Regioselectivity in Alcohol Dehydration The Zaitsev Rule

PROBLEM 5.12 Identify the alkene obtained on dehydration of each of the following alcohols  

SAMPLE SOLUTION (a) The hydrogen and the hydroxyl are lost from adjacent carbons in the dehydration of 3-ethyl-3-pentanol. 

The hydroxyl group is lost from a carbon that bears three equivalent ethyl substituents. Beta elimination can occur in any one of three equivalent directions to give the same alkene, 3-ethyl-2-pentene. 

In the preceding examples, inclnding those of Problem 5.12, only a single alkene could be formed from each alcohol by (3 elimination. What about elimination in alcohols such as 2-methyl-2-butanol, in which dehydration can occur in two different directions to give alkenes that are constitutional isomers Here, a double bond can be generated between C-1 and C-2 or between C-2 and C-3. Both processes occur but not nearly to the same extent. Under the usual reaction conditions 2-methyl-2-butene is the major product, and 2-methyl-1-butene the minor one. 

Dehydration of this alcohol is selective in respect to its direction. Elimination occurs in the direction that leads to the double bond between C-2 and C-3 more than between C-2 and C-1. Reactions that can proceed in more than one direction, but in which one direction is preferred, are said to be regioselective. 

7 CH3 Hydrogen is lost from p carbon having the fewest attached hydrogens 

Zaitsev s rule as applied to the acid-catalyzed dehydration of alcohols is now more often expressed in a different way elimination reactions of alcohols yield the most highly 

Each of the following alcohols has been subjected to acid-catalyzed dehydration and yields a mixture of two isomeric alkenes. Identify the two alkenes in each case, and predict which one is the major product on the basis of the Zaitsev rule. 

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