In the SnI reaction

A normal, a special, and a negative special salt effect have been detected in the SnI reaction between benzhydryl chloride and LiC104 in y-butyrolactone.71... 

Like other carbocation reactions, the El may be accompanied by rearrangement. Compare the following El reaction (with rearrangement) with the SnI reaction of the same substrate, shown in Mechanism 6-6. Note that the solvent acts as a base in the El reaction and a nucleophile in the SnI reaction. 

Now, the HSO4 is not involved in the rate-determining step—HSO4 is not at all basic and only behaves as a base (that is, it removes a proton) because it is even more feeble as a nucleophile. The rate equation will not involve the concentration of HSO4, and the rate-determining step is the same as that in the SnI reaction—unimolecular loss of water from the protonated t-BuOH. This elimination mechanism is therefore called El. 

If Sn2 is impossible, what about Sjsjl This is possible but very unfavourable. It would involve the unaided loss of the leaving group and the formation of an aryl cation. All the cations we saw as intermediates in the SnI reaction (Chapter 17) were planar with an empty p orbital. This cation is planar but the p orbital is full—it is part of the aromatic ring—and the empty orbital is an sp orbital outside the ring. 

The mechanism of the substitution reaction depends on the structure of the alcohol. Secondary and tertiary alcohols undergo SnI reactions. The carbocation intermediate formed in the SnI reaction has two possible fates It can combine with a nucleophile and form a substitution product, or it can lose a proton and form an elimination product. However, only the substitution product is actually obtained, because any alkene formed in an elimination reaction will undergo a subsequent addition reaction with HX to form more of the substitution product. 

So far we have added heteroatoms only—bromine, nitrogen, or sulfur. Adding a carbon substituent to a reluctant aromatic nucleophile requires reactive carbon electrophiles and that means carbocations. In Chapter 15 you learned that any nucleophile, however weak, will react with a carbocation in the SnI reaction benzene rings are no exception. The classic S l electrophile is the f-butyl cation, which is generated from ferf-butanol with acid. 

The abbreviation El staixls for Elimination, unimolecular. The mechanism is called unimolecular because the rate-limiting transition state involves a single molecule rather than a collision between two molecules. The slow step of an EI reaction is the same as in the SnI reaction unimolecular ionization to form a carbocation. In a fast second step, a base abstracts a proton fr n the carbon atom adjacent to the C . The electrons that once fwmed the carbon-hydrogen bond now form a pi bond between two carbon attxns. The general mechanism for the E1 reaction is shown in the following Key Mechanism box. 

In this reaction, 36 is an intermediate carbocation and the reaction medium contains both water and hydroxide, which are nucleophiles. If a nucleophile attacked carbocation 36, this would constitute an SnI reaction. Therefore, it is reasonable to ask if an SnI reaction using water or hydroxide as a nucleophile can compete with the El reaction. This important question is resolved by considering whether the hydroxide ion is more attracted to Hj, or to C+ in 36. If hydroxide collides with the positive carbon in 36, this nucleophilic reaction is formally analogous to SnI and the product will be 2-methyl-2-butanol. In fact, the attraction of hydroxide to C-i- is largely a function of the solvent, and in a protic solvent such as water, elimination is usually faster. If a base is rather nucleophilic and the reaction is done in an aprotic solvent, the SnI reaction competes with El if there is a carbocation intermediate. If the reaction is done in a protic solvent, elimination is usually faster than in the SnI reaction, but this obviously depends on the nucleophile. 

As anticipated on the basis of the idea that the rate-determining step in the El reaction is the same as that in the SnI reaction, representations such as that of Figures 7.17 and 7.18, shown for the path of El elimination from a 2-halo-2-methyl substrate (L = halogen) to the corresponding alkene product (i.e., 2-methylpropene [(CH3)2C=CH2]), will be strikingly similar to that seen for an SnI reaction on the same substrate. Thus, except for the product-determining step, it would be anticipated that the paths seen in Figures 7.6 (or 7.7) and 7.17 (or 7.18) would be the same. 

Eq. 11.31 shows one example where the only product found in the SnI reaction derives from a carbocation rearrangement. In this example, a primary carbenium ion is formed first, but a methyl migration creates the more stable tertiary carbenium ion. However, the precise timing of such migrations is debated, as we will see below. Since Sn2 reactions are concerted, rearrangements are impossible (Eq. 11.32). 

Sometimes racemization is the actual result, and there is 50% inversion and 50% retention in the product. Usually, however the stereochemical results are not so clean, and an excess of inversion is found. For example, the two optically active molecules of Figure 7.57 give 21 and 18% excess inversion in the SnI reaction with water. 

So, tertiary compounds, which form the most stable, tertiary carbocations, commonly react through the S l mechanism, and methyl and primary compounds, which would have to form extremely unstable carbocations, do not. Naturally enough, secondary compounds react faster than primary ones in the SnI reaction but more slowly than tertiary compounds (Fig. 7.59). 

FIGURE 7.60 The rate-determining step and product-determining step are sharply distinguished in the SnI reaction. 

FIGURE 8.21 The first step in the SnI reaction of tert-huty iodide in water. The formation of the cation is highly endothermic. 

This slowest step is the rate-determining step, sometimes called the rate-limiting step. It is always the step in the reaction with the highest-energy transition state. We cannot measure the rates of faster steps in the reaction. In the SnI reaction, for example, the slowest step is the ionization of the substrate to a carbocation. This ion may then be captured by a host of nucleophiles in following faster steps. There will be a different product for every capturing nucleophile (p. 295 Fig. 8.26). 

The rate-determining step of a reaction is the step with the highest-energy transition state. It may or may not be the same as the product-determining step. For example, in the Sn2 reaction it is the same, but in the SnI reaction it is not. 

PROBLEM 13.25 Why is triptycenyl chloride not nearly as reactive in the SnI reaction as trityl chloride ... 

In the SnI reaction, the rate-determining step is the formation of the carbocation, an endothermic reaction. According to the Hammond postulate, the stability of the carbocation determines the rate of its formation. 

Because carbocations are formed in the SnI reaction of 2° and 3° alcohols with HX, carbocation rearrangements are possible, as illustrated in Sample Problem 9.4. 

Note that in the SnI reaction, which is often carried out under acidic conditions, neutral water is sometimes the leaving group. This occurs, for... 

What about solvent Do solvents have the same effect in S l reactions that they have in 5 2 reactions The answer is both yes and no. Yes, solvents have a large effect on SnI reactions, but no, the reasons for the effects on SnI and Sn2 reactions are not the same. Solvent effects in the Sn2 reaction are due largely to stabilization or destabilization of the nucleophile reactant. Solvent effects in the SnI reaction, however, are due largely to stabilization or destabilization of the transition state. 

El eliminations begin with the same unimolecular dissociation we saw in the SnI reaction, but the dissociation is followed by loss of H" " from the adjacent carbon rather than by substitution. In fact, the El and SnI reactions normally occur together whenever an alkyl halide is treated in a protic solvent with a nonbasic nucleophile. Thus, the best El substrates are also the best SnI substrates, 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-methylpropan-2-ol (SnI) and 2-methylpropene (El) results ... 

Untmolecular reaction (Section 12.8) A reaction that occurs by spontaneous transformation of the starting material without the intervention of other reactants. For example, the dissociation of a tertiary alkyl halide in the SnI reaction is a unimolecular process. 

Figure 3.1 Solvent effects on the ground state and transition state in the SnI reaction of r-butyl chloride (in kcalmor relative to methanol).

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