Question

Propose a mechanism for the acid-catalyzed hydration of 1 -methylcyclohexene to give 1 -methylcyclohexanol. Which step in your mechanism is rate- determining?

Short answer

Answer: The rate-determining step in the acid-catalyzed hydration of 1-methylcyclohexene is the formation of the carbocation intermediate (Step 1). This step involves protonation of the alkene, causing the double bond to become an electron-rich nucleophile that attacks the proton (H+), resulting in a stable carbocation intermediate.

Step-by-step solution

Protonation of the alkene

In the presence of an acid (H+), the alkene's double bond will act as an electron-rich nucleophile and attack the proton (H+), leading to the formation of a more stable carbocation intermediate. In this case, since 1-methylcyclohexene has a tertiary carbon adjacent to the double bond, a tertiary carbocation will be formed after protonation. The reaction can be written as: 1-methylcyclohexene + H+ -> [1-methylcyclohexyl]+

Nucleophilic attack by water

The carbocation intermediate ([1-methylcyclohexyl]+) now has a positive charge on the tertiary carbon, making it an electrophile. A nucleophile, in this case, water (H2O), will attack the positively charged carbon, forming a bond and resulting in an oxonium ion intermediate. The reaction is as follows: [1-methylcyclohexyl]+ + H2O -> [1-methylcyclohexyl-OH2]+

Deprotonation

The oxonium ion intermediate ([1-methylcyclohexyl-OH2]+) has a positive charge on the oxygen atom, making it unstable. Another water molecule acts as a base and removes a proton (H+) from the oxygen atom, leading to the formation of the product, 1-methylcyclohexanol, and regeneration of the acid catalyst (H+). The reaction can be written as: [1-methylcyclohexyl-OH2]+ + H2O -> 1-methylcyclohexanol + H+ Now, let's identify the rate-determining step: The rate-determining step is the slowest step in a reaction mechanism, and it determines the overall rate of the reaction. In this mechanism, the first step (protonation of the alkene) involves breaking the pi bond and forming a carbocation, which is an energy-intensive process. So, the formation of the carbocation intermediate (Step 1) is the rate-determining step in the proposed mechanism.