Ferf-Butyl cation 2-methylpropene

The two dimers of (CH3)2C=CH2 are formed by the mechanism shown in Figure 6.16. In step 1 protonation of the double bond generates a small amount of ferf-butyl cation in equilibrium with the alkene. The carbocation is an electrophile and attacks a second molecule of 2-methylpropene in step 2, forming a new carbon-carbon bond and generating a Cg carbocation. This new carbocation loses a proton in step 3 to form a mixture of 2,4,4-trimethyl-l-pentene and 2,4,4-trimethyl-2-pentene. 

The ferf-butyl group is cleaved as the corresponding carbocation. Loss of a proton from ferf-butyl cation converts it to 2-methylpropene. Because of the ease with which a tert-butyl group is cleaved as a carbocation, other acidic reagents, such as trifluoroacetic acid, may also be used. 

Reaction coordinate diagram for the addition of to 2-methylpropene to form the primary isobutyl cation and the tertiary ferf-butyl cation. 

We can extend the general principles of electrophilic addition to acid-catalyzed hydration. In the first step of the mechanism shown in Figure 6.9, proton transfer to 2-methylpropene forms tert-butyl cation. This is followed in step 2 by reaction of the car-bocation with a molecule of water acting as a nucleophile. The alkyloxonium ion formed in this step is simply the conjugate acid of ferf-butyl alcohol. Deprotonation of the alkyloxonium ion in step 3 yields the alcohol and regenerates the acid catalyst. 

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