Question

Following are two reaction sequences for converting 1,2 -diphenylethylene into 2,3-diphenyloxirane. Suppose that the starting alkene is trans-1,2-diphenylethylene. (a) What is the configuration of the oxirane formed in each sequence? (b) Will the oxirane formed in either sequence rotate the plane of polarized light? Explain.

Short answer

Answer: For both reaction sequences (A and B), the same 2,3-diphenyloxirane product is obtained, having S configuration at C2 and R configuration at C3. As the product has chiral centers (C2 and C3), it will rotate the plane of polarized light.

Step-by-step solution

Stereochemistry of the starting alkene

The starting alkene, trans-1,2-diphenylethylene, is a geometric isomer with the two phenyl groups on opposite sides of the double bond. We can represent the stereochemistry using the following notation: (E)-1,2-diphenylethylene.

Identify the reaction sequences and determine the oxirane configuration

We have not been provided with the reaction sequences explicitly in the exercise. However, let's assume we have two different reaction sequences, Sequence A and B. For illustration purposes, let's assume the following sequences: # Sequence A: trans-1,2-diphenylethylene reacts with a peroxy acid (e.g., MCPBA) to form the 2,3-diphenyloxirane. # Sequence B: trans-1,2-diphenylethylene reacts with a halogenation reagent (e.g., Br_2), followed by an intramolecular substitution reaction (S_N2) with a strong nucleophile (e.g., hydroxide ion, -OH). In Sequence A, the reaction is stereospecific, and the oxirane is formed with a retention of configuration. So, the product will have S configuration at C2 and R configuration at C3. In Sequence B, the halogenation followed by S_N2 reaction would also be stereospecific. The first step adds a bromine to the double bond with anti-addition, and in the second step, the hydroxide ion attacks from the opposite side (back-side attack) displacing the bromine. So, the product will have the same configuration as in Sequence A: S configuration at C2 and R configuration at C3. In both sequences, the same oxirane product with S and R configurations at C2 and C3, respectively, is obtained.

Investigate the optical activity of the formed oxiranes

In order to determine whether the formed 2,3-diphenyloxirane will rotate the plane of polarized light, we need to look at the presence of chiral centers in the product. Chiral centers have four different ligands attached to a carbon and show optical activity. In the formed 2,3-diphenyloxirane from both sequences, C2 is attached to a phenyl, methyl, R-C3, and the oxygen atom. So, C2 is a chiral center. Similarly, C3 is also a chiral center as it is attached to a phenyl, methyl, S-C2, and the oxygen atom. As the oxirane product has chiral centers (C2 and C3), it will indeed rotate the plane of polarized light.

Key concepts

Stereochemistry of Alkenes

The stereochemistry of alkenes plays a critical role in determining the properties and reactivity of these compounds. The spatial arrangement of substituents around the double bond can significantly affect the outcome of chemical reactions. In the case of trans-1,2-diphenylethylene, we observe that the two phenyl groups are situated on opposite sides of the double bond.

This configuration is also referred to as the (E)-configuration based on the Cahn-Ingold-Prelog priority rules, where the higher priority groups are on opposite sides. Understanding the (E)-configuration is vital because it directly influences how the molecule interacts with reagents, leading to specific products such as 2,3-diphenyloxirane.

When teaching the concept of alkene stereochemistry, it's essential to emphasize the geometric isomerism and how it affects the reaction pathways and products. Visual aids, like molecular models, can improve students' comprehension of the three-dimensional orientation of substituents.

Stereospecific Reactions

Stereospecific reactions are a class of chemical reactions where the mechanism dictates the configuration of the product based on the configuration of the starting material. In our example, 2,3-diphenyloxirane synthesis via Sequence A or B both exhibit stereospecificity.

Sequence A uses a peroxy acid and follows a mechanism that retains the stereochemistry of the starting material. Hence, if the starting alkene is (E)-1,2-diphenylethylene, the configuration of the product will maintain the relative positioning of substituents.

In Sequence B, the bromination and subsequent back-side attack (S_N2 reaction) also leads to a product with a predictable configuration. A clear understanding of stereospecific reactions helps students predict product configurations by analyzing the starting material and reaction mechanism.

To enhance comprehension, juxtapose stereospecific reactions against reactions that are not stereospecific, such as some addition reactions to alkenes where the product's configuration is not dependent on the starting material's configuration.

Optical Activity in Organic Compounds

Optical activity is a phenomenon where certain organic molecules can rotate plane-polarized light, a property that arises due to chirality. A compound is considered chiral if it has an asymmetric carbon atom—usually one that is attached to four different substituents.

Students should recognize that the presence of chiral centers is what imparts optical activity to a compound. In the case of 2,3-diphenyloxirane synthesized from trans-1,2-diphenylethylene, both carbon atoms C2 and C3 are chiral centers.

Each chiral center in the molecule can exist in two configurations (R or S), which are mirror images and cannot be superimposed onto each other. These mirror images, known as enantiomers, rotate light in opposite directions. For educational purposes, it's crucial to demonstrate with models or diagrams, how chiral centers are identified and to discuss enantiomers as well as their physical and chemical properties which often differ despite having similar structural formulas.

With 2,3-diphenyloxirane, the presence of chiral centers means the compound can rotate the plane of polarized light. The direction and magnitude of rotation depend on the specific arrangement of the substituents, which is determined by the reaction mechanism.