Cyanamide

Cyanamide

Cyanamide, m.p.42^oC, may be prepared by the action of ammonia on cyanogens chloride:

ClCN   +   NH₃    ⟶    NH₂CN   +   HCl

It is also readily prepared by the action of water and carbon dioxide on calcium cyanamide.

Cyanamide is converted into guanidine (q.v.) by ammonia and into thiourea (q.v.) by hydrogen sulphide. When cyanamide is melted it forms the dimer dicyandiamide, (NH₂)₂C=NCN, and the trimer melamine, which is a cyclic compounds. X-ray analysis has shown that all the C-N bond lengths are the same. This can be explained on the assumption that melamine is a resonance hybrid of the amino-form (cf. cyanuric acid).

Melamine is manufactured by heating urea and passing the gaseous products over a heated catalyst:

CO(NH₂)₂    (-NH₂) ⟶    HNCO    (6 mol) ⟶   C₃H₆N₆   +   3CO₂

Melamine is used for making melamine-formaldehyde plastics.

Cyanamide itself is a tautomeric compounds (amidine system), and Raman spectra investigations have shown that two forms are present in equilibrium in the solid or fused state, or in solution:

H₂N-C=N   ⥨   NH=C=NH

Cyanamide forms salts, the most important of which is the calcium salt. This is manufactured by heating calcium carbide mixed with 10 per cent it weight of calcium chloride in a stream of nitrogen at 800^oC:

CaC₂   +   N₂     ⟶     CaNCN   +   C

The calcium cyanamide-carbon mixture is used as a fertilizer; it is hydrolyzed in the soil to cyanamide, which is then hydrolysed to urea, which in turn, is converted into ammonium carbonate by bacteria in the soil:

CaNCN  (H₂O) ⟶  Ca(OH)₂  +  NH₂CN  (H₂O) ⟶  CO(NH₂)₂  (H₂O) ⟶  (NH₄)₂CO₃