Concept 01 / 09The p-Block Elements
Phosphorus: Allotropes, Phosphine, Halides and Oxoacids
White, red and black phosphorus and their structures and reactivity, preparation and properties of phosphine, phosphorus trichloride and pentachloride (structure and hydrolysis), and the structures, basicity and reducing properties of phosphorus oxoacids.
Allotropes.
White phosphorus: discrete tetrahedral molecules with P-P-P angles of only , so it is highly strained and very reactive. Translucent waxy solid, poisonous, insoluble in water but soluble in ; glows in the dark (chemiluminescence) and catches fire in air at about , so it is stored under water.
Red phosphorus: made by heating white phosphorus at in an inert atmosphere; polymeric chains of linked tetrahedra. Much less reactive, non-poisonous, does not glow, insoluble in . Used in safety matches.
Black phosphorus: layered ( and forms), the most stable and least reactive, obtained by heating red phosphorus under pressure.
Reactivity: white > red > black.
Phosphine, . Preparation: ; or (disproportionation, in an inert atmosphere). Colourless, rotten-fish smell, highly poisonous. Weakly basic (). Impure phosphine containing ignites spontaneously; this is used in Holme's signals ( and ), and as smoke screens.
Phosphorus halides.
: from white phosphorus and dry chlorine, ; pyramidal (, one lone pair). Hydrolyses in moist air: . Converts alcohols and acids to chlorides.
: . Trigonal bipyramidal in the gas (); the two axial bonds () are longer than the three equatorial bonds () because they suffer more repulsion. In the solid it is ionic, . Partial hydrolysis gives : ; complete hydrolysis gives . On heating it dissociates: . It converts OH groups to Cl () and finely divided metals to chlorides.
Oxoacids (see figure). Phosphorus is tetrahedral in all of them, with one P=O. Only hydrogens attached to oxygen are acidic.
(hypophosphorous, P ): one P-OH and two P-H, so monobasic; strongly reducing.
(phosphorous, P ): two P-OH and one P-H, so dibasic; reducing; disproportionates on heating: .
(orthophosphoric, P ): three P-OH, tribasic; not reducing.
(pyrophosphoric): tetrabasic, with a P-O-P linkage. (metaphosphoric): cyclic or chain.
Acids with P-H bonds reduce to Ag: .
Key Takeaways:
- White P (, angles) is the most reactive; black P is the most stable.
- : trigonal bipyramidal gas (axial bonds longer), ionic solid .
- Basicity of P oxoacids number of P-OH groups: , , .
- P-H bonds make and reducing agents.
- Pitfall: the number of H atoms in the formula is not the basicity.
Brain-Teaser 1: How many moles of NaOH are needed to neutralise one mole each of , and ?
Solution: , and respectively, equal to the number of P-OH groups.
Brain-Teaser 2: Why is white phosphorus more reactive than red phosphorus?
Solution: White phosphorus consists of discrete tetrahedra with bond angles, far below the normal -orbital angle, so the bonds are highly strained and easily broken. In red phosphorus, one P-P bond of each tetrahedron is opened to link units into chains, relieving strain and giving a polymeric, less reactive solid.
Brain-Teaser 3: What is formed when is heated in a sealed tube, and why are its axial bonds longer than its equatorial bonds?
Solution: It dissociates reversibly into and (the weaker axial bonds break). Each axial bond pair makes three contacts with the equatorial pairs, whereas each equatorial pair makes only two contacts (plus ones), so axial bonds experience more repulsion and lengthen.
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