Cyanohydrin Formation

The last example of reversible addition is that of hydrogen cyanide (HC≡N), which adds to aldehydes and many ketone to give products called cyanohydrins.

RCH=O   +   H–C≡N     RCH(OH)CN     (a cyanohydrin)

Since hydrogen cyanide itself is an acid (pK_a = 9.25), the addition is not acid-catalyzed. In fact, for best results cyanide anion, C≡N^(-) must be present, which means that catalytic base must be added. Cyanohydrin formation is weakly exothermic, and is favored for aldehydes, and unhindered cyclic and methyl ketones. Two examples of such reactions are shown below.

The cyanohydrin from benzaldehyde is named mandelonitrile. The reversibility of cyanohydrin formation is put to use by the millipede Apheloria corrugata in a remarkable defense mechanism. This arthropod releases mandelonitrile from an inner storage gland into an outer chamber, where it is enzymatically broken down into benzaldehyde and hydrogen cyanide before being sprayed at an enemy.