Reactions and Rearrangement Involving Carbocation Intermediates

In this section, the emphasis is on carbocation reactions that modify the carbon skeleton, including carbon-carbon bond formation, rearrangements, and fragmentation reactions. The fundamental structural and reactivity characteristics of carbocations toward nucleophilic substitution were explored in Chapter 4 of Part A. 

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This reaction is related to the Wagner-Meerwein Rearrangement, Demjanov Rearrangement, Tiffeneau-Demjanov Ring Expansion and Retropinacol Rearrangement involving carbocation intermediates. 

Functional group transformations that rely on substitution by the S l mechanism are not as generally applicable as those of the Sn2 type. Hindered substrates are prone to elimination, and there is the possibility of rearrangement when carbocation intermediates are involved. Only in cases in which elimination is impossible are S l reactions used for functional group transformations. 

The mechanism of these reactions often involves carbocation intermediates, and the first-formed cation (e.g. primary or secondary) can rearrange to a more stable cation (e.g. tertiary). 

The acid-catalyzed dehydration of alcohols with H2SO4 or TsOH yields alkenes, too (Sections 9.8 and 9.9). The reaction occurs via an El mechanism for 2° and 3° alcohols, and an E2 mechanism for 1° alcohols. El reactions involve carbocation intermediates, so rearrangements are possible. Dehydration can also be carried out with POCI3 and pyridine by an E2 mechanism (Section 9.10). 

Unlike carbocations, however, less stable radicals generally do not rearrange to more stable radicals. This difference can be used to distinguish between reactions involving radical intermediates and those involving carbocations. 

In this section, we will discuss reactions which involve carbocation intermediates. These include carbon-carbon bond-forming reactions and also rearrangements and fragmentation reactions. The discussion will also include processes which are closely related in reactivity pattern but which avoid free carbocation intermediates. 

The reactions of copper salts with diacyl peroxides have been investigated quite thoroughly, and the mechanistic studies indicate that both radicals and carbocations are involved as intermediates. The radicals are oxidized to carbocations by Cu(II), and the final products can be recognized as having arisen from carbocations because characteristic patterns of substitution, elimination, and rearrangement can be discerned " ... 

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... 

Carbocation intermediates are involved and the structure and stereochemistry of the product are determined by the factors that govern substituent migration in the carbocation. Clean, high-yield reactions can be expected only where structural or conformational factors promote a selective rearrangement. Boron trifluoride is frequently used... 

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