Acid-Catalyzed Dimerization of 2- Methylpropene

Step 1 Protonation of the carbon-carbon double bond in the direction that leads to more stable carbocation  

Step 2 The carbocation acts as an electrophile toward the alkene. A carbon-carbon bond is formed, resulting in a new carbocation—one that has eight carbons  

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that form tertiary carbocations. In some cases reaction conditions can be developed that favor the formation of higher molecular weight polymers. Because these reactions proceed by way of carbocation intermediates, the process is referred to as cationic polymerization. 

We made special mention in Section 5.1 of the enormous volume of ethylene and propene production in the petrochemical industry. The boxed essay Ethylene and Pro-pene The Most Important Industrial Organic Chemicals summarizes the principal uses of these alkenes. Most of the ethylene is converted to polyethylene, a high-molecular-weight polymer of ethylene. Polyethylene cannot be prepared by cationic polymerization, but is the simplest example of a polymer that is produced on a large scale by free-radical polymerization. 

Ethylene and Propene The Most Important Industrial Organic Chemicals 

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FIGURE 6 16 Mechanism of acid catalyzed dimerization of 2 methylpropene... 

Hydroboration of 1-Methylcyclopentene 248 6.10 Acid-Catalyzed Dimerization of 2-Methylpropene 262... 

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