Cyanide cyanohydrins

Aldehydes and ketones react with HCN to give 2-hydroxynitriles, compounds that are generally termed cyanohydrins. HCN is only a weak acid (pAfa 9.1), and proton availability is insufficient to initiate a typical acid-catalysed reaction via the conjugate 

The reaction is reversible, and cyanohydrin formation is more favoinable with aldehydes than with ketones, as with other addition reactions. The reverse reaction is easily effected by treating a cyanohydrin with aqueous base, since cyanide is a reasonable leaving group (see Section 6.1.4). 

Cyanohydrin formation is a useful synthetic reaction, in that it utilizes a simple reagent, cyanide, to create a new C-C bond. The cyano (nitrile) group may easily be modified to other functions, e.g. carboxylic acids via hydrolysis (see Box 7.9) or amines by reduction. 

The main cyanogenic glycoside in laurel is prunasin, the P-o-glucoside of benzaldehyde cyanohydrin. The enzymic hydrolysis of prunasin may be visualized as an acid-catalysed process, first of all hydrolysing the acetal linkage to produce glucose and the cyanohydrin. Further hydrolysis results in reversal of cyanohydrin formation, giving HCN and benzaldehyde. 

Bitter almonds contain amygdalin, which is the P-D-glucoside of prunasin, so it hydrolyses sequentially to the same products. Cassava, which is used in many parts of the world as a food plant, contains linamarin, which is the P-D-glucoside of acetone cyanohydrin. Preparation of the starchy tuberous roots of cassava for food involves prolonged hydrolysis and boiling to release and drive off the HCN before they are suitable for consumption. 

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Nitrile groups m cyanohydrins are hydrolyzed under conditions similar to those of alkyl cyanides Cyanohydrin formation followed by hydrolysis provides a route to the preparation of a hydroxy carboxylic acids... 

Addition of hydrogen cyanide to aldehyde and ketone forms cyanohydrin. The reaction is usually carried out using sodium or potassium cyanide with HCl. Hydrogen cyanide is a toxic volatile liquid, and a weak acid. Therefore, the best way to carry out this reaction is to generate it in situ by adding HCl to a mixture of aldehydes or ketones and excess sodium or potassium cyanide. Cyanohydrins are useful in organic reaction, because the cyano group can be converted easily to an amine, amide or carboxylic acid. 

Fig. 2.17 Galls growing on an oak leaf. By production of polyphenols the tree tries to cut off the parasite from trace metal supply yet the wasp larva is capable of degrading them, in effect making use of the complexes as a means of metal enrichment. Thus a process used by many kinds of plant (photosynthetic organs and seeds likewise, often based on cyanide/cyanohydrines or on alkaloids) is turned in favor of a parasite (an outcome of co-evolution). Southern Germany, November 2008 photo taken by the author...

The addition of hydrogen cyanide to carbonyl compounds gives a-hydroxy cyanides (cyanohydrin synthesis). The reaction is reversible, and the extent of the cyanohydrin formation depends upon the structure of the Carbonyl compound. The equilibrium highly favors the formation of aliphatic and alicyclic cyanohydrins however, aryl alkyl ketones react to a lesser extent, and diaryl ketones, not at all. The reaction may be accomplished by mixing the carbonyl compound with liquid hydrogen cyanide in the presence of a basic catalyst. The equilibrium... 

Cyano halides, see Halo cyanides Cyanohydrins, see Hydroxy cyanides Cyanohydrin synthesis, 414, 604 Cyano iniides, preparation, 434 Cyano ketones, see Keto cyanides a-Cyano lactones, preparation, 534 Cyclodehydration of ketones and alcohols, 15, 843... 

Nucleophilic additions of inorganic anions to carbonyl groups also proceed efficiently under PTC conditions. Syntheses of acyl azides, acyl cyanides, cyanohydrin derivatives, and the Reissert compounds belong to this category of reactions ... 

The hydrogen of benzaldehyde is not acidic, so it cannot attack a second molecule of the same. However, by addition of cyanide (cyanohydrin formation), the hydrogen atom becomes acidic (the carbanion will be resonance stabilized by the cyano function) and the reaction proceeds. 

Despite, or perhaps because of, the high toxicity of cyanide, cyanohydrins are widely distributed in nature. The cyanohydrin of benzaldehyde, PhCH(OH)CN, is used as a defense mechanism by the millipede Apheloria corrugata. When the millipede is attacked, it excretes the cyanohydrin, which is then enzymatically converted to benzaldehyde and HCN, the latter being toxic to the attacker. 

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