Hydrogen cyanide equilibrium constants

Table 3. Equilibrium Constants for Formation of Cyanohydrins from Hydrogen Cyanide Plus Carbonyl Compounds ...

Druliner et al. (46) studied the addition of hydrogen cyanide to a variety of NiL4 complexes and found that both electronic and steric factors are important in the stability of the HNiL3CN complexes (Table III). For example, Ni[P(0-o-tolyl)3]4 and Ni[PEt3]4 react immediately and quantitatively with HCN to give HNiL3CN, whereas the equilibrium position of Eq. (17) remains far to the left with Ni[P(0-p-tolyl)3]4 the equilibrium constant in the latter case has been estimated to be 4 x 10-4 from visible/UV spectra in benzene at 25°C (47). The corresponding K for L = P(0-o-tolyl)3,... 

Exercise 16-10 One possible way of carrying out the cyanohydrin reaction would be to dispense with hydrogen cyanide and just use the carbonyl compound and sodium cyanide. Would the equilibrium constant for cyanohydrin formation be more... 

As an example of this effect, the equilibrium constant for the reaction of acetone with hydrogen cyanide is 32, whereas the equilibrium constant for a similar reaction of acetophenone is 0.77 ... 

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. 

Randall and Halford (loc. cit.) record K = 2.62 x 10 mole 1 obtained by combining data for the acidity constant of hydrogen cyanide, the solubility of silver chloride, and the equilibrium constant for the reaction AgCl (c) + 2HCN Ag(CN)7 + 2H+ + Cl". A value could also be obtained by a study of e.m.f. cells of the type discussed in Problem 127, but this method has not been used. 

The reactions in which hydrogen-bonded compounds are formed are exothermic and often possess very low equilibrium constants at room temperature. For example, c(T) for th reaction between hydrogen fluoride and hydrogen cyanide gases. 

Hydrogen cyanide adds to an aldehyde or ketone to give a compound called a cyanohydrin. In the product, the nucleophihc cyano group is bonded to the carbonyl carbon and the proton is bonded to the carbonyl oxygen. As we noted above, because ketones are more stable than aldehydes, the addition reactions of ketones are less favorable (have smaller equihbrium constants) than addition reactions of aldehydes. Thus, the equilibrium constant for the addition of HCN is much more favorable for aldehydes than for ketones. 

In all applications discussed in the following chapters the many-dimensional energy surfaces have been Scanned pointwise as a function of some appropriately chosen set of internal coordinates. Energy values thus obtained have been subjected to polynomial fits in order to find the equilibrium geometry and internal, harmonic force constants. In a few cases (polyyne, hydrogen fluoride, hydrogen cyanide) these force constants have been used for an evaluation of vibrational frequencies and phonon dispersion curves within the framework of the harmonic approximation using standard methods of polymer vibrational spectroscopy (see e.g. refs. l8,19 ). 

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