Question

Following are two reaction sequences for converting 1,2 -diphenylethylene into 2,3 -diphenyloxirane.

Short answer

Question: Compare the two reaction sequences for converting 1,2-diphenylethylene into 2,3-diphenyloxirane and describe the steps involved in each. Answer: In Reaction Sequence 1, 1,2-diphenylethylene reacts with a peracid to directly form the epoxide ring, resulting in 2,3-diphenyloxirane. In Reaction Sequence 2, 1,2-diphenylethylene first forms a halohydrin by reacting with bromine and water. Then, an intramolecular SN2 reaction takes place, where the oxygen atom from the hydroxyl group displaces the bromine atom, forming 2,3-diphenyloxirane.

Step-by-step solution

Reaction Sequence 1:

In this reaction sequence, 1,2-diphenylethylene (C6H5-CH=CH-C6H5) is converted into 2,3-diphenyloxirane (C6H5-CH-CH-C6H5 with an oxygen atom bridging between two carbons). Step 1: Formation of the epoxide C6H5-CH=CH-C6H5 + RCO3H → C6H5-CH-O-CH-C6H5 In this step, a peracid (RCO3H) reacts with 1,2-diphenylethylene to form an epoxide ring, which is an oxygen atom bridging between the two carbons.

Reaction Sequence 2:

In this reaction sequence, 1,2-diphenylethylene is converted into 2,3-diphenyloxirane through two main steps: halohydrin formation and intramolecular SN2 reaction. Step 1: Formation of the halohydrin C6H5-CH=CH-C6H5 + Br2, H2O → C6H5-CH(OH)-CHBr-C6H5 In this step, bromine (Br2) and water (H2O) are added to 1,2-diphenylethylene to form a halohydrin containing a bromine atom on one carbon and a hydroxyl group on the other carbon. Step 2: Intramolecular SN2 reaction C6H5-CH(OH)-CHBr-C6H5 → C6H5-CH-CH-C6H5 with an oxygen atom bridging between two carbons In this step, the halohydrin undergoes an intramolecular nucleophilic substitution (SN2) reaction, where the oxygen atom from the hydroxyl group displaces the bromine atom, forming the 2,3-diphenyloxirane.

Key concepts

Organic Reaction Mechanisms

Organic reaction mechanisms involve understanding the step-by-step transformation of reactants to products. This includes evaluating how bonds are broken and formed, and which intermediates or transition states are involved during a reaction. By unraveling these mechanisms, chemists can predict the outcome of organic reactions and apply this knowledge in synthesizing desired compounds.

In the synthesis of 2,3-diphenyloxirane from 1,2-diphenylethylene, the reaction mechanisms of epoxide formation and halohydrin conversion are crucial. These multi-step processes are determined by the specific reactants, reagents, and conditions chosen. Delving into the individual mechanisms provides an insight into the nature of the bonds involved and the rearrangements that take place.

Epoxide Formation

Epoxide formation is a crucial transformation in organic chemistry. Epoxides are three-membered cyclic ethers characterized by an oxygen atom bonded to two carbon atoms. The synthesis of epoxides, such as 2,3-diphenyloxirane, often involves the use of peracids in a reaction known as the Prilezhaev reaction.

In the provided exercise, 1,2-diphenylethylene reacts with a peracid (RCO3H). The peracid donates an oxygen atom to the carbon-carbon double bond, resulting in the formation of the epoxide ring. This transformation can be visualized as the reaction of an oxygen species with a pair of pi electrons from the double bond, giving birth to the strained, yet energetic ring structure of the epoxide.

Halohydrin Formation

Halohydrin formation is an intermediate step in the synthesis of epoxides from alkenes when using halogenating agents and water. A halohydrin features a halogen and a hydroxyl group attached to adjacent carbons.

In our exercise scenario, the addition of bromine (Br2) and water (H2O) to 1,2-diphenylethylene initiates the formation of a halohydrin, where the alkene double bond undergoes a concerted reaction to accommodate both the bromine atom and hydroxyl group. This process illustrates the electrophilic addition of Br2 to the electron-rich alkene bond, followed by the nucleophilic attack of water, resulting in a halogen at one carbon and an alcohol group at the other. This intermediate is strategically set up for the final transformation into an oxirane.

SN2 Reaction

The SN2 (Substitution Nucleophilic Bimolecular) reaction is a fundamental type of nucleophilic substitution where a nucleophile simultaneously attacks a carbon atom and displaces a leaving group. The SN2 reaction proceeds with inversion of the stereocenter at the carbon where the substitution occurs.

In the creation of 2,3-diphenyloxirane, the SN2 step involves the conversion of the halohydrin to the epoxide. The oxygen from the hydroxyl group acts as the nucleophile, attacking the carbon attached to the bromine, ousting it from the molecule. This intramolecular reaction is efficient as the reactants are within the same molecule, thereby facilitating the ring closure to form the epoxide. SN2 reactions are characterized by their concerted mechanism, requiring a strong nucleophile and an electrophilic carbon with a suitable leaving group, as observed in halohydrin.