Compounds with halogens, sulfur, and nitrogen Carbon

Compounds WITH SULFUR

Carbon disulfide is industrially important compound.

Preparation. In industry: 1. passing the vapor of sulfur through a layer of incandescent coal:

С + 2S СS₂ Н₂₉₈ = 87,9 kJ/mol

2. from CH₄ and vapors of S at 500 C with the use of silicagel as a catalyst:

CH₄ + 4S CS₂ + 2H₂S

Properties. CS₂ is a colourless liquid, in a pure state it smells pleasantly, but admixtures give it very unpleasant smell. As well as for CO₂, its molecule is linear and nonpolar. Dispersion of intermolecular forces at it is small and CS₂ boils at +46,2. It burns with light blue flame:

CS₂ + 3O₂ = CO₂ + 2SO₂

Interestingly, that the temperature of its flame is so low, that it does not even char a paper.

Higher of 150 it is decomposed by water:

CS₂ + 2H₂O_(steam) = CO₂ + 2H₂S

KMnO₄ oxidizes it with elimination of sulfur:

4KMnO₄ + 5CS₂ + 6H₂SO₄ = 10S + 4MnSO₄ + 5CO₂ + 2K₂SO₄ + 6H₂O

CS₂ interacts with soluble sulfides, forming thiocarbonates:

CS₂ + Na₂S = Na₂CS₃

Thiocarbonates of alkaline metals are stable enough. At action of strong acids on concentrated solutions of thiosalts it is possible to prepare the existing in the free state thiocarbonic acid H₂CS₃. It is a yellow oily liquid considerably stronger then carbonic acid (K₁ = 2•10^-3). It is slowly decomposed by water onto SO₂ and H₂S.

CS₂ is also used for preparation of carbon tetrachloride CCl₄:

CS₂ + 2Cl₂ = CCl₄ + 2S

Compounds of carbon with halogens

Tetrahalogenides CHal₄ are known with all halogens, among them most important is CCl₄.

Carbon tetrachloride, CCl₄. It is colourless uncombustible liquid. B.p. = 77. In water it is practically insoluble and water does not decompose it. It does not interact neither with bases, nor with acids.

CCl₄ is used as a very good solvent of organic substances - fats, oils, resins. Its uncombustibility is sometimes used for extinguishing of fires.

From other compounds of carbon with halogens important is CF₄ (gas, b.p. = –128), and CF₂Cl₂ - freons. They have a considerable heat of evaporation at low b.p. and are used as the working substances in refrigeration machines. They are used also in production of dezodorants.

Freons are very stable and heavily decomposed. They are oxidized only by ozone and one of theories of thinning of ozone layer of Earth links ozone depletion with occurence of freons which lately gets much into the atmosphere.

Compounds of carbon with nitrogen

C with N₂ forms various compounds.

Dicyan, C₂N₂. This is the simplest compound forming from elements at the voltaic arc conditions.

Preparation. As its G = +309,2 kJ/mol, direct preparation is impossible, therefore it is prepared by the decomposition of some compounds:

2AgCN 2Ag + C₂N₂

2CuSO₄ + 4KCN = 2CuCN + C₂N₂ + 2K₂SO₄

P

roperties. C₂N₂ is gas, b.p. = –21C, m.p. = –28C. It is very poisonous. It ignites easily. Its mixture with oxygen gives temperature of burning 4500C.

A molecule of C₂N₂ is extraordinarily stable, it is determined by the strength of bonds in it. It is linear, the central atoms of carbon are found in the state of sp-hybridization

and form three bonds: one with an atom C and two with an atom of N. Thus at all atoms remains by one p-electrons, which form delocalized -bonds.

At 500S it polymerizes in umber insoluble mass - paracyane.

C₂N₂ is reactive very much. By the chemical conduct and compositions of substances formed it reminds halogens, therefore they are attributed to so called pseudohalogens

H₂ + C₂N₂ = 2HCN, (like H₂ + Cl₂ = 2HCl)

C₂N₂ + KOH = KCN + KCNO + H₂O, (like Cl₂ + KOH)

hydrogen CYANIDE, HCN is a colourless volatile liquid, b.p.= 26C. It has a smell of bitter almond. A molecule of HCN is polar ( = 0,96•10^-29 C^.m). Interfuses with water in any relations. Its water solution is weak acid, C = 2,1•10^-9. In a pure state it is strongly associated via hydrogen bonds between molecules.

I

t is one of the strongest poisons. MAC in air is equal 0,0003 mg/l.

Preparation. In industry:

CO + NH₃ HCN + H₂O

2CH₄ + 3O₂ + 2NH₃ 2HCN + 6H₂O

Structure. HCN consists of molecules of two types which are found in a tautomeric equilibrium:

In the normal form the atom of H is bound to the tetravalent atom of carbon, and in isoform H is bounded to N, and C has covalency = 3.

At boiling point an equilibrium is displaced to the left (99,5% norm. forms). The toxicity is related to iso-form.

Properties. Water-soluble cyanides in solution are strongly hydrolysed:

CN^- + H₂O  HCN + OH^‑

Іон CN^- has structure, isoelectronic to CO. It forms the enormous number of very stable complexes with the ions of d-metals, for example, K₃[Cr(CN)₆], K₄[Fe(CN)₆], Na[Au(CN)₂], Na₂[Zn(CN)₄] and many others.

This ligand often stabilizes the unusual for a metal oxidation states in complexes: K₆[Cr(CN)₆] - zero; K₅[Mn(CN)₆] - one; K₄[Cr(CN)₆] - two and others like that.

Ion CN- has reductive propertiess. On air it eventually passes into cyanate:

2KCN + O₂ = 2KCNO

This salt corresponds to cyanic acid, HCNO, the chemical structure of which may be expressed by three structures:

First two forms are bound by tautomeric transition. At ordinary conditions an equilibrium is displaced to the left. The third form is fulminic acid ,it includes three-covalent C and four covalent N. Extremely unstable it exists in solutions only. Salts of fulminic acid (fulminates) AgONC and Hg(ONC)₂ very easy decompose with an explosion.

In the diluted solution cyanogen acid is quickly decomposed:

HNCO + H₂O = CO₂ + NH₃

with the following formation of urea (carbamide):

NH₃ + HNCO = CO(NH₂)₂

Ammonium cyanate NH₄NCO played an exceptional role in development of chemistry, as it was the initial inorganic matter for the synthesis of urea which is produced by living organisms. A bridge between the inorganic and organic matters was so laid.

The cyanides of alkaline metals can be oxidized by S into thiocyanates (rhodanides) - salts of thiocyanic (rhodanic) acid:

KCN + S KCNS

Waterless HCNS (hydrogen thiocyanide ) is prepared at heating of thiocyanates of lead or mercury in a stream of H₂S:

Pb(SCN)₂ + H₂S = 2HNCS + PbS

At ordinary terms HSCN - gas with a strong smell. At temperature lower than -90^оC it turns into a liquid. In vapor it is monomolecular, in a liquid and solid state - polymer. It is well dissolved in water and forms strong acid (K = 0,14), therefore the soluble thiocyanates do not hydrolyse

As well as HCN, hydrogen thiocyanate exists in two tautomeric forms:

Covalence of atoms in these structures are identical. The order of atoms differs only. Derivative, which answer to the first structure has name thiocyanates, and the second - isothiocyanates.

Like cyanide ion, the thiocyanate ion also is a ligand, which forms numerous complex compounds, for example, K₃[Cr(NCS)₆], K₃[Fe(NCS)₆], K₂[Hg(SCN)₄] and much other. During co-ordination it behaves as ambidentate ligand, that can be coordinated either through nitrogen, or through sulfur.

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