Nucleophilicity of halide ions

Hard and soft acid and base theory was not universally accepted by organic chemists. For example, Parker noted the reversal of nucleophilicity of halide ions in protic and nonprotic solvents, adding that "one cannot sensibly claim that saturated carbon is soft in protic solvents and hard in dipolar solvents, and yet this is required if the hard aclds-soft bases theory is to be applied to this observation" (reference 80). 

The apparent intrinsic nucleophilicities of halide ions have been determined by the thermal decomposition of quaternary ammonium salts (19). Complica-... 

The relative nucleophilicities of halide ions in polar protic solvents are quite different from those in polar aprotic solvents (Table 9.8). 

I Table S.8 Relative Nucleophilicities of Halide Ions in Polar Aprotic and Protic Solvents... 

Chloride ions are comparatively weak nucleophiles and do not react with azoles. In general, there is also no interaction of halide ions with azolium compounds. 

The acid-catalyzed additions of bromide and chloride ion to thiiranes occurs readily, with halide preferentially but not exclusively attacking the most substituted carbon atom of the thiirane. The reaction of 1-substituted thiiranes with acetyl chloride shows a slight preference for halide attack at the less substituted carbon atom (80MI50601). For further discussion of electrophilic catalysis of halide ion attack see Section 5.06.3.3.2. The reaction of halogens with thiiranes involves electrophilic attack on sulfur (Section 5.06.3.3.6) followed by nucleophilic attack of halide ion on carbon. 

Grignard reagents are rapidly hydrolysed by water or acid to give the parem hydrcxatbon, RH, but this reacdon is rarely of synthedc importance. Hydrocarbons can also be syndiesized by nucleophilic displacement of halide ion from a reacdve alkyl halide, e.g. 

Nucleophilic displacement of halide ion from a saturated carbon atom by alkali-metal diphenylphosphide reagents occurs with inversion of configuration at carbon, as is found in normal Sn2 displacements.19 Thus menthyl chloride or bromide gives ... 

These nucleophilic substitution reactions are rather facile when better leaving groups, e.g. halide ions, are present. Reaction occurs by addition of the nucleophile to the C=N bond, followed by loss of halide ion from the anion intermediate. 

The formation of Si-X bonds was confirmed by measurements of XPS spectra and flat band potentials. For (100) surface, back-bond oxidation occurs simultaneously with the Si-X formation under the same conditions, though it is negligible for Si fill). The mechanism of the surface reactions can be explained by hole injection by an oxidant, followed by the nucleophilic attack of halide ions or water molecules.32,33) For dihydride (=Si H2) bonds on H-Si (100), for example, the reactions at the initial stage can be expressed as follows ... 

The nucleophilic displacement of halide ions from M—X bonds by carbonylate anions (either mononuclear or polynuclear) is a general synthetic route to metal-metal bonded species (1,2), and numerous hetero-Pt clusters have been obtained in this way. The resulting products are not often those of expected stoichiometry, although under optimized experimental conditions this method can provide very useful syntheses, particularly of high-nuclearity clusters. Some examples are shown in Eqs. (7)—(11) (5,51-54). 

A similar picture holds for other nucleophiles. As a consequence, there might seem little hope for a nucleophile-based reactivity relationship. Indeed this has been implicitly recognized in the popularity of Pearson s concept of hard and soft acids and bases, which provides a qualitative rationalization of, for example, the similar orders of reactivities of halide ions as both nucleophiles and leaving groups in (Sn2) substitution reactions, without attempting a quantitative analysis. Surprisingly, however, despite the failure of rate-equilibrium relationships, correlations between reactivities of nucleophiles, that is, comparisons of rates of reactions for one carbocation with those of another, are strikingly successful. In other words, correlations exist between rate constants and rate constants where correlations between rate and equilibrium constants fail. 

The displacement of halide ion by a wide variety of other nucleophiles has also been demonstrated. A particularly dramatic example is seen in the reaction of 6,6 -dichloro-2,2 -bipyridine (or 2,9-dichloro-1,10-phenanthroline) with ammonium tetrachlorozincate. 

Soft nucleophiles or halide ions may also coordinate to C to give [Pd(l, 2,3-r)3-octa-2,7-dien-l-yl)(X)(L)] (F) (X = Nu, Cl ), which is known to be less active [47] and may even lead to catalyst deactivation. Finally, the by-product 1,3,7-octatriene can also be formed from intermediate C, by proton abstraction at the 4-position of the r 3,r l-octadienyl ligand. Note that the proton that is lost upon 1,3,7-octatriene formation is different from the proton added by the nucleophile (4 position vs. 6 position, respectively), which points at the general irreversibility of the step going from B to C. 

Since the first report in 1979 of gas-phase displacement reactions of nucleophiles, including halide ions, with acyl halides283 this field has not received great attention. In the original work the mechanistic proposal was advanced that reaction proceeds according to the double-well energy model via ion-dipole complexes as intermediates rather than the classical tetrahedral adduct of solution chemistry283. A claim of further evidence for this... 

Nitroalkanes may be readily prepared by nucleophilic displacement of halide ion by sodium nitrite in DMF or DMSO402. Chiral nitro compounds have been prepared in this way and the reader is directed to a review of these compounds for further details403. 

Another factor affecting the nucleophilicity of these ions is their solvation, particularly in protic solvents. A protic solvent is one that has acidic protons, usually in the form of O—H or N—H groups. These groups form hydrogen bonds to negatively charged nucleophiles. Protic solvents, especially alcohols, are convenient solvents for nucleophilic substitutions because the reagents (alkyl halides, nucleophiles, etc.) tend to be quite soluble. 

Direct nucleophilic displacement of halide ion by a group R, not necessarily organic, attached to copper is considered 31) to be important only in the presence of ligands, L, which strongly coordinate with copper and aid the formation of the species (CuL4) + R . 

The mechanism of the above elimination reaction is similar to the E2 mechanism of dehydrohalogenation. The reaction proceeds by nucleophilic attack of a base on silicon, simultaneous elimination of halide ion and formation of double bond (Scheme 4.4). 

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