Question

Using your roadmap as a guide, show how to convert the starting trans-alkene to the cis-alkene in high yield. Show all intermediate molecules synthesized along the way.

Short answer

Question: Briefly describe the three-step process to convert a trans alkene to a cis alkene, and state the intermediate molecules synthesized along the way. Answer: The process to convert a trans alkene to a cis alkene involves halogenation, halohydrin formation, and reduction. First, a halogen is added to the trans alkene to form a vicinal dibromo compound. Next, water is added, generating a halohydrin intermediate. Finally, the halohydrin is reduced using a reducing agent such as zinc, resulting in the desired cis alkene. The intermediate molecules synthesized along the way are the vicinal dibromo compound and the halohydrin.

Step-by-step solution

Halogenation

In the first step, we will add a halogen, for example, bromine (Br2), to the trans alkene. This process is called halogenation, and it results in the formation of a vicinal dibromo compound. The reaction occurs via a cyclic halonium ion intermediate. Trans alkene + Br2 -> Vicinal dibromo compound

Halohydrin Formation

In the second step, water is added to the vicinal dibromo compound, generating a halohydrin intermediate. The reaction occurs through the anti-addition of water across the bromine-bromine bond. In this step, the oxygen from the water will attack the more substituted carbon, displacing one of the bromines. Vicinal dibromo compound + H2O -> Halohydrin

Reduction

In the final step, we will use a reducing agent, such as zinc metal, to reduce the halohydrin to the desired cis alkene. The reaction occurs through a single electron transfer process, which results in the formation of a radical intermediate. The radical intermediate will quickly undergo a second single electron transfer process, resulting in the formation of the desired cis alkene. Halohydrin + Zn -> Cis alkene Following these steps, we have successfully converted the trans alkene to a cis alkene in a high yield, while also showing the intermediate molecules synthesized along the way: the vicinal dibromo compound and the halohydrin.

Key concepts

Halogenation

Halogenation is a fundamental reaction in organic chemistry where a halogen, such as bromine (Br2), is added to an alkene. In this process, the alkene's double bond is attacked by the halogen, leading to the formation of a cyclic halonium ion intermediate. This is a three-membered ring structure where the halogen is temporarily bonded to both carbon atoms of the alkene. Subsequently, another halogen ion attacks this intermediate, opening the ring and resulting in a vicinal dibromo compound, with the two bromine atoms adjacent to each other on the carbon chain. This step is crucial as it sets the foundation for subsequent reactions in the transformation process.

An important aspect to consider during halogenation is that the addition of the halogen atoms occurs in a 'trans' fashion, meaning that they are added on opposite sides of the double bond, which is critical to maintaining the stereochemistry required for further reactions.

Halohydrin Formation

Following halogenation, halohydrin formation is the next step in the transformation of an alkene. This reaction involves the addition of water (H2O) to the vicinal dibromo compound. When the water molecule approaches the dibromo intermediate, the oxygen end acts as a nucleophile and attacks the more-substituted carbon, leading to the displacement of one bromine atom.

The result is a halohydrin, which features a hydroxyl group (-OH) and a halogen on adjacent carbon atoms. The orientation is again important, as the addition of water occurs in an 'anti' fashion, meaning the OH group and the bromine atom end up on opposite sides. This configuration is critical for the subsequent reduction step that will ultimately invert the stereochemistry necessary to achieve the cis-alkene as the final product.

Alkene Reduction

To obtain the desired cis-alkene, the halohydrin undergoes a reduction. Zinc metal is commonly employed as a reducing agent in this reaction. When zinc is introduced, it participates in a single-electron transfer process, producing a radical intermediate. The presence of the radical indicates that the reaction involves the movement of single electrons rather than electron pairs, which is typical in many other organic reactions.

After the first electron transfer, the intermediate quickly undergoes a second electron transfer, leading to the removal of the halogen and the formation of the cis-alkene. The process not only reduces the number of substituents on the alkene but also ensures that the final product preserves the correct stereochemical arrangement, with the substituents ending up on the same side of the double bond in the cis configuration.

Chemoselectivity

The concept of chemoselectivity is integral to the successful transformation of an alkene from trans to cis configuration. Chemoselectivity refers to the preferential reaction of a chemical reagent with one functional group in the presence of other functional groups. In the context of our alkene problem, chemoselective conditions must be established during the addition of bromine and water to ensure that the reactions occur at the desired sites without affecting other potential reactive sites on the molecule.

For instance, during the halogenation step, the reagent bromine selectively reacts with the double bond of the trans-alkene instead of other parts of the molecule. Similarly, during the formation of halohydrin, water selectively displaces a bromine without affecting the other bromine atom. Ensuring chemoselectivity is essential for achieving a high yield of the targeted cis-alkene product.

Stereochemistry

Stereochemistry is the study of the spatial arrangement of atoms in molecules and how this arrangement affects the chemical behavior of the molecules. In the process of converting a trans-alkene to a cis-alkene, stereochemistry is a key consideration because it dictates the physical and chemical properties of the end product.

The halogenation and halohydrin formation steps involve 'anti' additions, where the new atoms are added on opposite sides of the double bond. This helps in obtaining the right stereochemical setup for the eventual reduction step. When reducing the halohydrin to a cis-alkene, the stereochemistry must be controlled to ensure that both substituents end up on the same side of the double bond, ultimately determining the 'cis' orientation of the product. Attention to stereochemistry is therefore crucial throughout the conversion process to ensure that the final product has the desired spatial arrangement of atoms.