Question

Propose a mechanism for addition of HI to 1 -methylcyclohexene to give 1 -iodo1 - methylcyclohexane. Which step in your mechanism is rate-determining?

Short answer

Question: Propose a mechanism for the addition of HI to 1-methylcyclohexene to form 1-iodo-1-methylcyclohexane and identify the rate-determining step in the mechanism. Answer: The mechanism involves two steps: 1) Protonation of the alkene by HI, forming a secondary carbocation and releasing iodide ion. This step is the rate-determining step due to its higher energy barrier. 2) Nucleophilic attack of iodide ion on the carbocation, leading to the formation of 1-iodo-1-methylcyclohexane.

Step-by-step solution

Protonation of the alkene by HI

In this step, the nucleophilic double bond (C=C) in 1-methylcyclohexene reacts with the acidic hydrogen in HI. The alkene will be protonated, forming a carbocation and breaking the C=C bond. One of the carbon atoms in the double bond gains the hydrogen atom from HI, while the other retains a positive charge, forming a cyclohexyl carbocation. Meanwhile, the iodide ion (I-) is released. The protonation can happen at both carbons of the double bond. However, protonation at the carbon attached to the methyl group will lead to a more stable secondary carbocation (more substituted), which will be the major product. 1-methylcyclohexene + HI → secondary carbocation + I-

Nucleophilic attack of iodide ion

In the second step, the iodide ion (I-) acts as nucleophile and attacks the positively charged carbocation. Since iodide is a strong nucleophile, this step will be relatively fast. The final product, 1-iodo-1-methylcyclohexane, is formed when the iodide ion forms a covalent bond with the carbocation. secondary carbocation + I- → 1-iodo-1-methylcyclohexane

Identifying the rate-determining step

To identify the rate-determining step, we need to look at the energy barriers involved in the elementary steps of the reaction. The rate-determining step is generally the step with the highest energy barrier. In this case, the first step (protonation of the alkene to generate the carbocation) is usually slower than the second step (nucleophilic attack of the iodide ion), making it the rate-determining step. The formation of a carbocation, which is a high-energy species, is the slowest and most energetically demanding step in this reaction mechanism.

Key concepts

Protonation of Alkene

Alkenes are characterized by a carbon-carbon double bond, which can react with acids to form new products. This reaction starts with the protonation of the alkene, which is essentially the addition of a proton (H⁺) from an acid to one of the carbon atoms in the double bond, converting it into a single bond. In the context of our example with 1-methylcyclohexene and hydrogen iodide (HI), the hydrogen ion (the proton) from HI bonds to one of the carbon atoms in the double bond, and as a result, a positively charged carbocation is formed.

This step is pivotal because it opens the double bond and creates a site for further reaction - the carbocation. It's essential for students to understand that the stability of the formed carbocation significantly influences which carbon atom gets protonated. A secondary carbocation, which is formed when the proton adds to the carbon attached to more alkyl groups, is more stable than a primary carbocation, explaining why this path is preferred.

During the protonation stage, the double bond acts as a nucleophile, an electron-rich species, which is attracted to the electron-deficient proton (electrophile). Because different carbons in the double bond can be protonated, product distribution can vary, leading to what we call regioselectivity in the reaction.

Carbocation Intermediates

Once the alkene is protonated, we are left with a key intermediate called a carbocation. A carbocation is a positively charged ion with a carbon atom holding a trio of bonds and a vacant p-orbital. This electron deficiency makes it incredibly reactive, longing to be 'satisfied' by a nucleophile that can donate electrons.

The stability of carbocations is a fundamental topic because it helps predict the course of many reactions. Carbocations are classified by the number of carbons attached to the positively charged carbon: primary (attached to one other carbon), secondary (attached to two carbons), and tertiary (attached to three carbons). The more substituted the carbocation, the more stable it is due to both inductive effects and hyperconjugation.

These intermediates are central in many organic reaction mechanisms, and their formation is often the rate-determining step because creating such a high-energy species requires a significant amount of energy.

Nucleophilic Attack

Following the formation of the carbocation intermediate, the next phase of the reaction is the nucleophilic attack. A nucleophile is a chemical species rich in electrons and capable of 'attacking' a positive or electron-deficient centre. In our exercise, the iodide ion (I^-), which was released after the HI molecule donated its proton, serves as the nucleophile.

The nucleophile approaches the electron-deficient carbocation and donates a pair of electrons to form a new covalent bond. This step is typically fast and energetically favorable, resulting in the completion of the reaction with the formation of 1-iodo-1-methylcyclohexane. Understanding this step involves recognizing that the strength and stereochemistry of nucleophiles can greatly influence both the rate and the outcome of these kinds of reactions.

Rate-Determining Step

In every sequential multi-step reaction, there is one step that is slower than the others, and therefore determines the speed of the entire reaction; this is known as the rate-determining step. Identifying this step is crucial because it provides insight into how the reaction rate can be influenced by concentration, temperature, and the presence of catalysts.

In our example, the protonation of the alkene to form the carbocation is the rate-determining step. This is because creating the carbocation intermediate requires overcoming a significant energy barrier compared to the subsequent nucleophilic attack, which is a relatively fast and energetically downhill process. Enzymes and catalysts in chemistry are often designed to speed up this slowest step in a reaction mechanism, thereby increasing the overall rate of the reaction.