Hydride shift alcohol dehydration

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... 

Hydride shifts often occur during the dehydration of primary alcohols. Thus, although 1-butene would be expected to be the only alkene fonned on dehydration of 1-butanol, it is in fact only a minor product. The major product is a mixture of cis- and trans-2-butene. 

Despite the structural similarity of this alcohol to the alkyl halide in the preceding part of this problem, its dehydration is more complicated. The initially formed carbocation is secondary and can rearrange to a more stable tertiary carbocation by a hydride shift. 

The various methods of generating o-quinone methides,4-5 including the thermal or (Lewis) acid-catalyzed elimination of a phenol Mannich base,149 150-160-161163 the thermal or (Lewis) acid-catalyzed dehydration of an o-hydroxybenzyl alcohol (ether),147-149-151-153-156-157-162-163-165-168 171-175-178-183 the thermal 1,5-hydride shift of an o-hydroxy styrene,171-173 175 178-183 the thermal dissociation of the corresponding spirochromane dimer,158 163-164,166 oxidation of substituted o-alkylphenols,152-170 and the thermal or photochemical-promoted cheletropic extrusion154-155 159 of carbon monoxide, carbon dioxide, or sulfur dioxide (Scheme 7-III), as well as their subsequent in situ participation in regiospecific, intermolecular [4 + 2] cycloadditions with simple olefins and acetylenes,147 149-151 152 153159 162-164... 

The answer is B. Rearrangement is possible in acid-catalyzed dehydration of alcohols. The intermediate is a carbocation, not a carbanion. This eliminates statements I and III. Depending on the type of alcohol, the rearrangement may involve either methyl shift or hydride shift. 

To conclude, the mie-pot conversion of cellulose-to-lactic acid (or lactate ester in alcoholic media) thus follows a complex cascade reaction network involving at least six reactions. These reactions have different catalytic needs, but, in general, the presence of both Lewis and Brpnsted acidity are paramount for catalytic success. Br0nsted acidity is key to the hydrolysis of cellulose (step 1) at mild temperatures (<200°C), and to some extent to the dehydration of triose (step 4), whereas Lewis acid sites play a vital role in the isomerization reaction of glucose-to-fructose (step 2), the retro-aldol (step 3), and the 1,2-hydride shift (step 6). Steps 4 and 5 are relatively less demanding they are catalyzed by both acid types. 

In Section 6.4, we saw that 1,2 hydtide shifts occur in the carbocations formed in addition reactions. We have also seen that 1,2-hydtide shifts can also occur in carbocations that are generated in dehydration reactions if a more stable carbocation results. Such 1,2-hydtide shifts occur even in the dehydration of primary alcohols. For example, the dehydration of 1-decanol gives 1-decene as a minor product, which may result from an E2 mechanism. However, the major product is largely a mixture of cis- and rw r-2-decenes. This product could result from loss of a proton by an El mechanism from a secondary carbocation formed by a hydride shift of a primary carbocation. 

Reactions studied include dehydrations of alcohols, double bond shifts in olefins, isomerization of hydrocarbons, racemization of optically active compounds, etc.. In the literature a rather rigid separation is made between a Brested acid, which is actually a proton donor, and a Lewis acid, which works as a hydride abstractor. We may illustrate this difference by using the double bond shift in olefins as the model reaction. 

---