Electrophilic addition carbocation rearrangements

Evidence for the Mechanism of Electrophilic Additions 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. 

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. 

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... 

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. 

As a simple example, note that the major products obtained as a result of addition of HBr to the alkenes shown below are not always those initially expected. For the first alkene, protonation produces a particularly favourable carbocation that is both tertiary and benzylic (see Section 6.2.1) this then accepts the bromide nucleophile. In the second alkene, protonation produces a secondary alkene, but hydride migration then leads to a more favourable benzylic carbocation. As a result, the nucleophile becomes attached to a carbon that was not part of the original double bond. Further examples of carbocation rearrangements will be met under electrophilic aromatic substitution (see Section 8.4.1). 

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. 

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

Steps 1 and 2 are alkene electrophilic additions, and steps 3 and 4 involve carbocation rearrangements. 

Propagation proceeds by the electrophilic addition of carbenium ions to double bonds with the regeneration of carbocations. The transition state is relatively late, and it was estimated that approximately half of the charge is transferred into the developing carbocation (Chapter 2). This may explain the fact that dormant species (covalent esters and onium ions) do not react directly with alkenes. The charge on the a-C atoms in the dormant species is not sufficient for the formation of the transition state. A multicenter rearrangement process is additionally disfavored by entropy. In contrast, a two-step process in which carbocations are formed and then very rapidly add to alkenes is free of this difficulty. 

Aristolochene, a hydrocarbon found in both pepper and tobacco, is bio-s> nthesized by the following pathway. Add curved arrows to show the mechanism of each step. Which steps involve alkene electrophilic addition(s), and which involve carbocation rearrangement(s) (The abbreviation H—A stands for an unspecified acid, and "Base" is an unspecified base in the enzyme.)... 

---