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 is the first step, which involves the protonation of the alkene to form a carbocation intermediate. This step is energetically uphill and slow due to the high activation energy required to form the carbocation.

Step-by-step solution

Protonation of the alkene

The acid catalyst donates a proton (H+) to the double bond of 1-methylcyclohexene, forming a carbocation on the more substituted carbon. 1-methylcyclohexene + H+ -> 1-methylcyclohexyl carbocation

Nucleophilic attack by water

A water molecule acts as a nucleophile, attacking the carbocation intermediate, and forming a new bond to the carbon, while simultaneously breaking the bond with the positively charged hydrogen atom. 1-methylcyclohexyl carbocation + H2O -> 1-methylcyclohexanol-OH2(+) intermediate

Deprotonation of the oxonium ion intermediate

A nearby water molecule (or another molecule acting as a base) abstracts a proton from the 1-methylcyclohexanol-OH2(+) intermediate, releasing the positively charged hydrogen atom as a H+ ion which can act as another acid catalyst. 1-methylcyclohexanol-OH2(+) intermediate + H2O -> 1-methylcyclohexanol + H3O+ Now let's determine the rate-determining step based on the proposed mechanism.

Rate-determining step

The rate-determining step in the proposed mechanism is the first step. This is because: 1. The protonation of the alkene to form the carbocation is an energetically uphill process. 2. The formation of a carbocation is a slow process due to its high energy and the double bond must overcome the activation energy to form the carbocation. 3. The subsequent steps involving nucleophilic attack and deprotonation tend to be faster due to the high reactivity of the carbocation intermediate. In conclusion, the mechanism for the acid-catalyzed hydration of 1-methylcyclohexene involves the protonation of the alkene, nucleophilic attack of water on the carbocation and finally the deprotonation of the oxonium ion. The rate-determining step is the first step for which the carbocation intermediate is formed.

Key concepts

Carbocation Intermediate

In organic chemistry, the term carbocation intermediate refers to a species that plays a pivotal role in many reaction mechanisms. Specifically, it is a carbon atom bearing a positive charge due to the absence of one of its bonding electrons. This electron deficiency makes carbocations electrophilic, meaning they are 'electron-loving' and tend to attract electron-rich species.

During the acid-catalyzed hydration of alkenes such as 1-methylcyclohexene, the initial step involves the protonation of the double bond. This transforms the alkene into a carbocation intermediate, an unstable and high-energy species that greatly influences the reaction's pathway.

This intermediate is key to understanding how the reaction proceeds because it provides a site for subsequent nucleophilic attack. The stability of carbocation intermediates is determined by the number of alkyl groups attached to the positively charged carbon, with more substituted carbocations being more stable. For instance, the carbocation formed in our exercise is stabilized by adjacent carbon chains, which help to disperse the positive charge.

It's critical to note that the formation of the carbocation is an endothermic process – it requires energy input. This step's high energy barrier often makes it the rate-determining step of the reaction, a concept we will explore in more detail shortly.

Nucleophilic Attack

Following the formation of the carbocation intermediate, the next fundamental step is the nucleophilic attack. A nucleophile is essentially a 'nucleus-loving' species that donates an electron pair to form a chemical bond. It is typically rich in electrons and is attracted to positively charged or electron-deficient atoms.

In the case of acid-catalyzed hydration, water serves as the nucleophile. It 'attacks' the carbocation, offering an electron pair to form a new, stable bond with the carbon atom. The oxygen of the water molecule bonds to the electron-deficient carbocation, resulting in the formation of an oxonium ion intermediate. During this critical juncture, the oxygen's lone pair of electrons becomes shared with the carbon, thus elongating and eventually breaking the O-H bond, as the water molecule transfers one of its hydrogens to bond more strongly to the carbocation.

This step is often faster than carbocation formation because once the high-energy intermediate is in place, the nucleophilic attack proceeds with relative ease. The flow of electrons from the nucleophile to the carbocation is energetically favorable and typically happens quite swiftly compared to the initial formation of the carbocation.

Rate-Determining Step

In any sequential reaction, the rate-determining step (RDS) is the slowest step that governs the overall reaction rate. It is analogous to the narrowest part of an hourglass; just as the narrow neck controls the rate at which the sand passes through, the RDS controls how fast the reaction proceeds.

Identifying the RDS is vital for understanding the kinetics of a reaction. As we've seen with the acid-catalyzed hydration of 1-methylcyclohexene, the initial formation of the carbocation intermediate is the rate-determining step. This step has the highest activation energy, which is the minimum amount of energy required for the reaction to proceed. Think of it as a high-energy hill that the reactants must climb to be transformed into products.

Since the carbocation formation is the most energetically demanding, it happens at the slowest rate. Once the carbocation is available, the following steps, including nucleophilic attack and deprotonation, occur relatively quickly. Understanding the rate-determining step is crucial to predicting reaction mechanisms and manipulating reaction conditions to control the outcome.