2.3- Dimethyl-2-butene catalyzed hydration

Dimethyl-2-butanol is the major product in the acid-catalyzed hydration of 3,3 -dimethyl-1 -butene. 

Like other reactions that involve carbocation intermediates, hydration may take place with rearrangement. For example, when 3,3-dimethyl-l -butene undergoes acid-catalyzed hydration, the major product results from rearrangement of the carbocation intermediate. 

Similarly, oxymercuration-demercuration of 3,3-dimethyl-l-butene gives the Markovnikov product, 3,3-dimethyl-2-butanol, in excellent yield. Contrast this unre-arranged product with the rearranged product formed in the acid-catalyzed hydration of the same alkene in Section 8-4B. Oxymercuration-demercuration reliably adds water across the double bond of an alkene with Markovnikov orientation and without rearrangement. 

Analysis of the mixture of products by gas chromatography failed to reveal the presence of any 2,3-dimethyl-2-butanol. The acid-catalyzed hydration of 3,3-dimethyl-1-butene, by contrast, gives 2,3-dimethyl-2-butanol as the major product. 

You might compare the product of oxymercuration-reduction of 3,3-methyl-l-butene with the product formed by acid-catalyzed hydration of the same alkene (Section 6.3C). In the former, no rearrangement occurs. In the latter, the major product is 2,3-dimethyl-2-butanol, a compound formed by rearrangement. The fact that no rearrangement occurs during oxymercuration-reduction indicates that at no time is a free carbocation intermediate formed. 

Acid-catalyzed hydration of 3,3-dimethyl-1-butene produces 2,3-dimethyl-2-butanol. Show a mechanism for this reaction. 

FIGURE 3.81 The acid-catalyzed addition of water to 2,3-dimethyl-2-butene —a hydration reaction. 

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