SnI and Sn2 mechanisms

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

The role of nitronium ion in the nitration of benzene was demonstrated by Sir Christo pher Ingold—the same per son who suggested the SnI and Sn2 mechanisms of nu cleophilic substitution and who collaborated with Cahn and Prelog on the R and S notational system... 

Because the SnI and Sn2 mechanisms are so different from each other, let s examine each one separately. 

The greater the contribution of 4 to the transition state, the more firmly the system is placed in the N category likewise a large contribution from 5 is characteristic of the Lim category. Bentley and Schleyer state that the essential difference between the SnI and Sn2 mechanisms depends upon whether nucleophilic attack... 

Several distinct mechanisms are possible for aliphatic nucleophilic substitution reactions, depending on the substrate, nucleophile, leaving group, and reaction conditions. In all of them, however, the attacking reagent carries the electron pair with it, so that the similarities are greater than the differences. Mechanisms that occur at a saturated carbon atom are considered first. By far the most common are the SnI and Sn2 mechanisms. 

In the lUPAC system, the SnI mechanism is Dn -f An or Djy-f An (where denotes the rate-determining step). The lUPAC designations for the SnI and Sn2 mechanisms thus clearly show the essential differences between them AnDn indicates that bond breaking is concurrent with bond formation Dn + An shows that the former happens first. 

The difference between the SnI and Sn2 mechanisms is in the timing of the steps. In the SnI mechanism, first X leaves, then Y attacks. In the Sn2 case, the two things happen simultaneously. One could imagine a third possibility first the attack of Y, and then the removal of X. This is not possible at a saturated carbon, since it would mean more than eight electrons in the outer shell of carbon. However, this type of mechanism is possible and indeed occurs at other types of substrate (p. 424 Chapter 13). 

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. 

The difference between the SnI and Sn2 mechanisms is that in the former case the formation of the ion pair (ki) is rate determining, while in the Sn2 mechanism its destruction ( 2) i rate determining. Borderline behavior is found where the rates of formation and destruction of the ion pair are of the same order of magnitude. However, a number of investigators have asserted that these results could also be explained in other ways. ... 

Among the experiments that have been cited for the viewpoint that borderline behavior results from simultaneous SnI and Sn2 mechanisms is the behavior of 4-methoxybenzyl chloride in 70% aqueous acetone. In this solvent, hydrolysis (i.e., conversion to 4-methoxybenzyl alcohol) occurs by an SnI mechanism. When azide ions are added, the alcohol is still a product, but now 4-methoxybenzyl azide is another product. Addition of azide ions increases the rate of ionization (by the salt effect) but decreases the rate of hydrolysis. If more carbocations are produced but fewer go to the alcohol, then some azide must he formed by reaction with carbocations—an SnI process. However, the rate of ionization is always less than the total rate of reaction, so some azide must also form by an Sn2 mechanism. Thus, the conclusion is that SnI and Sn2 mechanisms operate simultaneously. ... 

Deuterium Substitution. The a and P secondary isotope effects affect the rate in various ways (p. 298). The measurement of a secondary isotope effects provides a means of distinguishing between SnI and Sn2 mechanisms, since for Sn2 reactions the values range from 0.95 to 1.06 per a D, while for S l reactions the values are higher. This method is especially good because it provides the minimum of perturbation of the system under study changing from a H to a D hardly affects the reaction, while other probes, such as changing a substituent or the polarity of the solvent, may have a much more complex effect. 

Finally, do appreciate that, depending upon conditions, it is quite possible that both SnI and Sn2 mechanisms might be operating at the same time. 

Technically, for solution-phase nucleophilic substitution reactions, SnI and Sn2 mechanisms are limiting cases on a continuum of possible interactions ... 

This evidently indicates the duality of SnI and Sn2 mechanisms or competitive unimolecular and bimolecular processes, and the scheme of overall reaction is illustrated in Scheme 15 (Kim et al., 1995,1998). 

Unsaturation at the a Carbon. Vinylic, acetylenic, and aryl substrates are very unreactive toward nucleophilic substitutions. For these systems, both the SnI and Sn2 mechanisms are greatly slowed or stopped altogether. One reason that has been suggested for this is that sp (and even more, sp) carbon atoms have a higher electronegativity than sp carbons and thus a greater attraction for the electrons of the bond. As we have seen (p. 388), an p-H bond has a higher acidity than an H bond, with that of an sp H bond in... 

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