Properties

Cyanuric acid can be viewed as the cyclic trimer of the elusive chemical species cyanic acid, HOCN. The ring can readily interconvert between several structures via lactam-lactim tautomerism. Although the triol tautomer may have aromatic character, the keto form predominates in solution.^[3] The hydroxyl (-OH) groups assume phenolic character. Deprotonation with base affords a series of cyanurate salts:

[C(O)NH]₃ ⇌ [C(O)NH]₂[C(O)N]⁻ + H⁺ (pK_a = 6.88)^[4]

[C(O)NH]₂[C(O)N]⁻ ⇌ [C(O)NH][C(O)N]₂²⁻ + H⁺ (pK_a = 11.40)

[C(O)NH][C(O)N]₂²⁻ ⇌ [C(O)N]₃³⁻ + H⁺ (pK_a = 13.5)

Cyanuric acid is noted for its strong interaction with melamine, forming insoluble melamine cyanurate. This interaction locks the cyanuric acid into the tri-keto tautomer.

Synthesis

Cyanuric acid (CYA) was first synthesized by Friedrich Wöhler in 1829 by the thermal decomposition of urea and uric acid.^[5] The current industrial route to CYA entails the thermal decomposition of urea, with release of ammonia. The conversion commences at approximately 175 °C:^[2]

3 H₂N-CO-NH₂ → [C(O)NH]₃ + 3 NH₃

CYA crystallizes from water as the dihydrate.

Cyanuric acid can be produced by hydrolysis of crude or waste melamine followed by crystallization. Acid waste streams from plants producing these materials contain cyanuric acid and on occasion, dissolved amino-substituted triazines, namely, ammeline, ammelide, and melamine. In one method, an ammonium sulfate solution is heated to the "boil" and treated with a stoichiometric amount of melamine, by which means the cyanuric acid present precipitates as melamine-cyanuric acid complex. The various waste streams containing cyanuric acid and amino-substituted triazines may be combined for disposal, and during upset conditions undissolved cyanuric acid may be present in the waste streams. ^[6][7]

Intermediates and impurities

Intermediates in the dehydration include both isocyanic acid, biuret, and triuret:

H₂N-CO-NH₂ → HNCO + NH₃

H₂N-CO-NH₂ + HNCO → H₂N-CO-NH-CO-NH₂

H₂N-CO-NH-CO-NH₂ + HNCO → H₂N-CO-NH-CO-NH-CO-NH₂

As temperature exceeds 190 °C, other reactions begin to dominate the process.

The first appearance of ammeline occurs prior to 225 °C and is suspected also to occur from decomposition of biuret but is produced at a lower rate than that of CYA or ammelide.

3 H₂N-CO-NH-CO-NH₂ → [C(O)]₂(CNH₂)(NH)₂N + 2 NH₃ + H₂O

Melamine, [C(NH₂)N]₃, formation occurs between 325–350 °C and only in very small quantities.^[8]

N-substituted isocyanurates from isocyanates

N-substituted isocyanurates can be synthesised by the trimerisation of isocyanates. This is utilised industrially in the formation of polyisocyanurates.

Precursors to chlorinated cyanurates

Cyanuric acid is mainly used as a precursor to N-chlorinated cyanurates, which are used to disinfect water. The dichloro derivative is prepared by direct chlorination:

This species is typically converted to its sodium salt, sodium dichloro-s-triazinetrione. Further chlorination gives trichloroisocyanuric acid, [C(O)NCl]₃.^[2]

These N-chloro compounds serve as disinfectants and algicides for swimming pool water.^[2] The aforementioned equilibrium stabilizes the chlorine in the pool and prevents the chlorine from being quickly consumed by sunlight.^[9]

Precursors to crosslinking agents

Because of its trifunctionality, CYA is a precursor to crosslinking agents, especially for polyurethane resins and polyisocyanurate thermoset plastics.

The experimental antineoplastic drug teroxirone (triglycidyl isocyanurate) is formed by reacting cyanuric acid with 3 equivalents of epichlorohydrin. It works by cross-linking DNA.^[10]

2007 pet food recalls

Cyanuric acid is implicated in connection to the 2007 pet food recalls, the contamination and wide recall of many brands of cat and dog foods beginning in March 2007. Research has found evidence that cyanuric acid, a constituent of urine, together with melamine forms poorly soluble crystals which can cause kidney failure (see Analysis section above).