Regioselectivity in the Dehydration of Alcohols

The dehydration of an alcohol is an elimination reaction that requires breaking the carbon-oxygen bond and a carbon-hydrogen bond on an adjacent, P-carbon atom. Thus, dehydration of alcohols having two nonequivalent P-carbon atoms adjacent to the OH-bearing carbon atom can form a mixture of products. In such cases, the more substituted alkene is the major product. For example, the dehydration of 2-methyl-2-butanol yields 2-methyl-2-butene as the major product. The isomeric 2-methyl-1-butene forms in a substantially smaller quantity. 

This product distribution is another example of Zaitsevs rule. Zaitsevs rule can be extended to mixtures of geometric isomers. That is, the major product is the more stable tnrw-alkene. For example, 3-pen tanol yields a mixture of cis- and r7a r-2-pentenes. The tram isomer is the major product. Thus, the dehydration of an alcohol is both rcgioselecrive and stereoselective. 

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