SnI mechanism of nucleophile

Ionization is obviously important in the SnI mechanism of nucleophilic substitution, and indeed two ion pair intermediates have been invoked.These are related as in Eq. (8-19), where (s) represents the solvent. 

Carbenium ions have three bonds to the carbon atom and are planar, with six outer electrons and a vacant p-orbital. Ions of this type are intermediates in a number of organic reactions (for example, in the SnI mechanism of nucleophilic substitution). Certain carbenium ions ate stabilized by de-localization of the charge. An example is the orange-red salt (CgHjlsC CI. Carbenium ions can be produced by supetacids. 

Comparison of SnI and Sn2 Mechanisms of Nucleophilic Substitution in Alkyl Halides... 

The most ideal version of the SnI mechanism substitution nucleophilic uni-molecular) consists of two steps (once again, possible charges on the substrate and nucleophile are not shown) ... 

In a few reactions, nucleophilic substitution proceeds with retention of configuration, even where there is no possibility of a neighboring-group effect. In the SNi mechanism (substitution nucleophilic internal) part of the leaving group must be able to attack the substrate, detaching... 

The reaction of (CHalaCOH with HBr illustrates the SnI mechanism of a 3° alcohol (Mechanism 9.6). Nucleophilic attack on the protonated alcohol occurs in two steps the bond to the leaving group (H2O) is broken before the bond to the nucleophile X is formed. 

Backside attack of the nucleophile suggests an 8 2 mechanism, but attack at the more substituted carbon suggests an Sn 1 mechanism. To explain these results, the mechanism of nucleophilic attack is thought to be somewhere in between SnI and 8 2. 

If we mix f-BuOH and HBr in an NMR tube and let the reaction run inside the NMR machine, we see no signals belonging to the cation. This proves nothing. We would not expect a reactive intermediate to be present in any significant concentration. There is a simple reason for this. If the cation is unstable, it will react very quickly with any nucleophile around and there will never be any appreciable amount of cation in solution. Its rate of formation will be less, much less, than its rate of reaction. We need only annotate the mechanism you have already seen, the SnI mechanism for nucleophilic substitution at saturated carbon stage 1 formation of the carbocation... 

The role of nitronium ion in the nitration of benzene was demonstrated by Sir Christopher Ingold—the same person who suggested the SnI and Sisi2 mechanisms of nucleophilic substitution and who collaborated with Cahn and Prelog on the R and S notational system. 

The Sni mechanism (substitution-nucleophilic-internal), as originally proposed, postulated a front-side displacement occurring via a four-center transition state. It is now clearly established that such a process has never been observed, and that all the reactions proposed to proceed by SnI mechanisms involve ion pairs. The Sni mechanism was initially suggested to explain the observed stereochemistry of chlorosulfite decomposition. Chlorosulfite esters are formed by reaction of alcohols with thionyl chloride, and are the key reactive intermediates in the conversion of alcohols to chlorides with thionyl chloride. Nucleophilic attack by the chloride of the chlorosulfite ester was considered to be concerted with cleavage of the C-O bond ... 

The two main mechanisms for nucleophilic substitution of halogenoalkanes (RX) are SnI and 8 2. These represent the extreme mechanisms of nucleophilic substitution and some reactions involve mechanisms which lie somewhere between the two. 

There are alternatives to the addition-elimination mechanism for nucleophilic substitution of acyl chlorides. Certain acyl chlorides are known to react with alcohols by a dissociative mechanism in which acylium ions are intermediates. This mechanism is observed with aroyl halides having electron-releasing substituents. Other acyl halides show reactivity indicative of mixed or borderline mechanisms. The existence of the SnI-like dissociative mechanism reflects the relative stability of acylium ions. 

The SnI mechanism is an ionization mechanism. The nucleophile does not participate until after the rate-deter-rnining step has taken place. Thus, the effects of nucleophile and alkyl halide structure are expected to be different from those observed for reactions proceeding by the Sn2 pathway. Flow the structure of the alkyl halide affects the rate of SnI reactions is the topic of the next section. 

This is the reverse of the first step in the SnI mechanism. As written here, this reaction is called cation-anion recombination, or an electrophile-nucleophile reaction. This type of reaction lacks the symmetry of a group transfer reaction, and we should therefore not expect Marcus theory to be applicable, as Ritchie et al. have emphasized. Nevertheless, the electrophile-nucleophile reaction possesses the simplifying feature that bond formation occurs in the absence of bond cleavage. 

The notion of concurrent SnI and Sn2 reactions has been invoked to account for kinetic observations in the presence of an added nucleophile and for heat capacities of activation,but the hypothesis is not strongly supported. Interpretations of borderline reactions in terms of one mechanism rather than two have been more widely accepted. Winstein et al. have proposed a classification of mechanisms according to the covalent participation by the solvent in the transition state of the rate-determining step. If such covalent interaction occurs, the reaction is assigned to the nucleophilic (N) class if covalent interaction is absent, the reaction is in the limiting (Lim) class. At their extremes these categories become equivalent to Sn and Sn , respectively, but the dividing line between Sn and Sn does not coincide with that between N and Lim. For example, a mass-law effect, which is evidence of an intermediate and therefore of the SnI mechanism, can be observed for some isopropyl compounds, but these appear to be in the N class in aqueous media. 

One after the other, step through the sequence of structures corresponding to the three nucleophile substitution reactions shown above (reaction 1, reaction 2, reaction 3). Decide whether loss of Br occurs with or without the assistance of RO /ROH. The nucleophile-assisted and unassisted mechanisms are called Sn2 and SnI mechanisms respectively. Label each reaction as Sn2 or SnI as appropriate. 

The substitution of fairly labile nucleophilic groups (halogen, methoxy group) in the 3- and 5-positions, and the noncatalytic substitution of the diazonium group at C-4, are to be considered as nucleophilic substitution reactions. As in the benzenoid series, these reactions are believed to proceed in the former case by the Sx2 mechanism and in the latter by the SnI mechanism. 

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