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EXPT 9 Dehydration

EXPT 9 Dehydration - EXPERIMENT 9 Elimination Reactions of...

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EXPERIMENT 9: Elimination Reactions of Alcohols: Dehydration Written by Albert T. Sneden, VCU Department of Chemistry with revisions by L.M. Moses In addition to their role in substitution reactions, alcohols play an important role in reactions which lead to the formation of alkenes via elimination reactions. Alkenes may be formed directly from alcohols via an acid-catalyzed E1 elimination route. Like the reaction of alcohols with hydrogen halides to give alkyl halides, this is a reaction which proceeds through a carbocation and is an equilibrium reaction. Consequently, to avoid the possibility of product mixtures, halogen acids are not used in elimination reactions. Instead, concentrated sulfuric acid or concentrated phosphoric acid is the acid of choice for most eliminations. To force the equilibrium to the product side, the reaction is carried out at elevated temperatures. In some cases, water or the alkene may be removed from the reaction mixture by distillation at these elevated temperatures. An example of E1 elimination using an alcohol, the formation of 2- methylpropene from 2-methyl-2-propanol is shown below (Eq. 1). OH conc. H 3 PO 4 heat (Eq. 1) Mechanism: O H H OPO 3 H 2 O H H H OPO 3 H 2 The reaction usually proceeds to give the more stable alkene as the major product when two or more products are possible. The more stable alkene is generally the alkene with the more substituted double bond. This is known as Zaitsev elimination. Of course, if two or more alkenes can be formed, usually all products will be formed. This can lead to separation problems, since all of the alkenes formed in a reaction will generally have boiling points or melting points fairly close to one another as well as similar solubilities in common solvents. Because of these
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