Alcohols carbocation rearrangements

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 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]... 

Carbocation rearrangement of primary amines via diazotization to give alcohols through C-C bond migration. 

Hydration of alkenes can also be achieved either by oxymerciiration-reduction (Markovnikov addition of water) or hydroboration-oxidation (a ti-Markovnikov addition of water). Addition of water by oxymercura-tion-reduction or hydroboration-oxidation has two advantages over the acid-catalysed addition of water. These procedures do not require acidic condition, and carbocation rearrangements never occur. Thus, they give high yields of alcohols. 

Compound B has a carbon skeleton different from the alcohol that produced it by dehydration. We are therefore led to consider a carbocation rearrangement. 

A-9. Solvolysis of the compound shown occurs with carbocation rearrangement and yields an alcohol as the major product. Write the structure of this product, and give a mechanism to explain its formation. 

The Focus On box in Chapter 8 on page 298 showed that when carbocations are generated in superacid solution, they undergo extensive rearrangements, usually forming a relatively stable tertiary carbocation. As an example, when 1-butanol is dissolved in superacid at — 60°C, the protonated alcohol is formed. Water does not leave at this temperature because the carbocation that would be formed is primary. When the temperature is raised to 0°C, water leaves but the carbocation rearranges rapidly to the more stable tert-butyl carbocation ... 

The mechanism follows the usual path cyclization of linalyl diphosphate, followed by attack of the n electrons of the second double bond, produce an intermediate carbocation. A carbocation rearrangement occurs, and the resulting carbocation reacts with water to form an alcohol that is oxidized to give a-fenchone. 

Many other chemical modifications have been made to this remarkable series of compounds in the exploration of structure-activity relationships/473) Tertiary alcohols of general structure 214 (R = Me) have been dehydrated(348) by 98-100% formic acid to alkenes (217) via a C-19 carbocation intermediate. The anticipated carbocation rearrangement byproducts were also isolated. Dehydration was directed invariably to the side chain rather than to C-7, and the resultant alkenes were much reduced in analgesic potency. Carbocation rearrangements were catalyzed by concentrated HC1, and various bridged products have been investigated/349 350)... 

Because carbocations are formed in the Sfjl reaction of 2° and 3° alcohols with HX, carbocation rearrangements are possible, as illustrated in Sample Problem 9.4. 

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. 

Similar to the pinacol rearrangement, this mechanism involves a carbocation next to an alcohol, with rearrangement to a protonated carbonyl. Relief of some ring strain in the cyclopropane is an added advantage of the rearrangement. 

Dehydration of tert-alcohols.2 This reaction can be effected with BF3 etherate (1-3 equiv.) in CH2CI2 at 25°. Yields are usually higher than those obtained with the Burgess reagent, thionyl chloride/amine, or p-tolucncsulfonic acid. When dehydration could result in two different alkencs, the more thermodynamically stable alkenc predominates. Noepcntyl tert-alcohols result in mixtures of alkencs, some resulting from a carbocation rearrangement. 

WESTPHALEN - LETRE E Rearrangement Carbocation rearrangement of steroidal tert alcohols (seeWagner-Meerwein). 

In Section 4.5, you learned that water adds to an alkene if an acid catalyst is present. This is the way aUcenes are converted into alcohols industrially. However, under normal laboratory conditions, water is added to an alkene by a procedure known as oxymercuration-reduction. The addition of water by oxymercuration-reduction has two advantages over acid-catalyzed addition It does not require acidic conditions that are harmful to many organic molecules, and because carbocation intermediates are not formed, carbocation rearrangements do not occur. 

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... 

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. 

The relatively harsh conditions (acid and heat) required for alcohol dehydration and the structural changes resulting from carbocation rearrangements may result in low yields of the desired alkene. Dehydration, however, can be carried out under milder conditions by using phosphorus oxychloride (POCI3) and pyridine. 

Reaction with POCI3 converts the OH group of the alcohol into OPOCI2, a good leaving group. The basic reaction conditions favor an E2 reaction, so a carbocation is not formed and carbocation rearrangements do not occur. Pyridine serves as a base to remove the proton in the elimination reaction and to prevent the buildup of HCl, which would add to the alkene. 

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