Organic reaction mechanisms anionic nucleophiles

Early attempts to establish mechanistic models for substitution at metal centers used the labels Sjp and SN2 (substitution, nucleophilic, unimolecular or bimolecular) inherited from Ingold s attempts26 to extend his classic studies in organic reaction mechanisms to substitution at elements other than carbon. Unfortunately, Ingold s attribution of the discontinuity in reaction rates in the replacement of one Cl in m-Co(en)2CI2 1 by various anions A in methanol in the sequence... 

In its simplest form, the phase-transfer catalysed nucleophilic substitution reaction, RX + Y -4 RY + X", in which the active nucleophile Y" is transferred from the aqueous into the organic phase, can be depicted by Scheme 1.5. The mechanism requires the extraction of the nucleophilic anion by the quaternary ammonium cation Q+ as the ion-pair [Q+Y ] into the organic phase, where the nucleophilic reaction can take place. Subsequent to the reaction the spent catalyst forms an ion pair with the released anion X- and equilibration of [Q+X-] between the two phases establishes a... 

This chapter presents computational studies of organic reactions that involve anions. These reactions are usually not grouped together in textbooks. However, these reactions are fundamentally variations on a theme. Anions, acting as nucleophiles, can attack sp carbon atoms we call these as nucleophilic substitution reactions that follow either the S l or 8 2 mechanism. Reactions where the nucleophile attacks sp or sp carbon atoms are addition reactions. The 1,2- and... 

Four aspects of these nucleophilic reactions of anionic metal complexes are discussed below (i) characteristics of the metal complexes (ii) characteristics of the organic halide or related species (iii) the mechanism of this reaction and (iv) applications. 

Nucleophilic anions, i.e. halides, pseudohalides, alkoxides, phenoxides, and thio-phenoxides, are particularly suitable for these reactions. Even anions of lower reactivity in nucleophilic displacements, i.e. carboxylates, nitrates, nitrites and hydroperoxides, find practical application under PTC conditions. Reactions are rigorously Sf,2 in mechanism primary substrates are thus most suitable, since secondary substrates afford elimination products in high yields, especially when reacted at high temperatures, and tertiary substrates only give rise to elimination. This behaviour is consistent with the low polarity of the organic phase, preventing unimolecular mechanisms and favouring elimination over substitution when the reaction center is not a primary carbon atom. 

How can these trends be explained An important consideration is the interaction of the solvent methanol with the anionic nucleophile. We have largely ignored the solvent in our discussion of organic reactions so far, in particular, radical halogenations (Chapter 3), in which they play an insignificant role. Nucleophilic substitution features polar starting materials and a polar mechanism, and the nature of the solvent becomes more important. Let us see how the solvent can become involved. 

Nucleophilic Catalysis. The presence of other ions in solution can influence the rates of substitution and elimination reactions in two principal ways. First, various anions may increase the rates of substitution reactions in proportion to their respective nucleophilicities (22.32). an effect referred to here as "nucleophilic catalysis." This phenomenon has been documented in a number of laboratory studies of nucleophilic substitution and dehydrohalogenation reactions which employed a phosphate buffer to maintain constant pH in solution (5.331. Using the hydrolysis of a halo-organic compound by HPO42- as an example, this mechanism may be portrayed as follows (3.341 ... 

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