Hydrogen cyanide polymerization

Hydrogen cyanide, H-C=N , is a colorless, extremely toxic, volatile liquid, mp -13.4°C, bp -1-25.6 °C, with a high dielectric constant (107 at 25 °C). It functions as a weak acid Ka = 4.9 x 10 °). It is made by acidifying aqueous solutions of cyanides or industrially by the exothermic reaction of methane with ammonia in a fast flow/rapid quench system (equation 16). Hydrogen cyanide polymerizes readily nnder a variety of conditions hydrogen cyanide oligomers include the trimer aminomalononitrile and the tetramer diaminomaleonitrile (Figure 8). 

Properties Water-white liquid at temperatures below 26.5C faint odor of bitter almonds. Usual commercial material is 96-99% pure. D (Liquid) 0.688 (20/4C), (gas) 0.938 g/L, bp26.5C, fp-13.3C, flash p OF (-17.7C). Soluble in water. The solution is weakly acidic, sensitive to light. When not absolutely pure or stabilized, hydrogen cyanide polymerizes spontaneously with explosive violence. Miscible with water, alcohol, soluble in ether, autoign temp 1000F (537C). 

Ferris, J. P. and Orgel, L. (1965) Aminomalononitrile and 4-amino-5-cyanoimidazole in hydrogen cyanide polymerization and adenine synthesis. J. Am. Chem Soc. 87,4976 977. 

Compounds with active hydrogen add to the carbonyl group of acetone, often followed by the condensation of another molecule of the addend or loss of water. Hydrogen sulfide forms hexamethyl-l,3,5-trithiane probably through the transitory intermediate thioacetone which readily trimerizes. Hydrogen cyanide forms acetone cyanohydrin [75-86-5] (CH2)2C(OH)CN, which is further processed to methacrylates. Ammonia and hydrogen cyanide give (CH2)2C(NH2)CN [19355-69-2] ix.orn. 6<55i the widely used polymerization initiator, azobisisobutyronitrile [78-67-1] is made (4). 

Under certain conditions hydrogen cyanide can polymerize to black soHd compounds, eg, hydrogen cyanide homopolymer [26746-21-4] (1) and hydrogen cyanide tetramer [27027-02-2], C H N (2). There is usually an incubation period before rapid onset of polymer formation. Temperature has an inverse logarithmic effect on the incubation time. Acid stabilizers such as sulfuric and phosphoric acids prevent polymerization. The presence of water reduces the incubation period. 

Specifications are 99.5 wt % hydrogen cyanide (min), 0.5 wt % water (max), 0.06—0.10% acidity, and color not darker than APHA 20. A combination of H2SO4 (or H PO and SO2 acts as a stabilizer to prevent polymerization H2SO4 stabilizes the Hquid phase and SO2 stabilizes the vapor phase. 

Chemical Reactivity - Reactivity with Water No reaction Reactivity with Common Materials No reaction Stability During Transport Stable, in presence of moisture, toxic hydrogen cyanide gas may collect in enclosed spaces Neutralizing Agents for Acids and Caustics Not pertinent Polymerization Not pertinent Inhibitor cf Polymerization Not pertinent. 

Chemical Reactivity - Reactivity with Water. When potassiiun cyanide dissolves in water, a mild reaction occurs and poisonous hydrogen cyanide gas is released. The gas readily dissipates, however if it collects in a confined space, then workers may be exposed to toxic levels. If the water is acidic, toxic amounts of the gas will form instantly Reactivity with Common Materials Contact with even weak acids will result in the formation of deadly hydrogen cyanide gas Stability During Transport Stable Neutralizing Agents for Acids and Caustics Not pertinent Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Copper-catalyzed monoaddition of hydrogen cyanide to conjugated alkenes proceeded very conveniently with 1,3-butadiene, but not with its methyl-substituted derivatives. The most efficient catalytic system consisted of cupric bromide associated to trichloroacetic acid, in acetonitrile at 79 °C. Under these conditions, 1,3-butadiene was converted mainly to (Z )-l-cyano-2-butene, in 68% yield. A few percents of (Z)-l-cyano-2-butene and 3-cyano-1-butene (3% and 4%, respectively) were also observed. Polymerization of the olefinic products was almost absent. The very high regioselectivity in favor of 1,4-addition of hydrogen cyanide contrasted markedly with the very low regioselectivity of acetic acid addition (vide supra). Methyl substituents on 1,3-butadiene decreased significantly the efficiency of the reaction. With isoprene and piperylene, the mononitrile yields were reduced... 

Note Inhibited with 35-45 ppm hydroquinone monomethyl ether to prevent polymerization during storage and transport (Acros Organics, 2002). Commercial grades may contain the following impurities acetone and acetonitrile (300-500 ppm), acetaldehyde and propionaldehyde (300-500 ppm), acrolein, methanol, isopropanol and hydrogen cyanide (300-500 ppm) (NICNAS, 2000)... 

Table 10.5 gives the uses of acetone. A very important organic chemical that just missed the top 50 list, methyl methacrylate, is made from acetone, methanol, and hydrogen cyanide. Approximately 1.2 billion lb of this compound is manufactured and then polymerized to poly(methyl methacrylate), an important plastic known for its clarity and used as a glass substitute. The synthesis is outlined as follows. 

Esterification of Hexacyanoferric(II) Acid. When hexacyanoferric (II) acid is heated with ethyl alcohol, esterification of the acid takes place (15, 21). The initial partially esterified hexacyanoferric (II) acid polymerizes with the evolution of hydrogen cyanide or is further esterified. Both reactions appear to take place concurrently. Addition of hydrogen cyanide to the reaction mixture causes liberation of ethyl isonitrile from the complex. Hence it is possible to synthesize isonitriles on a continuous basis—i.e., esterification of the strong hexacyanoferric (II) acid, replacement of the isonitrile in the complex by hydrogen cyanide, re-esterification, etc. (15). The over-all reaction is complex, and the precise course of the reaction has not been elucidated. 

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