Some kinetic mechanistic and synthetic studies of the reactions of cyanide ions with aromatic nitro-compounds

Abstract

The reactions of cyanide ions with some benzene derivatives activated with electron-withdrawing groups have been studied in methanol, and in DMSO, and in mixtures of these solvents. The studies have involved spectroscopic measurements of reactants, intermediates and products together with kinetic measurements and the isolation of reaction products. The techniques used include uv/visible spectrophotometry, proton N.M.R. spectroscopy, e.s.r. spectroscopy, polarography and H.P.L.C. There is evidence from the reaction of 1,3,5-trinitrobenzene with excess cyanide in methanol that methanolysis of the cyanide ion produces a significant concentration of methoxide ions, which may also attack the substrate. A methanolysis constant of approximately 4xl0(^-4)moldm(^-3) has been calculated, which leads to a value of 13.52 for the pK(_a) of hydrogen cyanide in methanol. The reaction of a slight excess of potassium cyanide with meta-dinitrobenzene in methanol has been previously reported to yield 2-methoxy-6-nitrobenzonitrile. The author has confirmed that this is the major product, although isomeric methoxy- nitrobenzonitriles are also produced in small quantities. The reactions of the possible intermediates, 2,4- and 2,6-dinitrobenzonitrile, with methoxide in methanol have been studied. There is evidence for initial partitioning of the parent between reversible production of solvate, via attack of the methoxide at the cyano group, and irreversible substitution of a nitro group by methoxide. In a second slower stage the solvate is converted, via parent, to substitution products. With the same reagents in DMSO there is evidence for the reversible formation of sigma-adducts by attack at unsubstituted ring positions, followed by the irreversible substitution of niffo groups. There is evidence for common intermediates and products in the reactions oimeta- dinitrobenzene, 2,4-dinitrobenzonitrile and l-fluoro-2,4-dinitrobenzene with cyanide in media rich in DMSO. Under such conditions mera-dinitrobenzene and l-fluoro-2,4- dinitrobenzene undergo reaction to produce 2,4-dinitrobenzonitrile, which then reacts further with excess cyanide to produce two isomeric nitro-dicyanophenols. The natures of these reactions, which involve substitution of a ring hydrogen by a cyano group, are discussed. With a large excess of cyanide in media rich in DMSO l-chloro-, 1-bromo- and l-iodo-2,4-dinitrobenzene yield a product with a uv/vis spectrum similar to 2,4-dinitro- phenoxide. With small excesses of cyanide there is NMR evidence for the production of 5-cyano-2,4-dinitrophenol. Tentative suggestions are made for the possible mechanisms for the production of these species, and comparisons drawn with the reactions of l-fluoro-2,4-dinitrobenzene under similar conditions. A common feature of these reactions is the unusual ability of the cyanide ion to replace a ring hydrogen atom. It is suggested that initial attack of cyanide at unsubstituted ring positions yields anionic adducts which may transfer an electron to yield radicals. The presence of the electron-withdrawing cyano group makes these radicals sufficiently acidic to lose a proton, yielding aromatic radical anions containing cyano substituents. The latter species give rise to the observed products. Evidence is presented for the observation of dicyano-diniffobenzene radical anions.