Nucleophilic substitution reactions competition among

Not shown in the description above, but assumed to be important, is solvation at the rear side of in the ion pairs. According to this scheme, attack by a nucleophile or solvent can occur on the covalent substrate, the intimate ion pair, the solvent-separated ion pair, or on the dissociated carbonium ion. Nucleophilic attack on the covalent substrate or on the intimate ion pair will be akin to a displacement process, and will take place with inversion of configuration. At the solvent-separated ion-pair stage, collapse of the solvent shell can occur from the front to produce retention of configuration or from the back to produce inversion, or the carbonium ion can become symmetrically solvated to produce racemic product. The macroscopic properties of the nucleophilic substitution reaction result from competition among these various rate processes. 

A third factor can affect the competition between base strength and atom size in determining relative nucleophilicity the solvent in which the reaction is performed. Some solvents do not affect nucleophiles. These are the polar, aprotic solvents, of which there are several including acetone, /V,/V-dimethylformamide (DMF), and dimethylsulfoxide (DMSO). When nucleophilic substitutions are carried out in polar, aprotic solvents, the competition between more basic nucleophiles and larger nucleophilic atoms is close but usually won by the stronger base. Therefore, fluoride, the strongest base among the halides, reacts fastest in Sn2 displacements carried out in polar aprotic solvents such as DMF. 

The face selectivity in stericaUy unbiased systems, exhibited in 2-adamantyl cation addition and elimination reactions, among others, has been reviewed, " and so have the elecuonic factors governing the diastereofacial selectivity of many reactions, including the nucleophilic capture of 5-substituted adamantyl cations. The standard enthalpy of formation of the 1-adamantyl cation (99) in the gas phase has been determined experimentally to be 162.0 2.0 kcalmoP, and the ab initio calculations on this species have been re-assessed. It has been found that the inuamolecular hydride transfer occurring in the cation derived from (198) is competitive with intermolec-ular processes, and that the rates of the hydride transfers and the reaction products observed were dependent upon R. The direct synthesis of stable adamantylide-neadamantane bromonium salts is reported the anions used contained or Mo, and the resulting salts proved to be more stable than the Brs salts. The synthesis, reactions, and properties of some 2,8-didehydronoradamantane derivatives are reported. " ... 

Nucleophilic Displacement at Acetylenic Carbon.— Among studies of nucleophilic displacement at acetylenic carbon are the reactions of phosphines and amines with halogenoacetylenes. Using phosphines, displacement occurs by competitive attack on halogen and carbon the general order of reactivity in substitution at carbon by phosphine nucleophiles is sp <- sp > sp. In... 

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