Acid-Catalyzed Dehydration of an Alcohol

Alcohol dehydrations usually involve El elimination of the protonated alcohol. 

Step 1 Protonation of the hydroxyl group (fast equilibrium). 

Step 2 Ionization to a carbocation (slow rate limiting). 

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You may have noticed that the acid catalyzed hydration of an alkene and the acid catalyzed dehydration of an alcohol are the reverse of each other... 

Mechanism of the acid-catalyzed dehydration of an alcohol to yield an alkene. The process is an El reaction and involves a carbocation intermediate. 

Mechanism 11-4 (Review) Acid-Catalyzed Dehydration of an Alcohol 487... 

CHAPTER 7 CHAPTER 8 CHAPTER 10 CHAPTER 11 CHAPTER 15 CHAPTER 17 CHAPTER 18 Acid-Catalyzed Dehydration of an Alcohol 313 Electrophilic Addition to Alkenes 330 Grignard Reactions 443 The Williamson Ether Synthesis 500 The Diels-Alder Reaction 684 Electrophilic Aromatic Substitution 757 Nucleophilic Additions to Carbonyl Groups 841 Formation of Imines 851 Formation of Acetals 856... 

Hydride Reduction of a Carbonyl Group 454 Reaction of a Tertiary Alcohol with HBr(S[ 1) 480 Reaction of a Primary Alcohol with HBr (SN2) 480 Reaction of Alcohols with PBr3 485 (Review) Acid-Catalyzed Dehydration of an Alcohol 487 The Pinacol Rearrangement 495 Cleavage of an Ether by HBr or HI 639 Acid-Catalyzed Opening of Epoxides in Water 649 Acid-Catalyzed Opening of an Epoxide in an Alcohol Solution 650... 

Process to view animations showing the El acid-catalyzed dehydration of an alcohol. 

It has been shown that acid-catalyzed dehydration of an alcohol to an olefin can proceed through the El mechanism, as shown in the following equations ... 

Explain why the acid-catalyzed dehydration of an alcohol is a reversible reaction, whereas the base-promoted dehydrohalogenation of an alkyl halide is an irreversible reaction. 

The acid-catalyzed dehydration of an alcohol to a rearranged alkene is known as a Wagner-Meerwein rearrangement. Propose a mechanism for the following Wagner-Meerwein rearrangement ... 

Hydrogen gas is adsorbed onto the surface of these metal catalysts, and the catalyst weakens the H—H bond. In fact, if H2 and D2 are mixed in the presence of a platinum catalyst, the two isotopes quickly scramble to produce a random mixture of HD, H2, and D2. (No scrambling occurs in the absence of the catalyst) Hydrogenation is an example of heterogeneous catalysis, because the (solid) catalyst is in a diffo-ent phase from the reactant solution. In contrast, homogeneous catalysis involves reactants and catalyst in the same phase, as in the acid-catalyzed dehydration of an alcohol. 

In the second step of the cycle, citrate is converted to the constitutional isomer isocitrate. This isomerization occurs in two steps, both catalyzed by aconitase. First, in a reaction analogous to the acid-catalyzed dehydration of an alcohol (Section 8.2E), citrate undergoes enzyme-catalyzed dehydration of aconitate. Then, in a reaction analogous to acid-catalyzed hydration of an alkene (Section 5.3B), aconitate undergoes enzyme-catalyzed hydration to give isocitrate. 

When isomeric alkenes are obtained in acid-catalyzed dehydration of an alcohol, fhe alkene having the greater number of substituents on the double bond (the more stable alkene) generally predominates (Zaitsev s rule. Section 9.5). 

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