Ionic compounds nucleophiles

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

Hydrocarbon or chlorinated hydrocarbon were seldom used for nucleophilic substitution reactions because of their inability to dissolve ionic compounds. 

Arguments for the intermediacy of penta- or hexacoordinate silicon intermediates have been discussed in detail in earlier reviews (10-13,247), but little attention has so far been devoted to Eq. (40). There is, however, a body of evidence that nucleophilic displacement at silicon may follow pathway (40). Evidence for this mechanism includes the following (1) the common existence of compounds postulated to be intermediates or modeling the intermediates i.e., positively charged ionic silane-nucleophile complexes containing tetracoordinate silicon (2) the dynamic behavior of these compounds when mixed with their components, and the behavior... 

Nucleophilic carbenes have been reported to react with Eewis acidic group 14 complexes with the isolation of neutral or ionic compounds via simple adduct formation or displacement of a halide ion. > The range of carbene complexes of silicon, germanium, tin, and lead, as well as some related cyclopropenylidene complexes are described below. 

Maartmann-Moe, K., Sanderud, K. A., Songstad, J. Reactions of benzylic compounds. Nucleophilicity, leaving group ability and carbon basicity of some ionic nucleophiles in acetonitrile. Comments on the utility of the Finkelstein reaction in synthesis. Acta Chem. Scand. 1982, 636,211-223. 

We shall conclude by saying that the activation phenomenon discussed here is not limited to the activation of an insoluble compound by a more soluble one, but also to the mixture of two or more ionic compounds. As a matter of fact, such property modifications were cited in the literature with organosodium22) and organo-lithium23) compounds for example. What is important, in our opinion, is that in a reaction medium formed by, for example, two basic or nucleophilic entities (A-, M+) and (B-, M+) the observed reactions must be the result of mixed bases or nucleophiles. It is easy to understand what misinterpretation of reaction mechanisms can happen if this possibility is neglected. 

Iodide salts of two novel four-co-ordinate aluminium cations have been prepared. They are (65 L-L = TMED or sparteine). They-are produced by nucleophilic displacement reactions on Me3N,AlH2l. The four-co-ordination was confirmed by the values of the v(AlH) wavenumbers (ca. 1890 cm ) five-coordination would have given lower values. Such a phenomenon was observed for the adduct TMED,AlHBr2 [v(AlH) at 1735 cm ], which must therefore be a molecular adduct rather than an ionic compound. 

A remarkable property of crown ethers is that they allow inorganic salts to be dissolved in nonpolar organic solvents, thus permitting many reactions to be carried out in nonpolar solvents that otherwise would not be able to take place. For example, the Sn2 reaction of 1-bromohexane with acetate ion poses a problem because potassium acetate is an ionic compound that is soluble only in water, whereas the alkyl halide is insoluble in water. In addition, acetate ion is an extremely poor nucleophile. 

Thus, the reaction mechanism is influenced by the nature of the alkyl group, R , since mercaptides of secondary and thertiary thiols and thiophenols cannot provide nucleophilic substitution (SN2). Thiophenol thermolysis usually results in a metal sulfide with diphenyl molecule formation as a by-product. However, these ionic compounds melt and decompose at high temperatures and therefore are not adequate for nanocomposite synthesis. 

Both sodium and potassium ions form more ionic bonds, and the NaCN and KCN bonds are essentially ionic. Sodium and potassium are charge-dense cations, and they prefer to coordinate to charge-dense anions (the N end of cyanide). The electrons on carbon are more available for donation in these ionic compounds, and carbon is the better nucleophile. To be certain that recustion occurs at the acyl carbon, all reactions in this hook will use sodium cyanide (NaCN) or potassium cyanide (KCN) as the reagent. 

Fluoride ion would be an even better nucleophile in a nonpolar solvent (such as hexane) because there would not be any ion-dipole interactions between the ion and the nonpolar solvent. Ionic compounds, however, are insoluble in most nonpolar solvents, but they dissolve in aprotic polar solvents. Fluoride ion is also a good nucleophile in the gas phase, where there are no solvent molecules. 

The iron chemistry which is related to these reversible carbonylations is most usefully revealed by an ionic compound, sodium tetracarbonylferrate. The highly carbonylated Fe(-2) anion can effect the conversion of alkyl halides and tosylates into aldehydes, ketones, and carboxylic acid derivatives. The reactions begin by oxidative addition of the halide to the very nucleophilic iron atom, followed by carbonyl insertion. This particular reagent illustrates several of the crucial reactions... 

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