Substitution, nucleophilic mechanism

In most cases the alkoxide or phenoxide 1 reacts with the alkyl halide 2 by a bimolecular nucleophilic substitution mechanism ... 

Conversion of Esters into Alcohols Grignard Reaction Esters and lactones react with 2 equivalents of a Grignard reagent to yield a tertiary alcohol in which two of the substituents are identical (Section 17.5). The reaction occurs by the usual nucleophilic substitution mechanism to give an intermediate ketone, which reacts further with the Grignard reagent to yield a tertiary alcohol. 

There are four principal mechanisms for aromatic nucleophilic substitution. Each of the four is similar to one of the aliphatic nucleophilic substitution mechanisms discussed in Chapter 10. 

Kinetic experiments have been performed on a copper-catalyzed substitution reaction of an alkyl halide, and the reaction rate was found to be first order in the copper salt, the halide, and the Grignard reagent [121]. This was not the case for a silver-catalyzed substitution reaction with a primary bromide, in which the reaction was found to be zero order in Grignard reagents [122]. A radical mechanism might be operative in the case of the silver-catalyzed reaction, whereas a nucleophilic substitution mechanism is suggested in the copper-catalyzed reaction [122]. The same behavior was also observed in the stoichiometric conjugate addition (Sect. 10.2.1) [30]. 

The yields of reaction products from thermal nucleophilic substitution reactions in DMSO of 0- and p-nitrohalobenzenes (Zhang et al. 1993) or p-dinitrobenzene (Liu et al. 2002) with the sodium salt of ethyl a-cyanoacetate were found to be markedly diminished from the addition of small amounts of strong electron acceptors such as nitrobenzenes. At the same time, little or no diminution effects on the yields of the reaction products were observed from the addition of radical traps such as nitroxyls. These results are consistent with the conclusion that such reactions proceed via a nonchain radical nucleophilic substitution mechanism (Scheme 4.26). 

When nucleophilic substitution reactions are attempted, the expected product may often be accompanied by one or more additional products that arise from competing reactions. Since these competing reactions share features of the nucleophilic substitution mechanism, they are readily rationalized. 

Hydrolysis of an ester can be achieved by either base- or acid-catalysed reactions, and the nucleophilic substitution mechanisms follow processes that should... 

The basic concepts of nucleophilic substitution reactions appeared in the first semester of organic chemistry. These reactions follow or Sp 2 mechanisms. (In aromatic nucleophilic substitution mechanism, we use the designation Sp Ar.) In Sfjl and Sp 2 mechanisms, a nucleophile attacks the organic species and substitutes for a leaving group. In aromatic systems, the same concepts remain applicable, but with some differences that result from the inherent stability of aromatic systems. 

Reaction of 2-chloropyridine gives 2-chloro-6-fluoropyridine as the major product which arises from the preferential substitution of hydrogen over chlorine and would be unexpected on the basis of the nucleophilic substitution mechanism described above. The product obtained was suggested, therefore, to arise from the addition of fluorine to the most electron rich carbon-nitrogen double bond, followed by elimination of HF [155]. 

For a monograph on aromatic nucleophilic substitution mechanisms, sec Miller Aromatic Nucleophilic Substitution. Elsevier New York, 1968. For reviews, see Bernasconi Chimia 1980, 34, 1-11, Acc. Chem. Res. 1978, II. 147-152 Bunnctt J. Chem. Educ. 1974, 51, 312-315 Ross, in Bamford Tipper Comprehensive Chemical Kinetics, vol. 13 Elsevier New York, 1972. pp. 407-431 Buck Angew. Chem. 1m. Ed. Engl. 1969,8. 120-131 Angetv. Chem. 81. 136-148) Bunccl Norris Russell Q. Rev., Chem. Soc. 1968, 22. 123-146 Ref. I. 

The second major type of nucleophilic substitution mechanism is the S 1 mechanism. This mechanism proceeds via two steps. The first step (the slow step) involves the breakdown of the alkyl halide into an alkyl carbocation and a leaving group anion. The second step (the fast step) involves the formation of a bond between the nucleophile and the alkyl carbocation. 

An acid-catalyzed cleavage that occurs when hydriodic acid (HI) mixes with ethers is the most significant reaction that ethers experience. This reaction proceeds via a nucleophilic substitution mechanism. Primary and secondary alkyl ethers react by an S 2 mechanism, while tertiary, benzylic, and alcylic ethers cleave by an S 1 mechanism. A typical S 2 reaction would be the reaction of ethylisopropyl ether with HI. The mechanism for this reaction is ... 

In homogeneous catalysis, one species can also coordinate to the metal before the other has left. The two situations (first dissociation, then coordination or first coordination, then dissociation) are similar to the SN1 and SN2 nucleophilic substitution mechanisms. Figure 3.4 illustrates these two cases, using as an example the reaction of the Ni(CO)4 complex with acetone (dimethyl ketone) in TH F. In the first case, a CO ligand dissociates from the complex, leaving a vacant position that is immediately filled by a solvent molecule. Then, the solvent is replaced by an acetone... 

Although a substantial number of reactions are described in the text, they belong to a relatively modest number of mechanistic types. The preparation of alkyl halides from alcohols and HX, the cleavage of ethers, and the preparation of amines from alkyl halides and ammonia (and many other reactions) all, for example, occur by a nucleophilic substitution mechanism. The following is a brief review of the main mechanistic pathways discussed in the text. 

This reaction, inaptly named condensation reaction43, proceeds either by a nucleophilic substitution mechanism or by insertion of dichlorogermylene into the C—Cl bond. The nucleophilic substitution mechanism is doubtful. In accordance with quantum-chemical calculations, the negative charge in the anion GeCh- is distributed on chlorine atoms and a movement of anion to the cation from the rear results in Cl- loss and GeCl2 formation39. 

To understand this difference in reactivity of various acid derivatives look at the first step in the nucleophilic substitution mechanism (involving the addition of a nucleophile to the electrophilic carbonyl carbon) which is the rate-determining step. Therefore, the more electrophilic this carbon is, the more reactive it will be. The nature of Y has a significant effect in this respect ... 

The authors of this review article suggest that the results indicated in Schemes 34, 35 and 36 are consistent with a stepwise nucleophile substitution mechanism (64JA107) as outlined in Scheme 37. When Z is nitrogen the driving force for the reaction is the ease of cleavage of the N—S bond by a soft nucleophile with a concerted elimination of phenylcyanamide. In the thiazole (Z = CH), however, the stable C—S bond precludes ring fission by a nucleophilic attack at sulfur and the initial adduct (79) (presumably trans) is obtained as a stable product. 

In this chapter, elimination reactions were presented both independently and in association with their related nucleophilic substitution mechanisms. Furthermore, the processes by which molecules undergo both El and E2 eliminations were presented and explained using bonding and nonbonding orbitals and their required relationships to one another. While much emphasis was placed on the planar relationships of orbitals required for both elimination reaction mechanisms, the special case of frans-periplanar geometries were described as necessary for efficient E2 eliminations to occur. 

S m1 mechanism (Sections 7.10, 7.13) A nucleophilic substitution mechanism that goes by a two-step process involving a carbocation intermediate. SnI is an abbreviation for Substitution Nucleophilic Unimolecular. ... 

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