SN I reactions

K. S. Peters, Acc. Chem. Res. 40, 1 (2007). Dynamic Processes Leading to Covalent Bond Formation for SN I Reactions. 

Free energy versus REACTION PROGRESS DIAGRAM FOR THE SN I REACTION OF TERT-BUTYL CHLORIDE (2-CHLORO-2-METHYLPROPANE) AND ACETATE ANION. 

O The carbocation formed in this Sn I reaction is sp2 hybridized and planar. It is not chiral, so the products formed from it must be racemic The nucleophile—water in this case—can approach equally well from either side, resulting in the formation of equal amounts of the R and S enantiomers of the product. Note that it is water, not hydroxide ion, that acts as a nucleophile because the concentration of hydroxide ion in neutral water is extremely low. 

Stereochemistry of the Sn I reaction of (S)-1 -chloro- I -phenylethane in acetic acid... 

Sn I reactions are thought to play a role in how nitrosamines, compounds having the general structure R2NN=0, act as toxins and carcinogens. Nitrosamines are present in many foods, especially cured meats and smoked fish, and they are also found in tobacco smoke, alcoholic beverages, and cosmetics. Nitrosamines cause many forms of cancer. 

Note that in the S l reaction, which is often carried out under acidic conditions, neutral water can act as a leaving group. This occurs, for example, w hen an alkyl halide is prepared from a tertiary alcohol by reaction with HBr or HCl (Section 10.6). The alcohol is first protonated and then spontaneously loses H2O to generate a carbocation, which reacts with halide ion to give the alkyl halide (Figure 11.13). Knowing that an Sn I reaction is involved in the conversion of alcohols to alkyl halides explains why the reaction works well only for tertiary alcohols. Tertiary alcohols react fastest because they give the most stable carbocation intermediates. 

The positively charged carbon atom in a carbocation is an extremely electron-dehcieni (electrophilic) carbon. As such, its behavior is dominated by a need to obtain an electron pair from any available source. The Sn I reaction illustrates the most obvir>"s fate of a < nr .- -t on c a.bination with an external Lewis base, forming a new bond to carbon. However, the electron deficiency of cationic carbon is so great that even under typical SnI solvolysis conditions, surrounded by nucleophilic solvent molecules, some of the cations won t wait to combine with external electron-pair sources. Instead, they will seek available electron pairs within their own molecular structures. The most available of the.se are electrons in carbon-hydrogen bonds one carbon removed from the cationic center (at llic so-called carbon) ... 

Experiments designed to study the effects of substituents, solvents, nucleophiles/bases, and leaving groups on the balance between Sn2 and Sn-I reactions most often involve secondary substrates. Explain why this is so. 

Some reactions of a given substrate under a given set of conditions display all the characteristics of Sn2 mechanisms other reactions seem to proceed by SnI mechanisms, but cases are found that cannot be characterized so easily. There seems to be something in between, a mechanistic borderline region. At least two broad theories have been devised to explain these phenomena. One theory holds that intermediate behavior is caused by a mechanism that is neither pure Sn I nor pure Sn2, but some in-between type. According to the second theory, there is no intermediate mechanism at all, and borderline behavior is caused by simultaneous operation, in the same flask, of both the SnI and Sn2 mechanisms that is, some molecules react by the SnI, while others react by the Sn2 mechanism. 

Just as the E2 mechanism shares features of the Sn2 mechanism, the El mechanism shares features of the Sn 1 reaction. The initial step is formation of a carbocation intermediate through loss of the leaving group. This slow step becomes the rate-determining step for the whole reaction, i.e. the El mechanism is unimolecular. In general terms, the reaction can be represented as follows. 

Such C—H activation reactions were extended to the related Sn systems. The rates of addition for this chemistry were 4 to 6 times slower for the latter compared with the Ge reactions, in order to avoid the competing formation of Sn(I)[N(SiMe3)2]2Ph. Thus, similar... 

For Sn 2 reactions, the extent of solvent reorganization is not so marked as indicated by smaller values for lACp I (Table 5). In the transition state for this mechanism the developing anion-water... 

The existence of Sn(I) may be inferred from reports of the reactions of H and e q with Sn022 , SnF3 , and certain complexes of Sn(II) (14a, 14b). However, rather little is known of these species. The chemistry of Sn(III) is better documented. As summarized by Cannon, this oxidation state is frequently found for intermediates in reactions of Sn(II) with one-electron oxidants (77). Asmus et al. generated Sn(III) in a pulse radiolysis study by reaction of OH with Sn(II) in an unspecified medium (18). By conductivity measurements over the range pH 3-2.5 they obtained data which were interpreted as hydrolysis according to... 

An interesting indium(I) alkoxide [(2,4,6-(CF3)3C6H20)In]2 (93), featuring two-coordinate, bent indium sites, has been reported." This compound is synthesized from Cpin and 2,4,6-(CF3)3C6H20H in hexane, and features a planar In202 core. The reaction of Sn(/i-t-BuO)3Tl or... 

Sulfur trioxide is a strong Lewis acid and an oxidant. Insertions into element-carbon bonds to yield alkylsulfonates [reaction (k)] are known for E = Li , AH , Si , Hg °, Sn i and Ge ... 

Bonifacic M., Hug G.L., Schoeneich C., Kinetics of the reaction between sulfide raducal cation complexes [SS]-i- and [SN]-i- and superoxide or carbon dioxide radical anion, J. Phys. Chem. A, 2000,104,1240-1245. 

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