Characteristics of the SN1 Reaction

According to the Hammond postulate (Section 6.10), any factor that stabilizes a high-energy intermediate also stabilizes the transition state leading to that inlermediate. Since the rate-limiting step in an S l reaction is the spontaneous, unimolecLilar dissociation of the substrate to yield a carbocation, the reaction is favored whenever a stabilized carbocation intermediate is formed. The more stable the carbocation intermediate, the faster the S l reaction. 

We saw in Section 6.9 that the stability order of alkyl carbocations is 3° 2° 1° —CH3. To this list we must also add the resonance-stabilized allvl and benzyl cations. Just as allylic radicals are unusually stable because the 

Because of resonance stabilization, a primary allylic or benzylic carbocation is about as stable as a secondary alkyl carbocation and a secondary allylic or benzylic carbocation is about as stable as a tertiary alkyl carbocation. This stability order of carbocations is the same as the order of S l reactivity for alkyl halides and tosylates. 

Methyl Primary Allylic Benzylic = Secondary Tertiary 

Parenthetically we might also note that primary allylic and benzylic substrates are particularly reactive in S 2 reactions as well as in SN-1 reactions. 

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The effects predicted are qualitative at best. There are other factors that must be taken into account when predicting how various characteristics of the metal and ligand affect substitution reactions. For example, increasing the size of the metal ion is predicted to assist the formation of the transition state in SN1,... 

The mechanisms of these acid-catalyzed epoxide openings are more complex than they at first appear. They seem to be neither purely SN1 nor SN2 but instead to be midway between the two extremes and to have characteristics of both. Take the reaction of 1,2-epoxy-l-methylcyclohexane with HBr shown in Figure 18.2, for instance. The reaction yields only a single stereoisomer of 2-bromo-2-methyl-cyclohexanol in which the —Br and —OH groups are trans, an S 2-li.ke result caused by backside displacement of the epoxide oxygen. But the fact that Br attacks the more hindered tertiary side of the epoxide rather than the less hindered secondary side is an SN1 -like result in which the more stable, tertiary carbocation is involved. 

The pronounced proclivity of phosphoric monoester monoanions to eliminate POf is not always recognizable from the characteristic pH profile of Fig. 1. The hydrolysis rate maximum at pH w 4 may be masked by a faster reaction of the neutral phosphoric ester, as in the case of a-D-glucose 1-phosphate63) or on hydrolysis of monobenzyl phosphate 64). In the latter case, the known ability of benzyl esters to undergo SN1 and SN2 reactions permits fast hydrolysis of the neutral ester with C/O bond breakage. The fact that the monoanion 107 of the monobenzyl ester is hydrolyzed some 40 times faster than the monoanion 108 of the dibenzyl ester at the same pH again evidences the special hydrolysis pathway of 107, rationalized by means of the metaphosphate hypothesis. 

However, kobs versus [C5H5N] is linear in the pH region used (5.2-5.3) in which the water reaction would be expected to be most important, and also AS = — 27 eu for the water reaction, whereas SN1 ester hydrolyses characteristically have values of AS of 0-10 eu. 

Platinum(IV) is kinetically inert, but substitution reactions are observed. Deceptively simple substitution reactions such as that in equation (554) do not proceed by a simple SN1 or 5 2 process. In almost all cases the reaction mechanism involves redox steps. The platinum(II)-catalyzed substitution of platinum(IV) is the common kind of redox reaction which leads to formal nucleophilic substitution of platinum(IV) complexes. In such cases substitution results from an atom-transfer redox reaction between the platinum(IV) complex and a five-coordinate adduct of the platinum(II) compound (Scheme 22). The platinum(II) complex can be added to the solution, or it may be present as an impurity, possibly being formed by a reductive elimination step. These reactions show characteristic third-order kinetics, first order each in the platinum(IV) complex, the entering ligand Y, and the platinum(II) complex. The pathway is catalytic in PtnL4, but a consequence of such a mechanism is the transfer of platinum between the catalyst and the substrate. 10 This premise has been verified using a 195Pt tracer.2011... 

The outstanding chemical characteristic of alkenyl halides is their general inertness in SN1 and SN2 reactions. Thus chloroethene fails to react with silver nitrate in ethanol (i.e., low SN1 reactivity), fails to react with potassium iodide in acetone (i.e., low SN-2 reactivity), and only reacts slowly with sodium hydroxide to give ethyne (low E2 reactivity). The haloalkynes, such as RC=C—Cl, are similarly unreactive. 

Up to this point we have confined our discussion of nucleophilic substitution to those reactions that appear to follow either an extreme Sw2 process (Chapter 4) or the limiting SN1 path. There is a middle ground many substitutions have some of the characteristics of each extreme but belong to neither. 

Aryl halides are relatively unreactive toward nucleophilic substitution reactions. This lack of reactivity is due to several factors. Steric hindrance caused by the benzene ring of the aryl halide prevents SN2 reactions. Likewise, phenyl cations are unstable, thus making SN1 reactions impossible. In addition, the carbon-halogen bond is shorter and therefore stronger in aryl halides than in alkyl halides. The carbon-halogen bond is shortened in aryl halides for two reasons. First, the carbon atom in aryl halides is sp2 hybridized instead of sp3 hybridized as in alkyl halides. Second, the carbon-halogen bond has partial double bond characteristics because of resonance. 

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