Hydrogen cyanide acid-dissociation constant

Hydrogen cyanide is a weak acid with a dissociation constant of 4.8 x 10 ° (pKa = 9.32) at 25°C. At pH values lower than 7, most of the cyanide is present as hydrogen cyanide acid gas, dissolved in the solution. It evaporates easily from these solutions as a high toxic gas. At pH values higher than 11, cyanide is completely dissociated into its ions and at a pH of 9.32, 50% of the hydrocyanic acid is in the form of free cyanide (CN ). 

The physical properties of cyanoacetic acid [372-09-8] and two of its ester derivatives are Hsted ia Table 11 (82). The parent acid is a strong organic acid with a dissociation constant at 25°C of 3.36 x 10. It is prepared by the reaction of chloroacetic acid with sodium cyanide. It is hygroscopic and highly soluble ia alcohols and diethyl ether but iasoluble ia both aromatic and aUphatic hydrocarbons. It undergoes typical nitrile and acid reactions but the presence of the nitrile and the carboxyUc acid on the same carbon cause the hydrogens on C-2 to be readily replaced. The resulting malonic acid derivative decarboxylates to a substituted acrylonitrile ... 

Hydrogen cyanide, mp —13.3° bp 25.7°, is an extremely poisonous compound of very high dielectric constant (p. 55). It is miscible with H2O, EtOH and Et20. In aqueous solution it is an even weaker acid than HE, the dissociation constant Ka being 7.2 x 10 ° at 25°C. It was formerly produced industrially by acidifying NaCN or Ca(CN)2 but the most modem catalytic processes are based on direct reaction between... 

Attention may be called, in conclusion, to the observation of Brunei 55 that cyclohexene oxide fails to condense with hydrogen cyanide ThiB is again compatible with the weakly nuoleopliilii-obaracter of the ON ion and the low dissociation constant of its conjugate acid. 

Write the equilibrium-constant expressions and obtain numerical values for each constant in (a) the basic dissociation of ethylamine, C2H5NH2. (b) the acidic dissociation of hydrogen cyanide, HCN. 

Hydrogen cyanide. HCN, like the hydrogen halides, is a covalent, molecular substance, but capable of dissociation in aqueous solution. It is an extremely poisonous (though less so than H2S), colorless gas and is evolved when cyanides are treated with acids. It condenses at 25.6° to a liquid with a very high dielectric constant (107 at 25°). Here, as in similar cases, such as... 

Hydrogen cyanide is a very weak acid in aqueous solution with a dissociation constant of approximately 10 . In neutral and acid media non-dissociated HCN predominates, so that hydrogen cyanide can be eliminated from simple cyanide solutions by such weak acids as H2CO3 or H3BO3. If non-dissociated HCN predominates it can be removed from the solution by simple aeration. 

Hydrogen cyanide exhibits physical and chemical properties that typify both inorganic cyanide and nitrile. It is a very weak acid the magnitude of dissociation constant is on the same order as that of amino acids. It is also a nitrile of formic acid and exhibits many reactions similar to nitriles. However, becanse its toxicology is similar to that of alkali cyanides, HCN is discussed in this chapter. 

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