Problem 33 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The number of stereoisomers obtained by bromination of trans-2butene is (a) 1 (b) 2 (c) 3 (d) 4

Step-by-Step Solution

Answer

(b) 2

Step 1: Understanding Stereochemistry

To determine the number of stereoisomers formed, we need to understand that stereochemistry involves molecules with the same formula but different spatial orientations. Trans-2-butene has double bonds, making it a crucial focus for understanding stereoisomerism.

Step 2: Analyzing Bromination

Bromination of trans-2-butene happens via an anti-addition mechanism, meaning bromine atoms are added to opposite sides of the double bond. This process results in the formation of a vicinal dibromide.

Step 3: Identifying Possible Configurations

The addition of bromine can create new chiral centers. Upon bromination, the produced molecule can exist in different spatial orientations based on where the two bromine atoms attach to the carbon atoms.

Step 4: Counting the Stereoisomers

Each stereocenter can lead to different stereoisomers due to the different possible arrangements of bromine atoms. Specifically, bromination of trans-2-butene will result in two distinct stereoisomers: one pair of enantiomers, leading to two configurations that are mirror images of each other.

Key Concepts

Stereoisomers

Stereoisomers are fascinating because they share the same molecular formula but differ in the 3D arrangement of their atoms. This results in molecules that can have different physical and chemical properties. Think of stereoisomers like the left and right hand—identical in composition but not in form. In organic chemistry, this concept is essential because many reactions result in the formation of different stereoisomers.

For trans-2-butene, the spatial arrangement of atoms is affected by the geometric isomerism due to the presence of double bonds, which restrict rotation and create different isomers based on how substituents are oriented. Understanding these different arrangements is key to predicting and identifying the products formed from reactions such as bromination.

Bromination

Bromination is a specific type of halogenation reaction where bromine is added to an alkene or alkyne. This reaction is crucial because it often introduces new functionalities and complexities into a molecule. When trans-2-butene undergoes bromination, bromine atoms are added across the double bond.

The process occurs via an anti-addition mechanism, meaning the two bromine atoms add to opposite sides of the double bond, contributing to the formation of vicinal dibromide. This has significant implications for the structure and type of stereoisomers formed.

• Anti-addition maintains the trans geometry of the original molecule, leading to particular outcomes in terms of stereoisomerism.
• Vicinal dibromides, where bromine atoms are on adjacent carbons, result from this reaction.

Understanding this mechanism is important because it dictates the possible stereoisomers that can result from the reaction.

Chiral Centers

Chiral centers play a pivotal role in determining stereoisomer configurations. A chiral center, usually a carbon atom, has four different groups attached, allowing for non-superimposable mirror image forms. In chemical reactions like bromination, existing or new chiral centers affect the stereoisomeric outcome.

During the bromination of trans-2-butene, new chiral centers are formed when bromine atoms are added. The configuration of these chiral centers determines the type of stereoisomers produced.

• The presence of a chiral center leads to the possibility of different enantiomers being formed.
• It's important to examine each chiral center to accurately determine all possible stereoisomers.

Once you understand how chiral centers influence isomer formation, it becomes much easier to predict the types of molecules you will end up with.

Enantiomers

Enantiomers are a specific type of stereoisomer, where two molecules are mirror images of each other but cannot be superimposed. This property makes them chiral, just like your left and right hand. Enantiomers have identical physical properties except for their interaction with plane-polarized light and reactions in chiral environments.

In the bromination of trans-2-butene, enantiomers are formed due to the presence of new chiral centers created during the addition of bromine.

• These mirror-image configurations result from the different spatial arrangements possible after bromine atoms bond to the carbon atoms involved in the double bond.
• The presence of a chiral environment will affect which enantiomer is formed and in what proportion.

Recognizing enantiomers is crucial for understanding the full stereochemical landscape of a reaction's products.