SN1 and Sn2 mechanisms,

Two reaction mechanisms, such as SN1 and SN2 mechanisms, seem to be possible for explaining formations of 158a-c (Scheme 25). The former requires a resonance-stabilized indolyl cation 165 as an intermediate, while the latter indicates the presence of a transition state like 167. The introduction of a methoxy group into the 5 position of 165 should stabilize the corresponding cation 166, in which nucleophilic substitution on indole nitrogen would become a predominant pathway. 

It is now easy to imagine that depending on the nucleophile and on various steric (e.g., steric hindrance) and electronic (e.g., stabilization by conjugation) factors, the relative importance of the nucleophile may well lie somewhere in between these two extremes. We may, therefore, simply look at such cases as exhibiting properties intermediate between SN1 and SN2 mechanisms. 

Because the SN1 and SN2 mechanisms are so different from each other, let s examine each one separately. 

Comparison of SN1 and SN2 Mechanisms of Nucleophilic Substitution in Alkyl Halides... 

The role of nitronium ion in the nitration of benzene was demonstrated by Sir Christopher Ingold—the same person who suggested the SN1 and Sn2 mechanisms of nucleophilic substitution and who collaborated with Cahn and Prelog on the ft and S notational system. 

To distinguish between SN1 and SN2 mechanisms of solvolysis requires other criteria, notably stereochemistry (Sections 8-5 and 8-6), and the elfect of added nucleophiles on the rate and nature of the reaction products. For example, it often is possible to distinguish between SN1 and SN2 solvolysis by adding to the reaction mixture a relatively small concentration of a substance that is expected to be a more powerful nucleophile than the solvent. If the reaction is strictly SN1, the rate at which RX disappears should remain essentially unchanged because it reacts only as fast as R forms, and the rate of this step is not changed by addition of the nucleophile, even if the nucleophile reacts with R . However, if the reaction is SN2, the rate of disappearance of RX should increase because RX reacts with the nucleophile in an SN2 reaction and now the rate depends on both the nature and the concentration of the nucleophile. (See Exercises 8-5 and 8-6.)... 

Reactivities comparable to allylic halides are found in the nucleophilic displacement reactions of benzylic halides by SN1 and SN2 mechanisms (Table 14-6). The ability of the benzylic halides to undergo SK1 reactions clearly is related to the stability of the resulting benzylic cations, the electrons of which are extensively delocalized. Thus, for phenylmethyl chloride,... 

Nucleophilic substitution nucleophile (Nu-) seeks a "+" center (C of R group or >C=0), displaces leaving group -L. SN1 and SN2 mechanisms... 

Scheme 2.25 Alternative Sn1 and Sn2 mechanisms for the reaction of p,p -dimethylbenzhydryl chloride (64) in aqueous acetone containing sodium azide.

We have already mentioned substitution reactions at saturated carbon atoms the familiar SN1 and SN2 mechanisms of the organic chemist. In the latter case, the five-coordinate intermediate scarcely has any real... 

An understanding of electron transfer and proton transfer is of great importance to the chemist, the first because it is concerned with a change in oxidation state and the second because to transform an organic compound it is usually necessary to disrupt the skin of hydrogen atoms that protect the compound. Equally familiar to the well-educated chemist are nucleophilic substitution reactions together with their SN1 and SN2 mechanisms. In recent 1 For the present addresses see p. v. 

Further support for this interpretation can be found by considering the data for the hydrolysis of isopropyl compounds in Table 28. The values of r are calculated from (112). It can be seen that the transition states for the isopropyl transfers are much looser than those for the methyl transfers. The solvolysis of isopropyl compounds is closer to the borderline between the SN1 and SN2 mechanisms and therefore we may expect the SN2 transition state to be looser. As discussed above there is supporting evidence from Ko and Parker s (1968) measurements of transfer activity coefficients for the transition state. 

Figure 8.1 Functional groups available from alkyl halides via SN1 and SN2 mechanisms.

This reaction will show competition between SN1 and SN2 mechanisms due to the fact that this center is less hindered than a tertiary center but more hindered than a primary center. An SN1 mechanism will be favored using highly polar, aprotic solvents to stabilize the forming carbocation. An SN2 mechanism will be favored when nonpolar solvents are used. 

Two mechanisms of isotopic exchange in alcohols can he considered, corresponding to the SN1 and SN2 mechanisms of nucleophilic substitution. Water is the nucleophile in this case, attacking a saturated carbon atom. In all the studies reported so far, exchange occurs only in acid solution and it has always been assumed that the conjugate acid of the alcohol is the reacting species. 

The basic concepts of nucleophilic substitution reactions appeared in the first semester of organic chemistry. These reactions follow SN1 or SN2 mechanisms. (In aromatic nucleophilic substitution mechanism, we use the designation SNAr.) In SN1 and SN2 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. 

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