Electrophilic addition reaction carbocation rearrangements

What evidence is there to support the carbocation mechanism proposed for the electrophilic addition reaction of alkenes One of the best pieces of evidence was discovered during the 1930s by F. C. Whitmore of the Pennsylvania State University, who found that structural rearrangements often occur during the reaction of HX with an alkene. For example, reaction of HC1 with 3-methyl-1-butene yields a substantial amount of 2-chloro-2-methylbutane in addition to the "expected" product, 2-chloro-3-methylbutane. 

When carbocations are involved as intermediates, carbon skeleton rearrangement can occur during electrophilic addition reactions. Reaction of f-butylethylene with hydrogen chloride in acetic acid gives both rearranged and unrearranged chloride.5... 

In some electrophilic addition reactions, products from carbocation rearrangements are formed. 

Addition of HX (Section 6.3A) HX is used to convert alkenes to haloalkanes in an electrophilic addition reaction. The two-step mechanism involves initial protonation of the alkene tt bond to form a carbocation, which reacts with X to give the product haloalkane. The X atom becomes bonded to the more highly substituted atom of the alkene, so it follows Markovnikov regioselectivity (derived from the preference for forming the more stable carbocation intermediate). Carbocation rearrangements are possible. 

Evidence in support of a carbocation mechanism for electrophilic additions comes from the observation that structural rearrangements often take place during reaction. Rearrangements occur by shift of either a hydride ion, H (a hydride shift), or an alkyl anion, R-, from a carbon atom to the adjacent positively charged carbon. The result is isomerization of a less stable carbocation to a more stable one. 

Many variations of the reaction can be carried out, including halogenation, nitration, and sulfonation. Friedel-Crafts alkylation and acylation reactions, which involve reaction of an aromatic ling with carbocation electrophiles, are particularly useful. They are limited, however, by the fact that the aromatic ring must be at least as reactive as a halobenzene. In addition, polyalkylation and carbocation rearrangements often occur in Friedel-Crafts alkylation. 

Rearrangements may also be observed in these carbocations if they have the appropriate stmctnral featnres. It does not matter how the carbocation is prodnced, subsequent transformations will be the same as we have seen where rearrangements are competing reactions in nucleophilic substitution. Thus, electrophilic addition of HCl to 3,3-dimethylbut-l-ene proceeds via protonation of the alkene, and leads to the preferred secondary rather than primary carbocation (see Section 8.1.1). However, this carbocation may then undergo a methyl migration to produce the even more favonrable tertiary carbocation. Finally, the two carbocations are quenched by reaction with chloride ions. The prodnct mixture is found to contain predominantly the chloride from the rearranged carbocation. 

The study of carbocations has now passed its centenary since the observation and assignment of the triphenylmethyl cation. Their existence as reactive intermediates in a number of important organic and biological reactions is well established. In some respects, the field is quite mature. Exhaustive studies of solvolysis and electrophilic addition and substitution reactions have been performed, and the role of carbocations, where they are intermediates, is delineated. The stable ion observations have provided important information about their structure, and the rapid rates of their intramolecular rearrangements. Modem computational methods, often in combination with stable ion experiments, provide details of the stmcture of the cations with reasonable precision. The controversial issue of nonclassical ions has more or less been resolved. A significant amount of reactivity data also now exists, in particular reactivity data for carbocations obtained using time-resolved methods under conditions where the cation is normally found as a reactive intermediate. Having said this, there is still an enormous amount of activity in the field. 

This chapter presented many reactions. Although most of them follow the same general mechanism in which an electrophile adds to the carbon-carbon double bond, there are several variations on this mechanism. In addition, the regiochemistry and/or the stereochemistry of the reaction may be important. Complications, such as carbocation rearrangements, may occur. You will have an easier time remembering all these details if you organize the reactions according to the three variations of the mechanism that they follow. 

These carbocation rearrangements are similar to those that occur dur- ing electrophilic additions to alkenes (Section 6.12). For example, the relatively unstable primary butyl carbocation produced by reaction of 1-chloro-butane with AICI3 rearranges to the more stable secondary butyl carbocation by shift of a hydrogen atom and its electron pair (a hydride ion, H ) from C2 to Cl. 1... 

This reaction has several limitations and problems. First, the reaction fails for deactivated rings. Second, if it can, the carbocation almost always rearranges to a more stable carbocation. Third, once the alkyl group is attached, it activates the ring for further attack so sequential electrophile additions to give multiple substituted aromatics are possible. The electrophiles are generated in a variety of ways ... 

In the first step of the oxymercuration mechanism, the electrophilic mercury of mercuric acetate adds to the double bond. (Two of mercury s 5d electrons are shown.) Because carbocation rearrangements do not occur, we can conclude that the product of the addition reaction is a cyclic mercurinium ion rather than a carbocation. The reaction is analogous to the addition of Br2 to an alkene to form a cyclic bromonium ion. 

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