Cyanogen and its derivatives

The CN^- radical is a pseudo-halogen, i.e. its chemistry resembles that of a halogen atom, X; it forms C₂N₂, HCN and [CN]^-, analogues of X₂, HX and X^-. Although C₂N₂ and HCN are thermodynamically unstable with respect to decomposition into their elements, hydrolysis by H₂O, and oxidation by O₂, they and [CN]^- are kinetically stable enough for them to be well-established and much studied  species.  Cyanogen, C₂N₂, is a toxic, extremely flammable gas (mp 245 K, bp 252 K) which is liable to react explosively with some powerful oxidants. Although Δ_fH^o(C₂N₂, 298K) = ‏297 kJ mol^-1, pure C₂N₂ can be stored for long periods without decomposition. the tow reactions that showen in below equation give two syntheses of C₂N₂;  below reaction illustrates the pseudo-halide like nature of [CN]^- which is oxidized by Cu(II) in an analogous fashion to the oxidation of I^- to I₂. Cyanogen is manufactured by air-oxidation of HCN over a silver catalyst.

Cyanogen has the linear structure  and the short C^_C distance indicates considerable electron delocalization. It burns in air with a very hot, violet flame and resembles the halogens in that it is hydrolyzed by alkali and undergoes thermal dissociation to CN^- at high temperatures.

    Hydrogen cyanide, HCN is an extremely toxic and flammable, colourless volatile liquid (mp 260 K, bp 299 K) with a high dielectric constant due to strong hydrogen bonding; it has a characteristic smell of bitter almonds. The pure liquid polymerizes to HC(NH₂)(CN)₂ and (H₂N)(NC)C=C(CN)(NH₂) mixed with higher molecular mass polymers, and in the absence of a stabilizer such as H₃PO₄, polymerization may be explosive. In the presence of traces of H₂O and NH₃, HCN forms adenine and on reduction, gives MeNH₂. It is thought that HCN was one of the small molecules in the early atmosphere of the Earth, and played an important role in the formation of many biologically important compounds. Hydrogen cyanide is prepared on a small scale by adding acid to NaCN, and industrially.

Many organic syntheses involve HCN, and it is of great industrial importance, a large fraction going into the production of 1,4-dicyanobutane (adiponitrile) for nylon manufacture, and cyanoethene (acrylonitrile) for production of acrylic fibres.   In aqueous solution, HCN behaves as a weak acid (pK_a = 9.31) and is slowly hydrolysed. An older name for hydrocyanic acid is prussic acid.

The neutralization of aqueous HCN by Na₂CO₃, NaHCO₃ or Na[HCO₂] generates NaCN, the most important salt of the acid. It is manufactured by reaction as in above and it has widespread uses in organic chemistry (e.g. for the formation of C^_C bonds); it is also used in the extraction of Ag and Au.

At 298 K, NaCN and KCN adopt the NaCl lattice, each [CN]^- ion freely rotating (or having random orientations) about a fixed point in the lattice and having an effective ionic radius of ≈190 pm. At lower temperatures, transitions to structures of lower symmetry occur, e.g. NaCN undergoes a cubic to hexagonal transition below 283 K. Crystals of NaCN and KCN are deliquescent, and both salts are soluble in water and are highly toxic. Fusion of KCN and sulfur gives potassium thiocyanate, KSCN. Mild oxidizing agents convert [CN]^- to cyanogen but with more powerful oxidants such as PbO or neutral [MnO₄]^-, cyanate ion is formed. Potassium cyanate reverts to the cyanide on heating .

Two acids can be derived from  HOCN (cyanic acid or hydrogen cyanate) and HNCO (isocyanic acid). It has been established that HOCN and HNCO are not in equilibrium with each other. Isocyanic acid (pK_a = 3.66) is obtained by heating urea  but rapidly trimerizes, although heating the trimer regenerates the monomer.

The fulminate ion, [CNO] ^-, is an isomer of the cyanate ion. Fulminate salts can be reduced to cyanides but cannot be prepared by oxidation of them. The free acid readily polymerizes but is stable for short periods in Et₂O at low temperature. Metal fulminates are highly explosive; mercury(II) fulminate may be prepared and is a dangerous detonator.

Cyanogen chloride (mp 266 K, bp 286 K), is prepared by the reaction of Cl₂ with NaCN or HCN, and readily trimerizes  which has applications in the manufacture of dyestuffs and herbicides.

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