Sn2 and SN1 Reactions

The reaction of an alkyl halide or los3 late with a nucleophiJe/base results eithe in substitution or in diminution. Nucleophilic substitutions are of two types S 2 reactions and SN1 reactions, in the SN2 reaction, the entering nucleophih approaches the halide from a direction 180° away from the leaving group, result ing in an umbrella-like inversion of configuration at the carbon atom. The reaction is kinetically second-order and is strongly inhibited by increasing stork bulk of the reactants. Thus, S 2 reactions are favored for primary and secondary substrates. 

Sn2 reactions take place with inversion of configuration, and SN1 reaction take place with racemization. The following substitution reaction, howeve occurs with complete retention of configuration. Propose a mechanism. 

Thus the salient difference between reaction by the SN2 and SN1 pathways is that SN2 proceeds in one step only, via a transition state while SN1 proceeds in two steps, via an actual (carbocation) intermediate. 

Reference has already been made (p. 82) to the fact that the reactions of some substrates, e.g. secondary halides, may follow a mixed first/second order rate equation. The question then arises whether such a reaction is proceeding via both SN2 and SN1 pathways simultaneously (their relative proportions depending on the solvent, etc.) or whether it is proceeding via some specific, in between mechanistic pathway. 

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 SN1 mechanisms respectively. Label each reaction as SN2 or SN1 as appropriate. 

Because the reactions we consider in this section are single-step and therefore elementary reactions, the rate law specified in Section 2.3 as Equation 2.1 is obtained for the rate of formation of the substitution product Nu—R in Figure 2.4. It says that these reactions are bimolecular substitutions. They are consequently referred to as SN2 reactions. The bimolecu-larity makes it possible to distinguish between this type of substitution and SN1 reactions, which we will examine in Section 2.5 nucleophile concentration affects the rate of an SN2 reaction, but not an SN1 reaction. 

Furfuryl chloride can undergo substitution by both SN2 and SN1 mechanisms. Since it is a 1° alkyl halide, we expect SN2 but not SN1 reactions. Draw a mechanism for the SN1 reaction shown below, with careful attention to the structure of the intermediate. How can this primary halide undergo SN1 reactions Why is there no competition with E2 or El mechanisms ... 

Phenyl Ethers Phenyl ethers (one of the groups bonded to oxygen is a benzene ring) react with HBr or HI to give alkyl halides and phenols. Phenols do not react further to give halides because the s/r-hybridized carbon atom of the phenol cannot undergo the Sn2 (or SN1) reaction needed for conversion to the halide. 

Overall, we have retention of stereochemistry. As you know, Sn2 reactions go with inversion, and SN1 reactions with loss of stereochemical information—so this result is possible only if we have two sequential Sn2 reactions taking place—in other words neighbouring group participation. 

Competitive SN1, SN2, and E2 reactions of 1-bromo-2-phenylpropane 195) have also been investigated in the absence and presence of cationic surfactants 190 ... 

Ethanol is a poor nucleophile and a weak base, and hence SN2 and E2 reactions are unlikely. As it is a polar protic solvent, El and SN1 reactions are favoured and hence an initial carbocation can be formed on the loss of Br (see below). This secondary carbocation can undergo rearrangement to form a more stable tertiary carbocation, which can react with ethanol to give E and F. 

Four types of mechanisms are inherent to Organic Chemistry I. These are substitution reaction mechanisms (SN1 and SN2) and elimination reaction mechanisms (El and E2). The principles of these four types apply to Organic Chemistry II, and no review would be complete without a few reminders about these processes. 

After a section on industrial methods, a review covering SN2 and SN1 routes to alcohols is presented. Primary alcohols may be prepared by SN2 displacement reactions of HO with appropriate substrates (e.g., primary haloalkanes). This approach sometimes works for secondary systems, but elimination often interferes. To a limited extent, both secondary and tertiary alcohols may be formed in SN1 reactions with water as the nucleophile. However, the chemistry described in the remainder of the chapter provides much more versatile and reliable means of synthesizing alcohols. 

LFER. Consider the Sn2 reactions of XC6H4CH2CI with I- (ki) and the SN1 reactions. with OH (fc0H)- The reaction constants are given in Table 10-2. Sketch the appearance of a plot of log ki versus log kon- What is its slope ... 

The etherification between alcohol 10 and imidate 67 was one of the key transformations in the successful preparation of compound 1. The use of HBF4 as the catalyst for the etherification was crucial for obtaining high levels of diastereose-lectivity and relatively high conversion to the desired product 18. The fact that sec-sec ethers have rarely, if ever, been obtained with high levels of diastereocontrol in Sn2 fashion under typical SN1 reaction conditions prompted us to investigate the complex mechanistic details of this exceptional reaction. 

In steric terms there is a relief of crowding on going from the initial halide, with a tetrahedral disposition of four substituents about the sp3 hybridised carbon atom, to the carbocation, with a planar disposition of only three substituents (cf. five for the SN2 T.S.) about the now sp2 hybridised carbon atom. The three substituents are as far apart from each other as they can get in the planar carbocation, and the relative relief of crowding (halide - carbocation) will increase as the substituents increase in size (H- Me- Me3C). The SN1 reaction rate would thus be expected to increase markedly (on both electronic and steric grounds) as the series of halides is traversed. It has not, however, proved possible to confirm this experimentally by setting up conditions such that the four halides of Fig. 4.1 (p. 82) all react via the SN1 pathway. 

Acid anhydrides, (RC0)20, will also often react with weaker nucleophiles, though more slowly than acid chlorides neither SN1 nor Sn2 types of reaction pathway normally occurs. Anhydrides are essentially intermediate in reactivity—towards a particular nucleophile—between acid chlorides and esters, reflecting the leaving group ability sequence ... 

The role of steric effects is unclear but the anomeric effect could also contribute to an increase in electron density at nitrogen. X-ray data for the two TV-acyloxy-TV-alkoxyamides, a urea and a carbamate outlined above show clear evidence, both from bond lengths and conformations, of an anomeric interaction RO-N bonds are short when compared to alkoxyamines. This interaction is responsible for SN1, SN2, homolytic and rearrangement reactions of /V-acyloxy-TV-alkoxyamides (vide infra) and has also been supported computationally. Acyloxylation of the hydroxamic esters results in both pyramidalisation as well as anomeric donation from the... 

Other terms that he invented include the system of classification for mechanisms of aromatic and aliphatic substitution and elimination reactions, designated SN1, SN2, El, and E2. "S" and "E" refer to substitution and elimination, respectively, "N" to nucleophilic, and "1" and "2" to "molecularity," or the number of molecules involved in a reaction step (not kinetic order, having to do with the equation for reaction rate and the concentration of reactants). Ingold first introduced some of these ideas in 1928 in a... 

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