Cyanic acid

Cyanic acid

Cyanic acid, HNCO. Urea, on dry distillation, gives cyanuric acid:

3CO(NH₂)₂    ⟶    H₃N₃C₃O₃   +   3NH₃

Cyanuric acid is a colorless, crystalline solid, not very soluble in water, and is strongly acid, reacting as a mono-, di- and tribasic acid. It has a cyclic structure (a triazine derivative) and X-ray analysis indicates that the acid is best represented as a resonance hybrid.

When cyanuric acid is heated, it does not melt but decomposes into cyanic acid vapour which, when condensed below 0^oC, gives a colorless condensate:

H₃N₃C₃O₃    ⟶    3HNCO

Cyanic acid is a colorless, volatile, strongly acid liquid which above 0^oC, readily polymerises to cyanuric acid and cyamelide, (HNCO)_n. Aqueus solutions of cyanic acid rapidly hydrolyse to give carbon dioxide and ammonia:

HNCO   +   H₂O     ⟶    CO₂   +   NH₃

There are two possible structures for cyanic acid, H-O-C=N and H-N=C=O. There is very little evidence for the existence of the former (cyanic acid), e.g., infra-red absorption spectra indicate the litter (isocyanic acid).

When a solution of potassium isocyanate (usually called potassium cyanate) and ammonium sulphate is evaporated to dryness, urea and potassium sulphate are obtained, the urea being formed by the molecular rearrangement of the intermediate product ammonium isocyanate:

NH₄NCO     ⟶     CO(NH₂)₂

Cyanic acid reacts with alcohols toform urethans; excess fo cyanic acid converts urethans into allophanates, which are esters of the unknown acid, allophonic acid, NH₂CONHCO₂H.

ROH   +   HNCO    ⟶    NH₂CO₂R    (HNCO) ⟶   NH₂CONHCO₂R

N-substituted urethans are formed bythe reaction between alkyl or aryl isocyanates and an alcohol. Phenyl isocyanate is also used to characterize primary and secondary amines, with which it forms substituted ureas.

Phenyl isocyanate is usually prepared by the action of carbonyl chloride on aniline:

C₆H₅NH₂  +  COCl₂   ⟶   HCl  +  C₂H₅NHCOCl   (Heat) ⟶   HCl  +  C₆H₅NCO

This is a general method for preparing isocyanates; another general method is the Curtius reaction.

Di-isocyanates, e.g., hexamethylene di-isocyanate, OCN(CH₂)₆NCO are used to prepare polyurethane plastics by reaction with di- and polyhydroxyalcohols, e.g., tetramethylene glycol:

HO(CH₂)₄OH  +  OCN(CH₂)₆NCO  +  HO(CH₂)₄OH  +  …. ⟶  

                                                HO(CH₂)₄OCONH(CH₂)₆NHCOO(CH₂)₄O-….