And the SnI reaction

The SnI and Sn2 reactions allow the conversion of alkyl halides into a host of new structures. These reactions were summarized earlier in Figure 7.94. Remember that the Sn2 reaction works only for primary and secondary halides and the SnI reaction works only for tertiary and secondary molecules. 

When a molecule has in an allylic position a nucleofuge capable of giving the SnI reaction, it is possible for the nucleophile to attack at the y position instead of the a position. This is called the SnI mechanism and has been demonstrated on 2-buten-l-ol and 3-buten-2-ol, both of which gave 100% allylic rearrangement when treated... 

TABLE 10.5 Relative Rates for the SnI Reaction Between ROTs and Ethanol at 25° C... 

Bridgeheads. The Sn2 mechanism is impossible at most bridgehead compounds (p. 392). Nucleophilic attack in [l.l.l]propellane has been reported, however. In general, a relatively large ring is required for an SnI reaction to take place (p. 396). " The SnI reactions have been claimed to occur for l-iodobicyclo[l.l.l]pentane via the bicyclo[l.l.l]pentyl cation, but this has been disputed and the bicyclo[1.1.0]butyl carbinyl cation was... 

The SnI reactions do not proceed at bridgehead carbons in [2.2.1] bicyclic systems (p. 397) because planar carbocations cannot form at these carbons. However, carbanions not stabilized by resonance are probably not planar SeI reactions should readily occur with this type of substrate. This is the case. Indeed, the question of carbanion stracture is intimately tied into the problem of the stereochemistry of the SeI reaction. If a carbanion is planar, racemization should occur. If it is pyramidal and can hold its structure, the result should be retention of configuration. On the other hand, even a pyramidal carbanion will give racemization if it cannot hold its structure, that is, if there is pyramidal inversion as with amines (p. 129). Unfortunately, the only carbanions that can be studied easily are those stabilized by resonance, which makes them planar, as expected (p. 233). For simple alkyl carbanions, the main approach to determining structure has been to study the stereochemistry of SeI reactions rather than the other way around. What is found is almost always racemization. Whether this is caused by planar carbanions or by oscillating pyramidal carbanions is not known. In either case, racemization occurs whenever a carbanion is completely free or is symmetrically solvated. 

The SnI reaction of (S)-3 -brom o-3 -m ethyl hexane proceeds with racemization because the intermediate carbocation is achiral and attacked by the nucleophile can occur from either side. 

Now, to return to the hydrolysis of the secondary alkyl halides, we will call the reactions (1) and (2), where the 1 relates to the SnI reaction and the 2 relates to the Sn2 reactions. (And we write the numbers with brackets to avoid any confusion, i.e. to prevent us from thinking that the T and 2 indicate first- and second-order reactions respectively.) We next say that the rate constants of the two concurrent reactions are k(p and k(2) respectively. As the two reactions proceed with the same 1 1... 

In the above expression, the quantity to the right of the first equal sign indicates that the order of the reaction with respect to the specific reactants is one and zero, respectively and because the order with respect to hydroxide is zero, then [OH ]° = 1, and the overall reaction order is the sum of the orders of the respective reactants. Hence, a SnI reaction is first-order overall. 

SnI reaction means substitution nucleophilic unimolecular. The SnI reaction occurs in two steps, with the first being a slow ionization reaction generating a carbocation. Thus, the rate of an S l reaction depends only on the concentration of the alkyl halide. First, the C—X bond breaks without any help from the nucleophile, and then there is quick nucleophilic attack by the nucleophile on the carbocation. When water or alcohol is the nucleophile, a quick loss of a proton by the solvent gives the hnal product. For example, the reaction of t-butylbromide and methanol gives t-butyl methyl ether. 

Stereochemistry of the SnI reactions The SnI reaction is not stereospecific. The carbocation produced is planar and 5p -hybridized. For example, the reaction of (S)-2-bromobutane and ethanol gives a racemic mixture, (S)-2-butanol and (R)-2-butanol. 

Shafizadeh5 has suggested that a unimolecular (SNO reaction is operative. In this type of reaction, the rate-determining step would be the formation of a carbonium ion, with the removal of the ethylthio group subsequent attack on this ion by the nucleophile would be rapid. In an SNj reaction, the removal of the ethylthio group and the attack by the nucleophile would be simultaneous. The SNi reaction seems more probable here. 

An overview of the reactions over zeolites and related materials employed in the fields of refining, petrochemistry, and commodity chemicals reviewed the role of carbocations in these reactions.15 An overview appeared of the discovery of reactive intermediates, including carbocations, and associated concepts in physical organic chemistry.16 The mechanisms of action of two families of carcinogens of botanical origin were reviewed.17 The flavanoids are converted to DNA-reactive species via an o-quinone, with subsequent isomerization to a quinone methide. Alkenylbenzenes such as safrole are activated to a-sulfatoxy esters, whose SnI ionization produces benzylic cations that alkylate DNA. A number of substrates (trifluoroacetates, mesylates, and triflates) known to undergo the SnI reaction in typical solvolysis solvents were studied in ionic liquids several lines of evidence indicate that they also react here via ionization to give carbocationic intermediates.18... 

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... 

At carbon, most leaving groups are good in SN2 reactions these go with inversion. At carbon, most leaving groups are poor in SE2 reactions these go with retention. As for the SNi reaction, this is usually strongly assisted, e.g. in (129) and (130) by the formation of sulfur dioxide (Mackenzie, 1964 de la Mare, 1963). The stereo-... 

An example of a reaction with rearrangement is the SnI reaction of 2-bromo-3-methylbutane in boiling ethanol. The product is a mixture of 2-ethoxy-3-methylbutane (not rearranged) and 2-ethoxy-2-methylbutane (rearranged). 

Stereochemistry The SnI reaction involves a flat carbocation intermediate that can be attacked from either face. Therefore, the SN1 usually gives a mixture of inversion and retention of configuration. 

The El reaction almost always competes with the SnI reaction. Whenever a carbocation is formed, it can undergo either substitution or elimination, and mixtures of products often result. The following reaction shows the formation of both elimination and substitution products in the reaction of tert-butyl bromide with boiling ethanol. 

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. 

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