Carbocation rearrangements primary alcohol rearrangement

Unbranched primary alcohols and tertiary alcohols tend to react with hydrogen halides without rearrangement The alkyloxonmm ions from primary alcohols react rap idly with bromide ion for example m an Sn2 process Tertiary alcohols give tertiary alkyl halides because tertiary carbocations are stable and show little tendency to rearrange... 

This elimination reaction is the reverse of acid-catalyzed hydration, which was discussed in Section 6.2. Because a carbocation or closely related species is the intermediate, the elimination step would be expected to favor the more substituted alkene as discussed on p. 384. The El mechanism also explains the general trends in relative reactivity. Tertiary alcohols are the most reactive, and reactivity decreases going to secondary and primary alcohols. Also in accord with the El mechanism is the fact that rearranged products are found in cases where a carbocation intermediate would be expected to rearrange ... 

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

The prominent role of alkyl halides in formation of carbon-carbon bonds by nucleophilic substitution was evident in Chapter 1. The most common precursors for alkyl halides are the corresponding alcohols, and a variety of procedures have been developed for this transformation. The choice of an appropriate reagent is usually dictated by the sensitivity of the alcohol and any other functional groups present in the molecule. Unsubstituted primary alcohols can be converted to bromides with hot concentrated hydrobromic acid.4 Alkyl chlorides can be prepared by reaction of primary alcohols with hydrochloric acid-zinc chloride.5 These reactions proceed by an SN2 mechanism, and elimination and rearrangements are not a problem for primary alcohols. Reactions with tertiary alcohols proceed by an SN1 mechanism so these reactions are preparatively useful only when the carbocation intermediate is unlikely to give rise to rearranged product.6 Because of the harsh conditions, these procedures are only applicable to very acid-stable molecules. 

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 ... 

A wide variety of aliphatic tertiary and secondary alcohols can be ionized to the corresponding alkyl cations by use of magic acid [46-52]. Formation of the t-butyl cation (Eq. 23) [46] and a cyclopropyl-stabilized di-cation [53] are representative examples. Primary (and some secondary) alcohols are protonated only at temperatures lower than -60 °C [54]. At more elevated temperatures, they might cleave to give the corresponding carbocations, which, however, immediately rearrange to the more stable tertiary cations [49,50]. 

The primary alcohol 16 reacts with HBr to give the rearranged bromide 19 (reaction 5.16). Protonation will give the oxonium ion 17 as the water molecule leaves in the second step, the methyl group migrates so that the tertiary carbocation 18 is formed, which adds a bromide ion to give the final product. 

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... 

Although the dehydration of a primary alcohol is an E2 reaction and therefore does not form a carbocation intermediate, the product obtained in most cases is identical to the product that would be obtained if a carbocation had been formed in an El reaction and then had rearranged. For example, we would expect 1-butene to be the product of the E2 dehydration of 1-butanol. However, we find that the product is actually... 

Dehydration requires an acid catalyst the order of reactivity of alcohols is tertiary > secondary > primary. Elimination is regioselective and proceeds in the direction that produces the most highly substituted double bond. When stereoisomeric alkenes are possible, the more stable one is formed in greater amounts. An El (elimination unimolecular) mechanism via a carbocation intermediate is followed with secondary and tertiary alcohols. Primary alcohols react by an E2 (elimination bimolecular) mechanism. Sometimes elimination is accompanied by rearrangement. 

Secondary alcohols may react by an or an mechanism, depending on the alcohol and experimental conditions. Primary alcohols with extensive jS-branching react by an S l mechanism involving formation of a rearranged carbocation. 

---