Problem 202 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following sequence of reactions, the alkene affords the compound 'B' \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3} \stackrel{\mathrm{O}_{3}}{\longrightarrow}\) (A) \(\stackrel{\mathrm{H}_{2} \mathrm{O}}{\mathrm{Zn}}\) (B) The compound (B) is (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\) (b) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COCH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{CHO}\)

Step-by-Step Solution

Answer

The compound (B) is CH3CHO (option d).

Step 1: Understanding the Reaction Sequence

The reaction sequence first involves the alkene undergoing ozonolysis with ozone (O3) and then reductive work-up with zinc and water.

Step 2: Analyzing the Ozonolysis Reaction

During ozonolysis, the double bond in the alkene CH3CH=CHCH3 is cleaved, forming intermediates that contain two carbonyl groups, specifically ketones or aldehydes. The alkene is symmetrical, leading to the generation of two identical aldehyde molecules.

Step 3: Identifying the Products After Reductive Work-up

The application of Zn and water in the reductive work-up ensures that any ozonide intermediates formed are converted into carbonyl compounds without further oxidation steps. In this case, both fragments obtained after cleavage are aldehydes, specifically acetaldehyde (CH3CHO).

Step 4: Determining Compound (B) from Given Options

Since the ozonolysis produces acetaldehyde (CH3CHO), the correct compound (B) corresponds to option (d) in the list: CH3CHO.

Key Concepts

Ozonolysis Reaction

Ozonolysis is a powerful chemical reaction that breaks down alkenes, which are hydrocarbons with at least one carbon-carbon double bond. This reaction utilizes ozone molecules to cleave the double bonds, resulting in the formation of smaller compounds known as carbonyl compounds. The process involves the initial formation of ozonides, which are unstable and eventually convert to these carbonyl compounds during further reaction steps. Ozonolysis is particularly useful in organic chemistry for determining the position and number of double bonds in an unknown alkene, as well as for synthesizing smaller, more manageable molecules from larger, complex structures. Typically, it can yield either aldehydes or ketones, depending on the specific structure of the starting alkene.

Alkene Reactions

Alkene reactions are fundamental in organic chemistry, focusing on the transformation of alkenes through various chemical reactions. Alkenes are characterized by their carbon-carbon double bond, which is highly reactive. This reactivity allows alkenes to undergo addition reactions, where atoms or groups are added to the carbon atoms involved in the double bond. In the context of ozonolysis, these double bonds are the primary sites of reaction. The versatility of alkene reactions makes them crucial for synthesizing a wide array of organic products, ranging from simple molecules to complex pharmaceuticals. Due to their reactivity, alkenes are often used in laboratory settings for various conversion processes.

Carbonyl Compound Formation

During ozonolysis of an alkene, one of the major products formed after the ozonide intermediate is a carbonyl compound. Carbonyl compounds, such as aldehydes and ketones, contain a carbon-oxygen double bond. The nature of the carbonyl compound formed depends largely on the structure of the original alkene. If the alkene is symmetrical, ozonolysis can yield identical carbonyl products. For example, the ozonolysis of but-2-ene, a symmetrical alkene, results in the formation of two molecules of acetaldehyde (\( \mathrm{CH}_3\mathrm{CHO} \)). Understanding the formation of these compounds is crucial in predicting the outcome of ozonolysis reactions and is useful in synthetic pathways in organic chemistry.

Reductive Work-up

Reductive work-up is an essential step after the initial ozonolysis in the reaction sequence. This step reduces ozonides into stable carbonyl compounds by using reducing agents such as zinc (Zn) and water (\( \mathrm{H}_2\mathrm{O} \)). Without reductive work-up, ozonides, which are temporary intermediates, could undergo further unwanted reactions, leading to oxidation and potentially damaging side reactions. The role of zinc in this process is to break down the ozonide and prevent oxidation, ensuring the formation of desired carbonyl compounds. In the context of but-2-ene, the reductive work-up assures that the ozonides are fully converted to acetaldehyde, making the overall reaction precise and controlled.

Symmetrical Alkene Products

Symmetrical alkenes are unique because they produce identical products when subjected to certain reactions like ozonolysis. This property simplifies the prediction of the products formed. When a symmetrical alkene, like but-2-ene, undergoes ozonolysis followed by reductive work-up, it splits into two identical acetaldehyde (\( \mathrm{CH}_3\mathrm{CHO} \)) molecules. Because the alkene has mirrored sides across its double bond, breaking this bond results in two equivalent halves. This feature makes symmetrical alkenes particularly advantageous in experimental chemistry, as they provide consistent results that simplify identification and analysis of reaction products.