Cyanocarbon acids

Cyanobond Cyanoborohydride Cyanocarbon acids Cyanocarbon anions Cyanocarbons Cyanocobalamin... 

The most important members of the cyanocarbon class are the alkenes tetracyanoethylene, hexacyanobutadiene, and tetracyanoquino dime than the alkanes tetracyanomethane and hexacyanoethane dicyano acetylene hexacyanobenzene tetracyanoquinone cyanocarbon acids oxacyanocarbons thiacyanocarbons and azacyanocarbons. Tetracyanoethylene is described first because its chemical versatility makes it a rich source of other polycyano compounds. Moreover, an understanding of its chemistry is helpful in understanding the chemistry of other cyanocarbons. 

These acids (51) are organic molecules that contain a plurality of cyano groups and are readily ionized to hydrogen ions and resonance-stabilized anions. Typical cyanocarbon acids are cyanoform, methanetricarbonitrile (5) 1,1,3,3-tetracyanopropene [32019-26-4] l-propene-l,l,3,3-tetracarbonitrile (52) 1,1,2,3,3-pentacyanopropene [45078-17-9], l-propene-l,l,2,3,3-pentacarbonitrile (51) l,l,2,6,7,7-hexacyano-l,3,5-heptatriene [69239-39-0] (53) 2-dicyanomethylene-l,l,3,3-tetracyanopropane [32019-27-5] (51) and l,3-cyclopentadiene-l,2,3,4,5-pentacarbonitrile [69239-40-3] (54,55). Many of these acids rival mineral acids in strength (56) and are usually isolable only as salts with metal or ammonium ions. The remarkable strength of these acids results from resonance stabilization in the anions that is not possible in the protonated forms. 

Cyan-kalium, n. potassium cyanide, -kalium-losung, /. potassium cyanide solution, -ko-balt, m. cobalt cyanide, -kohlensaure, / cyanocarbonic acid. -kupfer, n. copper cyanide, -laugerei, -laugung, /. cyaniding. cyanidation. -losung, / cyanide solution, -metall, n. metallic methyl cyanide, -natrium, n. sodium cyanide. -platin, n. platinum cyanide. 

The protocol described also can be used for the acylation of ketone enolates with carbonic acid derivatives (Figure 13.62). Especially good acylating agents are cyanocarbonic acid methyl ester (Mander s reagent, Figure 13.62, top) and dialkyl pyrocarbonates (bottom). Usually it is not possible to use dimethyl carbonate for the acylation of ketone enolates because dimethyl carbonate is a weaker electrophile than cyanocarbonic acid methyl ester or diethyl pyrocarbonates. 

Fig. 13.62. Acylation of ketone enolates with carbonic acid derivatives. Especially good acylation reagents are cyanocarbonic acid methyl ester (top) and dialkyl pyrocarbonates (bottom).

In the first study [69] of general base catalysed proton transfer from a cyanocarbon acid, the rate of detritiation of l,4-dicyano-2-butene was measured in aqueous phenolate and amine buffers. For PhO" the process is... 

Primary kinetic isotope effects in proton transfer from cyanocarbon acids in water kL ... 

To test further the hypothesis that proton transfer from cyanocarbon acids is approaching normal behaviour, measurements were made [19] around ApK ca. 0 by studying the ionization of bromomalononitrile (pK° 7.81) in phosphate (pK° 7.21) and morpholine (pK° 8.49) buffers using the temperature-jump technique. The results of these experiments... 

Fig. 5. Rates of proton transfer for cyanocarbon acids. Open and closed points represent forward (log10 k, ) and reverse (logI0 -i ) rate coefficients, respectively, and ApK is the statistically corrected difference in acidities between the cyanocarbon acid and base (B). Points are o and for f-butylmalononitrile reacting with carboxylate ions and H20 a and for malononitrile with formate ion and H20 and for bromomalononitrile with phosphate ion and morpholine x for 1,4-dicyano-2-butene with phenolate ions. Redrawn with permission from F. Hibbert and F. A. Long, J. Am. Chem. Soc., 94 (1972) 2647. Copyright by the American Chemical Society.

For p-nitrobenzyl cyanide (pK° 13.4) [117] kinetic results were obtained [19] which are similar to the slow proton transfers described for nitroparaffins and ketones. In aqueous solution rate coefficients for amine catalysed detritiation give a Bronsted plot with slope 13 = 0.61 and the rates of the thermodynamically favourable recombination of the car-banion with ammonium ions vary between ca. 103 and 10s 1 mole-1 sec-1. In 80/20 (v/v) ethei ethanol at —77°, a value j3 = 0.49 was observed for catalysis by phenolate ions [12]. The p-nitrophenyl group in this cyanocarbon acid considerably alters the proton transfer behaviour. 

Weaker cyanocarbon acids have also been studied. In methanol/ dimethylsulphoxide solutions containing methoxide ion, rates of racemiz-ation and deuterium exchange (uptake of deuterium from the solvent) for 2-methyl-3-phenylpropionitrile are identical [118, 119]. The pK for this acid has been roughly estimated as ca. 30 [119]. There are two possible mechanisms for exchange and racemization [120], both involving a low concentration intermediate (79), viz. 

A value of kH/kD = 1.4 was obtained [114] for the rate of proton transfer compared with deuteron transfer from chloroform to hydroxide ion and this result is similar to the values determined earlier for several haloforms [164, 166]. In the most recent work [171(b)] a value kH /kD = 1.11 0.05 was determined for chloroform. These values are close to those observed for reaction of cyanocarbon acids (though a different base catalyst is involved) and in Sect. 4.3 it was argued that isotope effects as low as these are expected for a transition state in which proton transfer is almost complete. The isotope effect for proton transfer from chloroform was measured using a new and useful method [114]. It can be shown that the ratio of initial rates of uptake of tritium for the first ten per cent of reaction from tritiated water into CHC13 and CDC13 is identical to the primary isotope effect for proton loss (feH /fcD). The procedure can be used for measuring isotope effects on proton transfer from carbon acids to hydroxide ion or buffer catalysts and is more convenient than other methods. Other methods which have been used, for example, involve the comparison of rates of detritiation and dedeuteration or the comparison of rates of bromination for isotopically different acids (RCH and RCD) [113]. 

The small isotope effects observed in proton transfer from cyanocarbon acids to various bases shown in Table 3 (for example feH/feD = 1.46 for proton transfer from malononitrile to water) are compatible with an extremely product-like transition state in which the proton is almost fully transferred [113] (Sect. 4.3). Similar conclusions may be reached from the small isotope effects observed for chloroform (feH/feD = 1.41 0.01 [114] and 1.11 0.05 [171]) and phenylacetylene (kH/kD = 0.95 0.09 [143]) for reaction with hydroxide ion, and for reaction of disulphones with water (feH/feD = 2.2 0.1 [65]). In all these cases the magnitude of the Bronsted exponent is close to the limiting value of unity as expected for a product-like transition state. 

Many pKa values for cyanocarbon acids are in the compilations of Palm and colleagues A comprehensive study of the ionization of many carbon acids in DM SO has been made by Bordwell and colleagues and many of them are cyano compounds. 

R. H. Boyd, J. Am. Chem. Soc. 1961, 83, 4288-4290. Strengths of cyanocarbon acids and a H-acidity scale for concentrated acid solutions. 

R. H. Boyd, J. Phys. Chem. 1963, 67,737-744. Cyanocarbon chemistry. XXIII. The ionization behavior of cyanocarbon acids. 

A thorough investigation of this reaction for three cyanocarbon acids has been carried out by Long and his co-workers. Figure 8 shows... 

Figure 8. Bronsted plots [60] for proton transfer to (solid line) and from (broken line) cyanocarbon acids with RC02H/RC02 shown by circles H30 /H20 shown by x for malonitrile and by +...

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