Cyanohydrins dissociation

Cyanogenic compounds are not limited to plants. The defense secretion of many species of millipedes contains the products of cyanohydrin dissociation. These millipedes (Figure 17.7) store either benzaldehyde cyanohydrin or a derivative of it, and the enzyme that catalyzes its hydrolysis in separate chambers within their bodies. When the millipede is under stress, the contents of the two chambers are mixed and the hydrolysis products— including HCN—are released through the millipede s pores to deter predatory insects and birds. 

Equilibrium constants for the dissociation of cyanohydrins according to the equation... 

The equilibrium constant for the dissociation of the cyanohydrin of 5-acetyl-2,3-dihydrobenzo[6]thiophene has been determined.435... 

In the first step, cyanogenic glucosides are cleaved by p-glucosidases. The resulting a-hydroxynitriles are unstable and dissociate to produce a carbonyl compound and HCN. In many plants, the dissociation of the cyanohydrins is accelerated by an HNL. In plants that accumulate diglucosidic compounds, the hydrolysis of these substances can be achieved by either a sequential or a simultaneous mechanism (Kuroki et al, 1984) (Fig. 3.5). 

Studies in the field of tetracyclic and pentacycMc triterpenes [gsa] show that the dissociation constants for C 3)-cyanohydrins vary with structural changes even in rings C and D. A comparison with kinetic data for formation of 2-benzylidene derivatives of the same ketones cf. p. 178) indicates that the effect is largely one of conformational transmission. 

The stability of cyanohydrins, expressed by the equilibrium constants iCn for their dissociation (Table 16) [gi,g2], covers a wide range according to the type of ketone. The range is... 

The anions of protected cyanohydrins derived from saturated aliphatic aldehydes undergo competitive 1,2- and 1,4-addition to unsaturated carbonyl electrophiles. The pnqmrtion of the adducts >pears to vary as a function of both structure and solvent Steric interactions that favor dissociation of the reversibly formed 1,2-addition product increase the proportion of the 1,4-addition product For example, increasing the size of the substituent R in a protect cyanohydrin (29) from methyl to n-pentyl increases the ratio of 1,4-addition product (30) to 1,2-addition inoduct (31) from 1.5 1 to 2.7 1 (equation 14). ... 

Q Aromatic aldehydes such as benzaldehyde (phenylmethanal), PhCH=0, react with HCN in water to give the cyanohydrin PhCH(OH)CN. Suggest possible mechanisms for this reaction. Hint HCN is partially dissociated in water, with the formation of H30+ and CN ions. 

When the tendency to cyanohydrin formation becomes very great, i.e. when K becomes very small, complexities arise which may often be very confusing. For example, the dissociation constant of... 

Hydrolysis of amygdalin gives benzaldehyde cyanohydrin, which then dissociates to give benzaldehyde and hydrogen cyanide. 

A survey of the many examples of Lewis acid-promoted Passerini-type reactions, finds that most are characterized by one of four distinct processes a) nucleophilic attack of an additional equivalent(s) of isonitrile to form oligomeric products " (17) b) nitrogen dealkylation to give cyanohydrin-type products (18) c) intramolecular capture of tethered-nucleophiles to form cyclic products (19) and d) intermolecular capture of other weakly-basic nucleophiles such as dissociable counterions of the Lewis acid (20) such as the chloride ion for TiCk. 

The a-aminonitrile can also be obtained by the treatment of the aldehyde with HCN followed by reaction of the formed cyanohydrin with ammonia. This method is known as Erlenmeyer modification. A more convenient route is to treat the aldehyde in one step with ammonium chloride and sodium cyanide (this mixture is equivalent to ammonium cyanide, which in turn dissociate into ammonia and HCN). This procedure is referred to as the Zelinsky-Stadnikott modification. The final step is the hydrolysis of the intermediate a-aminonitrile under acidic or basic conditions. 

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