state decreases with increasing atomic number (that is down the Group).
- The basicity of the oxides increases with increasing atomic number.
OXO ACIDS
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Phosphoric acid is commercially prepared by dissolving the pentoxide in water; or reacting phosphate rocks with sulphuric acid. The most common commercial grade is an 85% sirupy liquid. The pure form is a stable colourless crystalline solid (m.pt.= 42.3�C). At high temperature it behaves as an oxidising agent.
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H2(HPO3) |
H(H2PO2) |
The other acids are phosphorous acid, OPH(OH)2 commonly represented by H2[HPO3], and hypophosphorus acid, H[H2PO3]. By now you might have noticed that the protons outside the bracket are the replaceable protons in the acids.
The name of the salts where phosphorus is the anion is based on the oxidation state of the phosphorus. P(+V) is known as phosphates and P(+III) as phosphites. To compute the oxidation state of the phosphorus, oxygen is given the oxidation state of (-2, like in O‾�) and proton is (+1 like in H+).
I will work out the oxidation state for phosphorus for three ions.
| 1) [PO4]‾� | P + 4 x (-2) | = | -3 |
| P | = | +5 | |
So [PO4]‾� is a phosphate ion. | |||
| 2) [PF6]‾ | P + 6 x (-1) | = | -1 |
| P | = | +5 | |
So [PF6]‾ is a hexafluorophosphate ion. | |||
| 3) [HPO3]‾� | P + 3 x (-2) + (+1) | = | -2 |
| P | = | +3 | |
So [HPO3]‾� is a phosphite ion. | |||
Phosphate occur extensively in nature. Ammonium phosphate is an important fertiliser. Our teeth contain hexafluorophosphate ion. Many aspects of life is controlled by phosphates. Sugar phosphates are responsible for photosynthesis, and adenosine triphosphate supplies energy to our body.
No M(+III) acids are known for arsenic, antimony and bismuth, although M(+III) salts are known. As(+III) ions are known as arsenites and Sb(+III) ions as antimonites.
Arsenic acid, H3AsO4, and antimonic acid, H3SbO4, are known. The As(+V) ions are known as arsenates and the Sb(+V) ions as antimonates.
Bismuthic hydroxide, Bi(OH)3 is alkaline and Bi(+V) ions are known as bismuthates. Bismuthates are strong oxidising agents in acid.
ORGANOMETALLIC COMPOUNDS
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Tripheylphosphine reacts easily with primary and secondary organic halides to give a phosphonium salt. Since this is a SN2 reaction it works best with primary and secondary alkyl halide.
The α−proton to the phosphorus is weakly acidic and can be removed by a base, like
n-butyllithium, to give a phosphorane (sometimes also known as ylide).
