The SN1 Mechanism

Heterolysis of the C- Br bond forms an intermediate carbocation. This step is rate-determining because it involves only bond cleavage. 

Nucleophilic attack of acetate on the carbocation forms the new C - O bond in the product. This is a Lewis acid-base reaction the nucleophile is the Lewis base and the carbocation is the Lewis acid. Step [2] is fesferthan Step [1] because no bonds are broken and one bond is formed. 

To understand the stereochemistry of the SnI reaction, we must examine the geometry of the carbocation intermediate. 

To illustrate the consequences of having a trigonal planar carbocation formed as a reactive inter-me g t itip l igj g[2 ( alide A having the leaving group bonded to a ster en carbon. 

An energy diagram fortheSNl reaction (CHslsCBr + CHaCOO (CHslsCOCOCHg + Br- 

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POTENTIAL ENERGY DIAGRAMS FOR MULTISTEP REACTIONS THE Sn1 mechanism... 

Reference has already been made in the last chapter to the generation of carbocations, in ion pairs, as intermediates in some displacement reactions at a saturated carbon atom, e.g. the solvolysis of an alkyl halide via the SN1 mechanism. Carbocations are, however, fairly widespread in occurrence and, although their existence is often only transient, they are of considerable importance in a wide variety of chemical reactions. 

Increasing the temperature of the reaction favors reaction by the El mechanism at the expense of the SN1 mechanism. 

The kinetics of the esterification of potassium p-nitrobenzoate by benzyl bromide in dichloromethane-water catalysed by dicyclohexyl-18-crown-6 ([20] + [21]) has been studied by Wong (1978). At low catalyst concentrations (e.g. 5.0 x 10 3 M) he found that 22% of the product was formed by the SN1 and 78% by the SN2 mechanism. Higher catalyst concentrations increased the salt concentration in the organic phase, and the contribution of the SN1 mechanism becomes negligibly small. 

In this, as in many other cases in aqueous solution, OH" plays the role of the base. Note that for compounds such as 1,1,2,2-tetrachloroethane and pentachloroethane, the base catalyzed reaction is important at quite low pH values (/NB = 4.5, i.e., pH at which the neutral and base catalyzed reaction are equally important, see Table 13.7 and Section 13.3). In fact, for polyhalogenated alkanes a small7NB value (e.g., <7) is indicative of an E2 reaction, or, in special cases, of an E1CB reaction see below. Some other examples of compounds reacting by an E2-mechanism include 1,1,2-trichloro-ethane, 1,1,2-tribromoethane, and l,2-dibromo-3-chloroethane (see Table 13.7). A high /NB value (e.g., >10) does not, however, necessarily exclude ( elimination, because this reaction may also occur with water as base, or by an alternative to the SN1 mechanism (i.e., an El mechanism, see below). 

Potential Energy Diagrams for Multistep Reactions The SN1 Mechanism... 

FIGURE 4.12 Potential energy diagram for the reaction of terf-butyl alcohol and hydrogen chloride according to the SN1 mechanism. 

For a proposed reaction mechanism to be valid, the sum of its elementary steps must equal the equation for the overall reaction and the mechanism must be consistent with all experimental observations. The SN1 mechanism set forth in Figure 4.6 satisfies the first criterion. What about the second ... 

The rate-determining step in the SN1 mechanism is dissociation of the alkyloxo-nium ion to the carbocation. 

The SN1 mechanism is generally accepted to be correct for the reaction of tertiary and secondary alcohols with hydrogen halides. It is almost certainly not correct for methyl alcohol and primary alcohols because methyl and primary carbocations are believed to be much too unstable and the activation energies for their formation much too high for them to be reasonably involved. The next section describes how methyl and primary alcohols are converted to their corresponding halides by a mechanism related to, but different from SN1. 

The SN1 mechanism is an ionization mechanism. The nucleophile does not participate until after the rate-determining step has taken place. Thus, the effects of nucleophile and alkyl halide structure are expected to be different from those observed for reactions proceeding by the SN2 pathway. How the structure of the alkyl halide affects the rate of SK1 reactions is the topic of the next section. 

FIGURE 8.6 Energy diagram illustrating the SN1 mechanism for hydrolysis of tert-butyl bromide. 

SAMPLE SOLUTION (a) Isopropyl bromide, (CH3)2CHBr, is a secondary alkyl halide, whereas isobutyl bromide, (CH3)2CHCH2Br, is primary. Because the ratedetermining step in an SN1 reaction is carbocation formation and secondary car-bocations are more stable than primary ones, isopropyl bromide is more reactive Jthan isobutyl bromide in nucleophilic substitution by the SN1 mechanism. ... 

Rearrangements, when they do occur, are taken as evidence for carbocation intermediates and point to the SN1 mechanism as the reaction pathway. Rearrangements are never observed in SN2 reactions. 

Solvent Effects on the Rate of Substitution by the SN1 Mechanism. Table 8.6 lists the relative rate of solvolysis of tert-butyl chloride in several media in order of increasing dielectric constant (e). Dielectric constant is a measure of the ability of a material, in this case the solvent, to moderate the force of attraction between oppositely charged particles compared with that of a standard. The standard dielectric is a vacuum, which is assigned a value e of exactly 1. The higher the dielectric constant e, the better the medium is able to support separated positively and negatively charged species. Solvents... 

Functional group transformations that rely on substitution by the SN1 mechanism are not as generally applicable as those of the SN2 type. Hindered substrates are prone to elimination, and rearrangement is possible when carbocation intermediates are involved. Only in cases in which elimination is impossible are SN1 reactions used for functional group transformations. 

Rate is governed by stability of car-bocation that is formed in ionization step. Tertiary alkyl halides can react only by the SN1 mechanism they never react by the SN2 mechanism. (Section 8.9) Rate is governed by steric effects (crowding in transition state). Methyl and primary alkyl halides can react only by the SN2 mechanism they never react by the SN1 mechanism. (Section 8.6)... 

If the alcohol originally has the d configuration, what configuration would the resulting chloride have if formed (a) by the SN2 mechanism and (b) by the SN1 mechanism ... 

Why do tertiary alkyl compounds ionize so much more rapidly than either secondary or primary compounds The reason is that tertiary alkyl cations are more stable than either secondary or primary cations and therefore are formed more easily. You will appreciate this better by looking at the energy diagram of Figure 8-4, which shows the profile of energy changes for hydrolysis of an alkyl compound, RX, by the SN1 mechanism. The rate of... 

Exercise 8-15 Select the compounds from the following list that would be expected to hydrolyze more rapidly than phenylmethyl (benzyl) chloride by the SN1 mechanism ... 

Suppose one could hydrolyze pure c/s-1-chloro-2-butene exclusively by (a) the SN1 mechanism or (b) the SN2 mechanism. Would you expect the 2-butenol formed in each case to be the cis isomer, the trans isomer, or a mixture Explain. 

Some acid-catalyzed solvolysis reactions of oxacyclopropanes appear to proceed by SN1 mechanisms involving carbocation intermediates. Evidence for the SN1 mechanism is available from the reactions of unsymmetrically substituted oxacyclopropanes. For example, we would expect the conjugate acid of 2,2-dimethyloxacyclopropane to be attacked by methanol at the primary carbon by an SN2 reaction and at the tertiary carbon by an SN1 reaction ... 

The activating effect of Mg2+ upon the cleavage of the phosphoryl group from the ATP could reflect the enhancement of an SN2 reaction at phosphorus by electron withdrawal and charge neutralization via coordination to the metal (equation 1). Support for an SN2 mechanism comes from a consideration57 of the inhibition by vanadate. Coordination of the transferable phosphoryl group would inhibit the SN1 mechanism. 

The slow step, dissociation of the oxonium ion, is unimolecular. The reaction of 2-butanol with hydrogen bromide follows the SN1 mechanism. 

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