Rearrangements, carbocations

Why do carbocations rearrange The answer is straightforward once we recall that tertiary carbocations are more stable than secondary carbocations (Section 4 10) Thus rearrangement of a secondary to a tertiary carbocation is energetically favorable As... 

Like alcohol dehydrations El reactions of alkyl halides can be accompanied by carbocation rearrangements Eliminations by the E2 mechanism on the other hand nor mally proceed without rearrangement Consequently if one wishes to prepare an alkene from an alkyl halide conditions favorable to E2 elimination should be chosen In prac tice this simply means carrying out the reaction m the presence of a strong base... 

Alkene synthesis via alcohol dehydration is complicated by carbocation rearrangements A less stable carbocation can rearrange to a more sta ble one by an alkyl group migration or by a hydride shift opening the possibility for alkene formation from two different carbocations... 

Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes... 

CARBOCATION REARRANGEMENTS IN HYDROGEN HALIDE ADDITION TO ALKENES... 

Carbocations usually generated from an alkyl halide and aluminum chloride attack the aromatic ring to yield alkylbenzenes The arene must be at least as reactive as a halobenzene Carbocation rearrangements can occur especially with primary alkyl hal ides... 

Hydride shift (Section 5 13) Migration of a hydrogen with a pair of electrons (H ) from one atom to another Hydnde shifts are most commonly seen in carbocation rearrange ments... 

The alkyl-bridged structures can also be described as comer-protonated cyclopropanes, since if the bridging C—C bonds are considered to be fully formed, there is an extra proton on the bridging carbon. In another possible type of structure, called edge-protonated cyclopropanes, the carbon-carbon bonds are depicted as fully formed, with the extra proton associated with one of the bent bonds. MO calculations, structural studies under stable-ion conditions, and product and mechanistic studies of reactions in solution have all been applied to understanding the nature of the intermediates involved in carbocation rearrangements. 

Carbocation rearrangements occur in the reactions of some secondary alco hols with DAST, thus isobutyl alcohol gives a mixture of isobutyl fluoride and tert-hxAy] fluonde [95] (Table 6), and both bomeol and isoborneol rearrange to the same 3-fluoro-2 2,3-tnraethylbicyclo[2 2 IJheptane (72-74%) accompanied by camphene [95]... 

ThomsonNOW Click Organic Interactive to use a web-based palette to predict products from simple carbocation rearrangements. 

Evidence for the Mechanism of Electrophilic Additions Carbocation Rearrangements... 

Carbocation rearrangements can also occur by the shift of an alkyl group with its ejection pair. For example, reaction of 3,3-dimethyJ-l-butene with HCI Leads to an equal mixture of unrearranged 2-chloro-3,3-dimethyTbutane and rearranged 2-chloro-2,3-dimethyibutane. In this instance, a secondary carbocation rearranges to a more stable tertiary carbocation by the shift of a methyl group. 

The (Ticdcl-Crafts reaction of benzene with 2-chloro-3-methylbutane in the presence of AICI3 occurs with a carbocation rearrangement. What is the structure of the product ... 

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. 

Isoborneol (Problem 27.37) is converted into camphene on treatment with dilute sulfuric acid. Propose a mechanism for the reaction, which involves a carbocation rearrangement. 

Propose a mechanism for the biosynthesis of the sesquiterpene trichodiene from famesyl diphosphate. The process involves cyclization to give an intermediate secondary carbocation, followed by several carbocation rearrangements. 

---