And SnI reactions

Mannosides are difficult to obtain since here a 2-O-acyl group blocks the -position. 2-O-Benzyl-a-mannosyl bromides give, however, high yields of pure -glycosides with a heterogeneous silver silicate catalyst preventing anomerization and SnI reaction of the bromide H. Paulsen, 1981 B, Q. 

In type A, the adduct loses water (or, in the case of addition to C=NH, ammonia, etc.), and the net result of the reaction is the substitution of C=Y for C=0 (or C= NH, etc.). In type B, there is a rapid substitution, and the OH (or NH2, etc.) is replaced by another group Z, which is often another YH moiety. This substitution is in most cases nucleophilic, since Y usually has an unshared pair and SnI reactions occur very well on this type of compound (see p. 434), even when the leaving group is as poor as OH or NH2. In this chapter, we shall classify reactions according to what is initially adding to the carbon-hetero multiple bond, even if subsequent reactions take place so rapidly that it is not possible to isolate the initial adduct. 

In both Sn2 and SnI reactions, a nucleophile is attacking an electrophile, giving a substitution reaction. That explains the Sn part of the name. But what do the 1 and 2 stand for To see this, we need to look at the mechanisms. Let s start with Sn2 ... 

Bromomethylhenzene or henzylhromide can he subjected to further nucleophilic reactions. Bromine can be replaced by a variety of nucleophiles by means of an Sn2 and SnI reaction, resulting in various monosubstituted benzenes. 

The actual values of the rate constants are not important. The graph in Figure 17.1 has been plotted to put the rates of the Sn2 and SnI reactions of the secondary alkyl... 

Owing to the similarity of solvent effects for Ei and SnI reactions due to the common rate-controlling step, only some examples of E2 reactions are mentioned. Typical examples of the reaction types (c) to (f) in Table 5-6 are shown in Eqs. (5-22) to (5-25). 

According to the Hughes-Ingold rules, in E2 and Sn2 reactions the dispersal of charge is spread over more atoms than in Ei and SnI reactions, so increase in solvent... 

The E2 and El mechanisms differ in the timing of bond cleavage and bond formation, analogous to the Sn2 and SnI mechanisms. In fact, E2 and Sn2 reactions have some features in common, as do El and SnI reactions. 

I he fact that the rate is independent of the base concentration is interpreted as it was in Sec. 14.12 for the SnI reaction indeed, we see that the rate-determining step in El and SnI reactions is exactly the same. It follows that the order of reactivity of halides should be the same as in SnI reactions—and it is. 

El eliminations begin with the same unimolecular dissociation we saw in the S i reaction, but the dissociation is followed by loss of H from the intermediate carbocation rather than by substitution. In fact, the El and SnI reactions normally occur in competition whenever an alkyl halide is treated in a protic solvent with a nonbasic nucleophile. Thus, the best El substrates are also the best siibstrates, and mixtures of substitution and elimination products are usually obtained. For example, when 2-chloro-2-methylpropane is warmed to 65 C in 80% aqueous ethanol, a 64 36 mixture of 2-methyl-2-propanol (S[jl) and 2-methylpropene (El) results ... 

---