14.12A: Cyanogen and its Derivatives

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Compounds with C-N Bonds: Cyanides and Related Compounds

Cyanogen (NºC–CºN) is a poisonous and flammable gas (bp -21 ^oC. Although its heat of formation is strongly endothermic 297 kJ mol^-1 is is fairly stable. Impure cyanogen polymerizes to form "paracyanogen". Cyanogen is prepared by nitrogen dioxide catalyzed oxidation of hydrogen cyanide by oxygen:

\[\ce{2HCN + NO2 -> (CN)2 + NO + H2O}\]

\[\ce{NO + ½O2 -> NO2}\]

It is also formed by oxidation od CN^- with Cu²⁺:

\[\ce{Cu^{2+} + 2CN^{-} -> CuCN + ½(CN)2}\]

Notice the similarity between the above reaction and the one below:

\[\ce{Cu^{2+} + 2I^{-} -> CuI + ½(I)2}\]

The term "pseudo halogen/halide" is often applied to molecules and derived ions such as (CN)₂ and CN^-. Notice also the existence of HCN which has its parallel in the hydrohalic acids and the reaction of cyanogen with base:

\[\ce{(CN)2 + 2OH^{-} -> CN^{-} + OCN^{-} + H2O}\]

Compare:

\[\ce{(Cl)2 + 2OH^{-} -> Cl^{-} + OCl^{-} + H2O}\]

The reaction of cyanogen with oxygen produces one of the hottest flames known at about 5000 ^oC.

Hydrogen cyanide boils at 25.6 ^oC. It is very poisonous and has an odor of almonds which not everyone can smell. It is a very good solvent due to its high dielectric constant, e = 107. It is made on an industrial scale (~300 000 tons in 1980) as follows:

\[\ce{CH4 + 3O2 + 2NH3 -> 2HCN + 6H2O}\]

with Pt/Rh or Pt/Ir catalyst and 800 oC.

\[\ce{CH4 + NH3 -> HCN + 3H2}\]

with Pt catalyst and 1200 oC

Cyanides are made industrially via the calcium cyanamide salt by the processes:

\[\ce{CaC2 + N2 ->[1100 degrees] CaNCN + C}\]

\[\ce{CaNCN + C + Na2CO3 -> CaCO3 + 2NaCN}\]

\[\ce{NaNH2 + C -> NaCN (500-600 oC)}\]

(NCN²^- which yields cyanamide itself, H₂NCN, by hydrolysis of the salt, is isoelectronic with CO₂.)

Cyanide is important, among other things, as a very good p-acceptor ligand like CO.