CARBOCATION REARRANGEMENTS

Hydride Shift   

GREEN (Cl) = nucleophile     BLUE (OH) = leaving group     ORANGE (H) = hydride shift proton                                                  RED(H) = remaining proton

                  

Hydration of Alkenes: Hydride Shift

In a more complex case, when alkenes undergo hydration, we also observe hydride shift. Below is the reaction of 3-methyl-1-butene with H₃O⁺ that furnishes to make 2-methyl-2-butanol
                                                                                

 

We see this mechanism below:

    Alkyl Shift

Alkyl Shift acts very similarily to that of hydride shift. Instead of the proton (H) that shifts with the nucleophile, we see an alkyl group that shifts with the nucleophile instead. The shifting group carries its electron pair with it to furnish a bond to the neighboring or adjacent carbocation.

We see alkyl shift from a secondary carbocation to tertiary carbocation in S_N1 reactions:

#2, on the other hand, we can say that it undergoes a concerted mechanism. In short, this means that everything happens in one step. This is because primary carbocations cannot be an intermediate and they are relatively difficult processes since they require higher temperatures and longer reaction times. After protonating the alcohol substrate to form the alkyloxonium ion, the water must leave at the same time as the alkyl group shifts from the adjacent carbon to skip the formation of the unstable primary carbocation.

                  

Problems

 

Answers to Practice Problems