Editing Hydrogen cyanide (section)

==Chemical properties==
Hydrogen cyanide is weakly [[acidic]] with a [[Acid dissociation constant|p''K''<sub>a</sub>]] of 9.2. It partially [[ionization|ionizes]] in [[Aqueous solution|water]] to give the [[cyanide]] anion, {{chem2|CN−}}.  HCN forms hydrogen bonds with its conjugate base, species such as {{chem2|(CN-)(HCN)_{n} }}.<ref>{{cite journal |doi=10.1002/anie.201915206 |title=Salts of HCN-Cyanide Aggregates: &#91;CN(HCN)<sub>2</sub>&#93;<sup>−</sup> and &#91;CN(HCN)<sub>3</sub>&#93;<sup>−</sup> |date=2020 |last1=Bläsing |first1=Kevin |last2=Harloff |first2=Jörg |last3=Schulz |first3=Axel |last4=Stoffers |first4=Alrik |last5=Stoer |first5=Philip |last6=Villinger |first6=Alexander |journal=Angewandte Chemie International Edition |volume=59 |issue=26 |pages=10508–10513 |pmid=32027458 |pmc=7317722 }}</ref>

Hydrogen cyanide reacts with [[alkene]]s to give nitriles.  The conversion, which is called [[hydrocyanation]], employs nickel complexes as catalysts.<ref>{{cite book | last=Leeuwen | first=P. W. N. M. van | title=Homogeneous Catalysis: Understanding the Art | publisher=Kluwer Academic Publishers | publication-place=Dordrecht | date=2004 | isbn=1402019998 | oclc=54966334}}</ref>
:{{chem2|RCH\dCH2 + HCN → RCH2\sCH2CN}}

Four molecules of HCN will tetramerize into [[diaminomaleonitrile]].<ref>{{cite journal |first1=J. P. |last1=Ferris|first2=R. A.|last2=Sanchez|title=Diaminomaleonitrile (Hydrogen Cyanide Tetramer) |journal=Organic Syntheses |date=1968 |volume=48 |page=60 |doi=10.15227/orgsyn.048.0060 }}</ref>

[[Metal cyanide]]s are typically prepared by [[salt metathesis]] from alkali metal cyanide salts, but [[mercuric cyanide]] is formed from aqueous hydrogen cyanide:<ref>{{cite book|author1=F. Wagenknecht|author2=R. Juza|chapter=Mercury (II) cyanide|title=Handbook of Preparative Inorganic Chemistry|edition=2nd |editor=G. Brauer|publisher=Academic Press|year=1963|place=NY, NY|volume=2}}</ref>
:{{chem2|HgO  +  2 HCN  ->  Hg(CN)2  +  H2O}}