Problem 162 - A Complete Resource Book in Chemistry for JEE Main

Question

\(4-\) hexadiyne \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) is allowed to react with \(\mathrm{Li}\) n \(\mathrm{NH}_{3}(\) liq \()\). The product obtained is treated with 1 equivalent of \(\mathrm{Br}_{2}\) in \(\mathrm{CCl}_{4}\). Which of the following constitutional isomers are possible products? (a) CC=CC(Br)C(C)Br (b) CC(Br)C=CC(C)Br (c) CCCC=CC(C)Br (d) CCC=CC(Br)CBr

Step-by-Step Solution

Answer

The possible constitutional isomer is product (c).

Step 1: Identify the Starting Material and Reagent

The starting material is 4-hexadiyne (C6H6), which is a linear alkyne with triple bonds located at positions 1 and 4 of the hexane chain. The reagent is lithium in liquid ammonia (Li in NH3).

Step 2: First Reaction - Reduction with Lithium in NH3

The Li/NH3 reagent is known for performing the Birch reduction on triple bonds. This reaction is selective for reducing alkynes to trans-alkenes. Therefore, 4-hexadiyne will be reduced to trans-hexene with double bonds between carbons 1-2 and 3-4, forming 4-hexene.

Step 3: Second Reaction - Bromination with Br2 in CCl4

After reduction, the alkene (4-hexene) is treated with one equivalent of Br2 in CCl4, which selectively adds bromine atoms across one of the double bonds. Since there are two double bonds, only one double bond will react, and it can happen in either position, resulting in multiple possible dibromoalkene isomers.

Step 4: Identifying Possible Products

The bromination of 4-hexene can occur in different positions leading to constitutional isomers. The positions 1-2 or 3-4 can undergo addition of Br2. Evaluating options: - Product (a), CC=CC(Br)C(C)Br, has bromines added to the double bond at carbons 1-2. - Product (b), CC(Br)C=CC(C)Br, is incorrect as it implies a reaction not consistent with a single Br2 equivalent. - Product (c), CCCC=CC(C)Br, correctly represents bromines added to the double bond at carbons 1-2 and the preservation of targeting only one bond. - Product (d), CCC=CC(Br)CBr, is incorrect since this structure doesn't align with bromination at any of the initial double bonds.

Step 5: Choose the Correct Isomers

Evaluating each isomer against the expected reaction pathway, the correct constitutional isomer from the treatment of bromine on a single double bond of 4-hexene is product (c), based on the typical selectivity of the reaction.

Key Concepts

Alkyne Reduction

Alkyne reduction is a fascinating chemical transformation where alkynes, which contain a carbon-carbon triple bond, are converted into alkenes with a double bond. This is accomplished by adding hydrogen atoms. For the process using lithium in liquid ammonia, an approach known as Birch reduction is employed. In this reaction, the triple bond in the alkyne is specifically reduced to form a trans-alkene. This means that the newly formed double bond is shaped so that the substituent groups (the atoms or groups attached to the double bond) are on opposite sides. This selectivity is crucial for producing a particular desired geometric configuration of the molecule. Alkyne reduction is used in various synthetic pathways to prepare specific alkenes, which can further undergo other reactions to introduce more complexity into the molecule.

Bromination Reaction

The bromination reaction involves the addition of bromine (Br extsubscript{2}) to organic compounds. In the context of alkenes, this reaction takes advantage of the double bonds which act as sites of reactivity. When an alkene like the trans-4-hexene formed from alkyne reduction is treated with Br extsubscript{2} in a solvent such as carbon tetrachloride (CCl extsubscript{4}), the bromine molecules add across the double bond. This process typically results in a dibromo compound with two bromine atoms added to the carbons initially involved in the double bond.

• In the exercise, bromination is shown to occur across one of the alkene's double bonds.
• This can lead to the formation of various positional isomers, depending on which double bond undergoes the reaction.

The selectivity of bromination can significantly affect the product distribution, making it crucial to predict and analyze the potential outcomes in synthetic chemistry.

Birch Reduction

Birch reduction is named after chemist Arthur Birch and is a method of selectively reducing aromatic rings and alkynes. In the case of alkynes, it reduces the triple bond to a trans double bond. The reaction conditions involve the use of lithium, an alkali metal, in the presence of liquid ammonia. Electrons provided by lithium assist in breaking the triple bond, and the ammonia donates protons, resulting in the formation of a trans-alkene. This reaction is particularly useful because it leaves other functional groups in the molecule unaltered, making it a targeted and efficient method to selectively modify complex organic molecules.

• By reducing triple bonds to double bonds, Birch reduction serves as an essential tool for chemists looking to tailor the structure of hydrocarbons.
• The product of alkyne Birch reduction provides a unique geometric configuration crucial for subsequent reactions.

Understanding Birch reduction can aid in grasping how planners of synthetic routes will alter structures methodically to reach desired outcomes in materials and pharmaceuticals development.