Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

During debromination of meso-dibromobutane, the major compound formed is (a) n-butane (b) 1 -butane (c) cis-2-butene (d) trans-2-butene

Step-by-Step Solution

Answer

The major compound formed is trans-2-butene (d).

Step 1: Understand the Reaction

Debromination of meso-dibromobutane involves the removal of two bromine atoms from the molecule, typically through the application of a reducing agent like Zn. This reaction leads to the formation of a double bond.

Step 2: Identify Stereochemistry of the Starting Material

Meso-dibromobutane is a compound with two identical chiral centers but is achiral due to its internal plane of symmetry. The molecule can rotate to align the bromines on opposite sides, making it favorable for anti-elimination.

Step 3: Determine the Mechanism of Debromination

The mechanism mainly involves anti-elimination, where the bromines are removed from adjacent carbons. This type of elimination typically leads to the formation of the most stable alkene in the trans configuration.

Step 4: Predict the Product

Considering the stereochemistry and the anti-elimination mechanism, the removal of bromine from opposite sides within the meso compound leads to the formation of trans-2-butene, which is the more stable isomer due to less steric hindrance.

Key Concepts

Meso-dibromobutane

Meso-dibromobutane is an intriguing compound due to its unique stereochemical properties. It contains two chiral centers, which could suggest the potential for optical activity. However, meso-dibromobutane is achiral, meaning it does not exhibit optical activity. This occurs because the molecule has an internal plane of symmetry that bisects it, resulting in the two halves being mirror images of each other. In the case of meso-dibromobutane, the presence of a plane of symmetry simplifies the stereochemistry, making it an ideal candidate for specific types of chemical reactions. Understanding this compound's structure is crucial when predicting how it will behave during chemical reactions, such as debromination.

Anti-elimination mechanism

The anti-elimination mechanism is a common pathway in organic chemistry where two atoms or groups are removed from adjacent carbon atoms. This often occurs in a manner that is anti-periplanar, meaning the groups are on opposite sides of the molecular plane. During debromination of meso-dibromobutane, each bromine is removed from adjacent carbon atoms in an anti orientation. This is due to the fact that in most cases, such an arrangement is favored because it allows for more overlap between orbitals, which facilitates the formation of the double bond. Therefore, the anti-elimination mechanism is not only about removing the groups but also involves a strategic rearrangement of the molecule’s geometry to maximize stability. This reaction type often leads to a more stable product, particularly in more complex molecules.

Trans-2-butene formation

In the context of debromination reactions, the formation of trans-2-butene is a significant outcome. The product's stability is due in part to its geometric structure, where substituents are located on opposite sides of the double bond. This ensures minimal steric hindrance, allowing for lower energy configurations. During debromination of meso-dibromobutane, trans-2-butene becomes the primary product because it is the most stable isomer that can be formed under the given conditions. Trans-2-butene's stable conformation makes it a favorable product in chemical reactions involving alkenes. Its formation as the main product in this reaction exemplifies the consequences of both the compound's original stereochemistry and the anti-elimination mechanism utilized, leading to a reduction in potential energy and increased molecular stability.