Question

In the following reaction, configuration of di-bromo alkene obtained as a major product is \((\mathrm{A}) \mathrm{E}\) (B) Product cannot show geometrical isomerism (C) Z (D) Cannot decide

Short answer

The configuration of the di-bromo alkene obtained as a major product in this reaction is the Z configuration. This is determined by assigning priority to the substituents using the Cahn-Ingold-Prelog rules and finding that the two bromine atoms have higher priority and are added to the same side of the double bond, resulting in a Z isomer.

Step-by-step solution

Understand E/Z Isomerism

E/Z isomers (also known as cis/trans or geometric isomers) are stereoisomers that differ in the positions of atoms bonded to two sp2 hybridized carbon atoms with a double bond. E-isomers have the highest priority substituents on opposite sides of the double bond, while Z-isomers have the highest priority substituents on the same side of the double bond.

Determine the Reaction

We are given a general reaction involving a di-bromo alkene as the major product. Since no specific reaction is provided, we will use a generic reaction as a reference: R-CH=CH-R + Br2 -> R-CH(Br)-CH(Br)-R In this reaction, an alkene reacts with bromine, resulting in the formation of a di-bromo alkene product.

Assign Priority Of Substituents

To determine the configuration of the di-bromo alkene major product, we must assign priority to the substituents on each carbon atom with the double bond. The Cahn-Ingold-Prelog (CIP) rules are used to assign priority: 1. Priority is based on the atomic number of the directly bonded atom. The higher the atomic number, the higher the priority. 2. If a tie occurs, consider the next atoms along the substituent. This process is repeated until a difference is found.

Determine the Configuration

As both bromine atoms have a higher atomic number than any other atom directly attached to the carbons of the double bond, they both have higher priority. Since both bromine atoms are added to the same side of the double bond (syn addition), this results in a Z configuration. Therefore:(C) Z is the correct answer.

Key concepts

Geometrical Isomerism

Geometrical isomerism, also known as cis-trans isomerism, occurs in compounds with restricted rotation around a double bond. This leads to different spatial arrangements. In alkenes, the double bond creates a fixed structure where substituents can either be on the same side (cis/Z) or on opposite sides (trans/E). Geometrical isomerism is an essential concept as it affects the chemical properties and reactivity of compounds.
To identify geometrical isomers, look at each carbon in the double bond and determine what is attached. If the groups on each end are different, the compound can exhibit geometrical isomerism. Understanding this helps in predicting molecular behavior in chemical reactions.

Cahn-Ingold-Prelog Rules

The Cahn-Ingold-Prelog (CIP) priority rules are a system to determine the priority of substituents around a double bond. This method helps in deciding which isomer is present, labeled as E (entgegen, opposite) or Z (zusammen, together).

• First, look at the atoms directly attached to the double-bonded carbons. The atom with the highest atomic number gets higher priority.
• If there’s a tie, move to the next bonded atoms until a difference is found.

Once the priorities are established, compare the highest priority substituents on each carbon. If they are on the same side, the compound is a Z isomer. If on opposite sides, it's an E isomer. This system provides a clear method to classify complex molecules and clarify their 3D structure.

Stereochemistry

Stereochemistry is the study of the spatial arrangement of atoms in molecules and their effects on chemical reactions. It plays a crucial role in understanding how molecules interact and react. With stereochemistry, different isomers can have vastly different properties despite having the same molecular formula.
For example, E/Z isomerism is a type of stereochemistry where the spatial positioning around double bonds is crucial. Knowing both the geometric and Cahn-Ingold-Prelog rules help in predicting isomer's behavior.
Why is this important? Because even slight differences in configuration can lead to different chemical reactivities and biological activities. Mastering stereochemistry gives you a deeper insight into molecular science and its applications in pharmaceuticals, materials, and natural products.