The distillation of the product during the dehydration reaction results in impro

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The distillation of the product during the dehydration reaction results in improved yields of alkene. Explain why this is true. Dehydration of an Alcohol Alcohols can be dehydrated to produce alkenes. This is usually accomplished by heating the alcohol with concentrated sulfuric acid or concentrated phosphoric acid. Certairn Lewis acids (P:0s, ZnCl2, Al:0s) have also been used for this purpose. The ease of dehydration increases with branching at the alpha carbon of the alcohol. Hence tertiary alcohols are more easily dehydrated than secondary alcohols, and secondary alcohols are more easily dehydrated than primary alcohols. Tertiary and secondary alcohols are dehydrated by an El mechanism and yield very little, if any, substitution product. Primary alcohols probably follow an E2 path and their dehydration is often complicated by side reactions in which ethers are produced. In all cases, the dehydrations follow Saytzeff's rule. According to this rule, the more highly substituted alkene will be the favored product A major problem in the preparation of alkenes by the acid catalyzed dehydration of alcohols is that these reactions are often accompanied by molecular rearrangements. Carbocations, which are intermediates in the El mechanism, may rearrange to form more stable carbocations which, in turn, may lead to the formation of several isomeric alkenes El Mechanism The El dehydration mechanism involves the reversible protonation of the alcohol to form an oxonium ion. This ion then dissociates in a rate determining step to form a carbocation and water. The carbocation then loses a proton from a site adjacent to the positively charged carbon atom to form an alkene. The loss of the proton is aided by a base such as another molecule of the alcohol or a water molecule. ll these steps, as illustrated in Scheme 1, are reversible under the reaction conditions and hence the product alkene is often distilled from the reaction mixture in order to maximize the yield. oxonium ion alkene Seheme 1 Competing Reactions Two side reactions that often accompany the dehydration reaction are substitution and rearrangement of the carbocation, Scheme 2. Any nucleophile present in the reaction mixture may capture the carbocation resulting in the formation of an ether or another substitution product. R-OHHR-O-R+H Scheme 2

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The distillation of the product during the dehydration reaction results in impro

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