Halides and oxohalides of sulfur

Sulfur reacts directly with all halogens except for iodine.

Thus:

S₂F₂ (b.p. -38⁰)

SF₂ (-35⁰)

SF₄ (-40⁰)

SF₆ (-64⁰, sublimates)

S₂Cl₂ (13.7⁰)

SCl₂ (59⁰, decomposes)

SСl₄ (-30⁰, decomposes)

S₂Br₂ (54⁰ at 0.2 mm Hg)

In the series of compounds of sulfur with halogens from fluorine to bromine stability of these compounds sharply drops. The most stable halide of fluorine is SF₆, of chlorine is S₂Cl₂. The latter compound is decomposed by water:

2S₂Cl₂ + 2H₂O = SO₂ + 3S + 4HCl

Sulfur forms oxohalides:

SOCl₂ is thionyl chloride

SO₂Cl₂ is sulfuryl chloride

SOCl₂ is an extraordinarily active compound.

Preparation:

PCl₅ + SO₂ = SOCl₂ + POCl₃

It is hydrolysed readily:

SOCl₂ + H₂O = SO₂ + 2HCl

SO₂Cl₂ is an extremely active liquid.

Preparation:

SO₂ + Cl₂ = SO₂Cl₂

CuCl₂ + 2SO₃ = CuSO₄ + SO₂Cl₂

Hydrolysis of SO₂Cl₂ takes place:

SO₂Cl₂ + 2H₂O = H₂SO₄ + 2HCl

Therefore, SO₂Cl₂ is a mixed anhydride or halogenanhydride.

Oxygen compounds of Se(VI), Te(VI), and Po(VI). The most important of them are SeO₃ and TeO₃ oxides, relevant acids and their salts.

OXIDES. EO₃ are produced indirectly, whereas EO₂ are the products of direct synthesis from simple substances.

SeO₃ is separated during boiling of selenates with liquid SO₃:

K₂SeO₄ + SO₃ = K₂SO₄ + SeO₃,

or at dehydration H₂SeO₄ by P₂O₅.

Molecules of SeO₃ exist only in the gaseous state. Tetramers (SeO₃)₄ are condensed when cooling. The solid tetramer (SeO₃)₄ has two polymorphic modifications: vitreous and asbestos-like. It is difficult to obtain (SeO₃)₄ without impurities (i.e. to use recrystallisation) because it explodes while interacting with any solvent. SeO₃ is a crystalline substance (m.p. = 121 °C) under normal conditions, which decomposes at t > 180 °C as follows:

2SeO₃ = 2SeO₂ + O₂,

but sublimes in vacuum without decomposition. SeO₃ is an acid oxide (acid anhydride of selenic acid):

SeO₃ + H₂O = H₂SeO₄.

In concentrated solutions, it forms a mixture of polyselenic acids similar to polysulfuric acids (H₂Se₂O₇, m.p. = 19 °C, H₂Se₃O₁₀, m.p. = 39 °C). When diluting, polyselenic acids solutions are hydrolysed to final product H₂SeO₄.

TeO₃ is obtained by thermal dehydration of orthotelluric acid:

Н₆ТeO₆ ТeO₃ + 3Н₂O

TeO₃ forms two polymorphs in the solid state. The reaction above leads to the a-TeO₃, amorphous phase of yellow colour (density 5.1 g/cm³), which is low soluble in cold water (0.5 g /L), and is gradually transforming again into orthotelluric acid in hot water. The crystalline b- TeO₃ phase of gray colour (density 6.2 g/cm³), which no longer reacts with acids and alkali solutions even when heated, can be prepared at a long-term heating of a-TeO₃ in a sealed tube (300 °C).

In general, SeO₃ and TeO₃ oxides are soluble in alkalis forming the corresponding selenates and tellurates:

SeO₃ + NaOH = Na₂SeO₄

SeO₃ oxide’s oxidising ability is so high that oxidises HCl to Cl₂ even at cooling. TeO₃ oxidises HCl only when heated. Therefore, oxidising properties are weakened at the transition from SeO₃ to TeO₃.

ACIDS. Preparation. The highest acids of chalcogens are mostly produced by the action of strong oxidising agents:

H₂Se⁺⁴O₃ + H₂O^-1₂ = H₂Se⁺⁶O^-2₄ + H₂O^-2

Te + 3H₂O₂ = H₆TeO₆

Te + HClO₃ + 3H₂O = H₆TeO₆ + HCl.

H₂SeO₄ is also prepared by electrochemical oxidation of selenious acid.

H₂SeO₄. A convenient laboratory method of preparation.

H₂SeO₄ is obtained by processing Ag₂SeO₃ suspension with bromine water:

Ag₂SeO₃ + Br₂ + H₂O = 2AgBr¯ + H₂SeO₄

Selenic acid exists in the form of meta-acid, H₂SeO₄, telluric is the ortho-acid, H₆TeO₆. Both acids are colourless crystalline substances.

Structure. H₂SeO₄ (m.p. = 62.4 °C) can be mixed with water at any proportion; its concentrated solutions are viscous and similar to H₂SO₄. It has strong affinity to water and will remove 'combined' water in sugars and other organic compounds through the formation of solid hydrates H₂SeO₄∙H₂O (m.p. = 26 °C), H₂SeO₄∙2H₂O (m.p. = -24 °C), H₂SeO₄∙4H₂O (m.p. = -52 °C) like H₂SO₄. When heated over 260 °C, selenic acid is transformed into SeO₂:

2H₂SeO₄ = 2SeO₂ + O₂ + 2H₂O.

The H₂SeO₄ molecule structure (sp³ -hybridisation) and the acid strength are similar to H₂SO₄ (H₂SeO₄, K₂ = 1∙10^-2; H₂SO₄, K₂ = 1,2∙10^-2; both acids have K₁» 10³). The significant strength is due to the presence of two very electronegative atoms of oxygen in acid residues. They shift electron density causing the growth of H—O bonds polarity and their ability to dissociate. Moreover, EO₄^2- anions (the bond length Se—O is 0.161 nm), which appear after dissociation, are stabilised in a solution due to the negative charge uniform delocalisation between four oxygen atoms:

H₆TeO₆. Orthotelluric acid molecule has an octahedral structure (sp³d² -hybridisation of tellurium orbitals):

Unlike H₂SO₄ and H₂SeO₄, covalent bonds Te=O are absent in H₆TeO₆, which would be favourable to H—O bonds polarisation and facilitate their dissociation. Therefore, H₆TeO₆ is a very weak acid (K₁ = 2∙10^-8; K₂ = 9∙10^-12; K₃ = 3∙10^-15), which is well soluble in hot water, but its solubility is limited under normal conditions (25% at 20 °C).

If heated, H₆TeO₆ is transformed into metatelluric acid, H₂TeO₄. It is much stronger than ortho- H₆TeO₆ form and H₂TeO₄ gradually turns again into orthoacid in a solution. H₆TeO₆ is an illustration of the well-known rule: the more a p-element atom forming an acid the more the OH-groups number associated with it, i.e. the ability of more hydrated forms formation. Tellurium (VI), in addition to other p- elements of the fifth period, has the stable coordination number 6.

When neutralising H₆TeO₆ by alkalis, acid salts are formed. The most abundant among them are the low soluble Na₂H₄TeO₆ and well soluble K₂H₄TeO₆∙3H₂O. Orthotelluric acid H₆TeO₆ can be replaced by metals all six hydrogen atoms. For instance, Na₆TeO₆ neutral salt can be obtained at H₆TeO₆ fusion with NaOH. It gradually transforms into Na₂H₄TeO₆∙3H₂O in the humid atmosphere. There are also salts Ag₆TeO₆ and Hg₃TeO₆.

Properties. Selenic and telluric acids are powerful oxidising agents. In aqueous solutions, they are considerably stronger than H₂SO₄, as evidenced by E° comparison:

SeO₄^2-+ 4H⁺ + 2e^-= H₂SeO₃ + H₂O; E ° = 1,15 V

H₆TeO₆ + 2H⁺ + 2e^-= TeO₂ + 4H₂O; E ° = 1,02 V

SO₄^2-+ 4H⁺ + 2e^-= H₂SO₃ + H₂O; E ° = 0,17 V

Concentrated H₂SeO₄ and H₆TeO₆ unlike H₂SO₄ are able to oxidise not only I^‑ and Br^-, but also Cl^-, turning into the more stable oxidation state of these elements (+4):

2HCl + H₂SeO₄ = Cl₂ + H₂SeO₃ + H₂O.

Especially strong is the oxidising ability of H₂SeO₄. The hot anhydrous H₂SeO₄ dissolves well not only Ag (like H₂SO₄) but Au either:

2Au + 6H₂SeO₄ = Au₂(SeO₄)₃ + 3SeO₂ + 6H₂O.

Like aqua regia (HCl + HNO₃) the mixture of H₂SeO₄ + HCl oxidises even Pt:

Pt + 4HCl + 2H₂SeO₄ = PtCl₄ + 2SeO₂ + 4H₂O

Halides. They can be obtained by direct synthesis reaction from simple substances.

Composition, colour, state of halides of
Se	Te
SeF6 colourless gas	TeF6 colourless gas
SeF4 colourless	TeF4 colourless solid
SeCl4 colourless solid	TeCl4 colourless solid
SeCl2 liquid brown	TeCl2 solid green
SeBr4 solid yellow	TeBr4 solid orange
Se2Br2 liquid red	TeBr2 solid brown
	TeI4 gray-black

Molecules SeF₆ (m.p. = -46.6 °C) and TeF₆ (m.p. = -38.6 °), like SF₆, have octahedral structure, which shows sp³d²- hybridisation (Se—F and Te—F bond lengths are 0.170 nm and 0.184 nm, respectively). The most abundant are SeX₄ and TeX₄. By its nature, selenium halides are close to the corresponding derivatives of sulfur. For instance, Se₂Cl₂ and Se₂Br₂ are decomposed even at careful heating:

2Se₂Cl₂ = 3Se + Se₂Cl₄

Tellurium halides are significantly different from those of sulfur compounds. They have salt-like behaviour. Unlike hexafluorides of S and Se, TeF₆ hydrolyses by water easily:

TeF₆ + 6H₂O = H₆TeO₆ + 6HF (it is a halogenanhydride).

Thus, in the series S—Se—Te—Po nonmetallic properties are weakened, metallic ones are increased, and ionicity in molecules grow. The chemical nature of halides changes starting with covalent (Se halides) through ionic-covalent (saltlike TeX_n) to the ionic (salts PoX_n).

Дата добавления: 2014-01-11