Question

Propose a mechanism for addition of HI to 1 -methylcyclohexene to give 1 -iodo-1methylcyclohexane. Which step in your mechanism is rate-determining?

Short answer

Answer: The rate-determining step in this mechanism is the first step, where the carbocation is formed through the interaction of 1-methylcyclohexene with HI.

Step-by-step solution

Identify the reactants

In this reaction, the reactants are 1-methylcyclohexene and HI (hydroiodic acid).

Formation of Carbocation

The double bond in 1-methylcyclohexene acts as a nucleophile and attacks the electrophilic hydrogen atom of the HI molecule. This process breaks the double bond and results in the formation of a carbocation, along with iodide ion (I-). In this case, the carbocation will be formed at the tertiary position because this tertiary carbon is more sterically bulkier and more stable than a secondary carbocation. Reaction: 1-methylcyclohexene + HI -> [1-Methylcyclohexyl]+ + I-

Carbocation Interaction with Iodide Ion

The Iodide ion (I-) formed in the previous step now acts as a nucleophile and attacks the carbocation, resulting in the formation of a C-I bond. This step leads to the formation of the product 1-iodo-1-methylcyclohexane. Reaction: [1-Methylcyclohexyl]+ + I- -> 1-iodo-1-methylcyclohexane

Determining the Rate-determining Step

In this step, we need to identify which step in the mechanism is slower or rate-determining. The rate-determining step is generally associated with the highest energy species’ formation. In the proposed mechanism, the formation of carbocation occurs in the first step, which is usually the highest energy process. Therefore, the rate-determining step is most likely the formation of the carbocation. In conclusion, the rate-determining step in this mechanism is the first step, where the carbocation is formed through the interaction of 1-methylcyclohexene with HI.

Key concepts

Carbocation Formation

In organic chemistry, the formation of a carbocation is a critical step in many reactions, including the addition of hydrogen halides like hydroiodic acid (HI) to alkenes. A carbocation is a positively charged ion (a cation) that contains a carbon atom without a full octet of electrons, rendering it highly reactive.

During the addition of HI to 1-methylcyclohexene, the alkene's double bond functions as a nucleophile. A nucleophile is a species that donates an electron pair to an electrophile to form a chemical bond. In this case, the double bond electrons attack the electrophilic hydrogen atom in HI. The electron-rich double bond breaks, which leads to the formation of the carbocation. For our specific molecule, the most stable carbocation forms at the tertiary carbon; this is because tertiary carbocations are stabilized by the surrounding carbon atoms through hyperconjugation and inductive effects.

The stability of different carbocations increases in the following order: methyl < primary < secondary < tertiary. Hence, during the addition to alkenes, the structural arrangement around the double bond is crucial for dictating the site and stability of carbocation formation. The end result in our example is the formation of a tertiary carbocation and an iodide ion (I-), setting the stage for the next reaction step.

Nucleophilic Attack

Following the formation of the carbocation in the reaction of HI with alkenes, a nucleophilic attack is the subsequent crucial step that leads to the final product. The nucleophilic attack involves the iodide ion (I-), which is a good nucleophile due to its negative charge and ability to donate an electron pair.

In the case of our reaction, the iodide ion approaches the positively charged carbocation, and donates its electron pair to form a covalent bond, thereby creating the C-I bond of 1-iodo-1-methylcyclohexane. This stage is usually fast and energetically favorable because it involves the formation of a stable molecule from a highly reactive intermediate.

Importance of Nucleophilic Attack Direction

For some reactions, the direction from which the nucleophile attacks the carbocation can influence the final product's stereochemistry. However, in the case of 1-iodo-1-methylcyclohexane, the carbocation is planar, allowing the iodide ion to attack from either side, leading to a product without stereoselective concerns.

Rate-Determining Step

The rate-determining step (RDS) is the slowest step in a reaction mechanism, and it has the greatest influence on the overall reaction rate. This step typically involves the highest energy transition state and thus requires the most activation energy to proceed.

In the addition of HI to alkenes, the formation of the carbocation is often the RDS, as it involves breaking the stable pi bond of the alkene to form a high-energy carbocation intermediate. Since the energy barrier for this step is the highest, it governs the kinetics of the reaction. The rate of the overall reaction is a reflection of this step; consequently, the reaction will not proceed significantly faster than the rate at which the carbocation is formed.

Factors Affecting the Rate-determining Step

Several factors can influence the RDS, such as the stability of the carbocation formed, the leaving group ability, and the reaction conditions (e.g., temperature, solvent). In reactions where carbocation rearrangement can occur, such rearrangements are typically not part of the RDS and occur rapidly after the initial carbocation formation.