Solvent effects with cyanide

It should be mentioned briefly that solvation phenomena should also influence the outcome in the case of ambident nucleophiles, at least to the extent to which these reagents are sensitive to solvent effects. With an ambident anion, which is not manipulated by countercations (formation of ion pairs), the more electronegative center should attack preferentially. The more this area is blocked by hydrogen bridges formed in protic solvents, or shielded by countercations, the more likely it is that the less electronegative end will react. If dipolar aprotic solvents are used, which can only solvate the cations, a preferential attack of the nonshielded more electronegative center is to be expected. It must be realized, however, that in substitution reactions employing cyanide ions, dipolar aprotic solvents have not been reported to enhance the formation of isonitriles. " ... 

Contraindications Intestinal obstruction, GI tract not anatomically intact patients at risk of hemorrhage or GI perforation, if use would increase risk and severity of aspiration not effective for cyanide, mineral acids, caustic alkalis, organic solvents, iron, ethanol, methanol poisoning, lithium do not use charcoal with sorbitol in patients with fructose intolerance, hypersensitivity to charcoal or any component of the formulation... 

An investigation of the effect of various catalysts, the solvent, and alkyl groups on the oxirane ring28 has shown that for ring opening with cyanide ion the best yields are obtained when the catalyst is (16) and the solvent is ethylene glycol. The reactions are regiospecific at the least substituted carbon and occur rapidly at room temperature,... 

High-level ab initio calculations have shown that the AN2 reaction of the cyanide ion with ethyl chloride is catalysed by 1,4-benzenedimethanol in dipolar aprotic solvents through selective two hydrogen bonds.7 In non-polar solvents, combined with phase-transfer catalysis, the 1,4-benzenedimethanol could replace some water molecules hydrating the cyanide ion and induce a substantial rate acceleration effect. 

Although the solvent effects are small, the alkene formation diminishes as predicted with increasing water content (corresponding to increased solvent polarity). The Sn2/E2 reaction of 2-phenylpropyl tosylate with sodium cyanide (in hexamethyl-phosphoric triamide and in A,A-dimethylformamide as solvents at 100 °C) gives a-methylstyrene (elimination product) and l-cyano-2-phenylpropane (substitution product) [75]. It has been found, in accordance with the predictions of the Hughes-Ingold rules, that the elimination/substitution ratio decreases as the polarity of the solvents (measured by the relative permittivity) increases [75]. Theoretical investigations of the... 

The transformation of alkyl halides into alkanenitriles with cyanide ions has frequently been carried out in protic solvents such as methanol or ethanol, sometimes with the addition of water or acetone, and often at elevated temperatures. Under these conditions reaction rates decrease in the order iodides, bromides, chlorides, as would be expected. Accordingly iodide ions have a catalytic effect and increase reaction rates. The use of anhydrous ethylene glycol or di- and poly-ethylene glycols and their corresponding ethers allows the use of higher temperatures, which means better solubility of the alkali metal cyanides. There is probably additional help from the extensive solvation of the countercations by some of these hydroxy polyethers. While for primary halides yields for nitriles range up to 90% (Table 1), they drop sharply with secondary and tertiary halides. ... 

Nonactivated olefins fail to react even under strenuous conditions with cyanide anion catalysis. Due to this lack of reactivity coupled with the inherent desirability of the products, much research has focused on developing catalysts for the hydrocyanation of these nonactivated olefins. This has led to nickel, palladium, copper, and cobalt-based catalysts effective at 25-125°C with or without a solvent. Most were developed for the hydrocyanation of unactivated olefins, but many are equally applicable for oAer olefins. For example, much work has been reported on butadiene hydrocyanation employing all of the catalysts mentioned above except palladium. 

The effect of the nature of various di-imine ligands in reactivity has been studied for reaction with hydroxide and with cyanide. In the latter investigation the effects of ligand denticity were probed by employing a series of SchiflF-base ligands with denticities between two and six. Kinetic parameters for the reaction of the [Fe(5-Br-phen)3] + cation with cyanide in aqueous solution have been reported kinetics of reactions of di-imine-iron(ii) complexes with cyanide have also been studied in ethylene carbonate- and propylene carbonate-water mixtures. Effects of solvent variation on reactivity have been dissected into initial-state and transition-state components for the reaction of [Fe(phen)3] + with hydroxide in methanol- and in acetone-water mixtures (cf. p. 294). 

The bimolecular reaction of the [Fe(terpy)J + cation with cyanide ion shows the expected increase in rate on going from water (kt = 0.0191 mol s at 35 °C) to 50% aqueous ethanol (k = 0.068 1 mol s" at 35 °C). This increase can be ascribed to the decreased solvation, and thus increased chemical potential, of the cyanide ion in the aqueous ethanol. It is interesting to contrast this situation with the dissociative-interchange reaction between iron(iii) and thiocyanate mentioned above, where the small decrease in rate in going from water to less-solvating DMSO was used as evidence against bimolecular attack by thiocyanate. The bimolecular substitution redox reaction of iron(ii) with the [Co(NH3)6Br] + cation shows a more complicated reactivity pattern in mixed aqueous solvents. The pattern is discussed in terms of the effects of the organic co-solvents on water structure and thence on reaction rates. ... 

Entering Groups. Rates of reaction of [PtCl4], [PtCl3(OH2)], and [PtCl2(OH2)2] with ethene are all rather similar. Rates and activation parameters for the reaction of a variety of uncharged platinum(ii) complexes with cyanide have been reported. The complexes in question are [PtCl(N02)(NH3)2], [Pt(N02)2(NH3>2], [Pt(CN)a-(en)], and rra s -[Pt(CN)(N02)(NH3)2] cyanide substitution follows the usual rate law [equation (1) above]. The variation of the second-order term with solvent composition for the reaction of [PtCl2(bipy)] with thiourea in aqueous dioxan and in aqueous THE has been discussed in terms of solvent effects on the initial and transition states (see Chapter 5 of this Part). ... 

The separation of solvent effects on reactivities into constituent initial-state and transition-state effects by the use of appropriate kinetic and thermodynamic data has been successfully carried out for several organic reactions. Thus, for example, the solvolysis of t-butyl chloride and the Menschutkin reaction were treated in this manner some time ago a recent organic example is afforded by the solvolysis of isopropyl bromide in aqueous ethanol. For inorganic reactions, this approach was early used for reactions of tetra-alkyltin(iv) compounds with mercury(ii) halides. A more recent analysis of reactions of low-spin iron(n) complexes with hydroxide and with cyanide in binary aqueous mixtures was complicated by the need to make assumptions about single-ion values in such ion+ion reactions. Recent estimates of thermodynamic parameters for solvation of complexes of the [Fe(phen)3] + type are helpful in this connection. However, it is more satisfactory to work with uncharged reactants when trying to undertake this type of analysis of reactivity trends. A suitable system is provided by the reaction of [PtClaCbipy)] with thiourea. In dioxan-and tetrahydrofuran-water solvent mixtures, reactivity is controlled almost entirely... 

The addition of water to the methanol decreases the rate constant while increasing the dielectric constant of the medium. Likewise the addition of an inert salt as sodium chloride slows the reaction and the product, sodium thiocyanate, also slightly slows the reaction. Both the solvent effect and the salt effects are consistent with the reaction of an ionic species (cyanide ion) with a neutral molecule (sulfur, Sg). Had the rate-determining step involved the reaction of an anion with another anion, increasing the dielectric constant would have increased the rate and the presence of more salts would have also increased the rate. 

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