Rearrangement alcohol dehydration

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... 

Direct acid-catalyzed dehydration of 2,3,6-trimethoxy-6,l l-dihydrodibenz[6,/]oxepin-1()-ol leads to a mixture of the corresponding dibenz[b,/]oxepin and the rearranged alcohol with a 9/7-xanthene structure.262... 

Like other El reactions, alcohol dehydration follows an order of reactivity that reflects carbocation stability 3° alcohols react faster than 2° alcohols, and 1° alcohols are the least reactive. Rearrangements of the carbocation intermediates are common in alcohol dehydrations. In most cases, Zaitsev s rule applies The major product is usually the one with the most substituted double bond. 

We can account for rearrangements in dehydration in the following way. A carbonium ion is formed by the loss of water from the protonated alcohol. If a 1,2-shift of hydrogen or alkyl can form a more stable carbonium ion, then such a rearrangement takes place. The new carbonium ion now loses a proton to yield an alkene. 

Despite the high temperatures required for acetate and xanthogenate pyrolysis, the accessibility of these compounds from the corresponding alcohols provides for a useful route to alkenes. One advantage is that as no acidic or basic reagents are required, rearrangement reactions, which can interfere with alcohol dehydration, are avoided. For example, pyrolysis of lactol acetates has been used to prepare 4,5-dihy-drofurans, as illustrated in equation (58), - and pyrolysis of the corresponding thionocarbonate was used to dehydrate alcohol (128 Scheme 38). 

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. 

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