Carbocation rearrangements halides

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... 

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... 

Rearrangement is especially prevalent with primary alkyl halides of the type RCH2CH2X and R2CHCH2X Aluminum chloride induces ionization with rearrangement to give a more stable carbocation Benzylic halides and acyl halides do not rearrange... 

Well, that depends. You have now seen a few useful carbocation rearrangements that give single products in high yield. But you have also met at least one reaction that cannotbe done because of carbocation rearrangements Friedel-Crafts alkylation using primary alkyl halides. 

Strategy Carbocation rearrangements of alkyl halides occur (1) if the initial carbocation is primary or secondary, and (2) if it is possible for the initial carbocation to rearrange to a more stable secondary or tertiary cation. 

A 2° alcohol reacts with HBr by an SnI mechanism. Because substitution converts a 2° alcohol to a 3° alkyl halide in this example, a carbocation rearrangement must occur. 

Because the carbon skeletons of the starting material and product are different— the alkene reactant has a 4° carbon and the product alkyl halide does not—a carbocation rearrangement must have occurred. 

The result in Equation [1] is explained by a carbocation rearrangement involving a 1,2-hydride shift the less stable 2° carbocation (formed from the 2° halide) rearranges to a more stable 3° carbocation, as illustrated in Mechanism 18.8. 

Alkyl halide Can be methyl, ethyl, 2°, or 3° primary halides undergo carbocation rearrangement. 

As a result of carbocation rearrangement, two alkyl halides are formed—one from the addition of the nucleophile to the unrearranged carbocation and one from the addition to the rearranged carbocation. The major product is the rearranged one. Because a shift of a hydrogen with its pair of electrons is involved in the rearrangement, it is called a hydride shift. (Recall that H is a hydride ion.) More specifically it is called a 1,2-hydride shift because the hydride ion moves from one carbon to an adjacent carbon. (Notice that this does not mean that it moves from C-1 to C-2.)... 

A shift involves only the movement of a species from one carbon to an adjacent electron-deficient carbon 1,3-shifts normally do not occur. Furthermore, if the rearrangement does not lead to a more stable carbocation, then a carbocation rearrangement does not occur. For example, when a proton adds to 4-methyl-1-pentene, a secondary carbocation is formed. A 1,2-hydride shift would form a different secondary carbocation. Because both carbocations are equally stable, there is no energetic advantage to the shift. Consequently, rearrangement does not occur, and only one alkyl halide is formed. 

The addition of hydrogen halides and the acid-catalyzed addition of water and alcohols form carbocation intermediates. Hyperconjugation causes tertiary carbocations to be more stable than secondary carbocations, which are more stable than primary carbocations. A carbocation will rearrange if it becomes more stable as a result of the rearrangement. Carbocation rearrangements occur by... 

When an alkyl halide can undergo substitution by both mechanisms, what conditions favor an SnI reaction What conditions favor an Sn2 reaction These are important questions to synthetic chemists because an Sn2 reaction forms a single substitution product, whereas an SnI reaction can form two substitution products if the leaving group is bonded to an asymmetric carbon. An SnI reaction is further complicated by carbocation rearrangements. In other words, an Sn2 reaction is a synthetic chemist s friend, but an SnI reaction can be a synthetic chemist s nightmare. 

The rate of an SnI reaction depends only on the concentration of the alkyl halide. The halogen departs in the first step, forming a carbocation that is attacked by a nucleophile in the second step. Therefore, carbocation rearrangements can occur. The rate of an SnI reaction depends on the ease of carbocation formation. Tertiary alkyl halides, therefore, are more reactive than secondary alkyl halides since tertiary carbocations are more stable than secondary carbocations. Primary carbocations are so unstable that primary alkyl halides cannot undergo SnI reactions. An SnI reaction takes place with racemization. Most SnI reactions are solvolysis reactions The solvent is the nucleophile. 

Because the reaction of a secondary or a tertiary alcohol with a hydrogen halide is an SnI reaction, a carbocation is formed as an intermediate. Therefore, we must check for the possibility of a carbocation rearrangement when predicting the product of the substitution reaction. Remember that a carbocation rearrangement will occur if it leads to formation of a more stable carbocation (Section 4.6). For example, the major product of the reaction of 3-methyl-2-butanol with HBr is 2-bromo-2-methylbutane, because a 1,2-hydride shift converts the initially formed secondary carbocation into a more stable tertiary carbocation. 

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