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

Question

In the following sequence of reactions, the alkene affords the compound 'B' \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3} \stackrel{\mathrm{O}_{3}}{\longrightarrow}(\mathrm{A}) \stackrel{\mathrm{H}_{2} \mathrm{O}}{\longrightarrow}\) (B) \(\mathrm{Zn}\) The compound (B) is [2008] (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 acetaldehyde, \\(\mathrm{CH}_{3}\mathrm{CHO}\\), which is option (d).

Step 1: Understand Ozonolysis

Ozonolysis is the reaction of an alkene with ozone to form an ozonide intermediate, which can then be cleaved to form carbonyl compounds. Here, the alkene 2-butene is subjected to ozonolysis.

Step 2: Identify the Starting Alkene

The given alkene is but-2-ene, \(\mathrm{CH}_{3}\mathrm{CH}\!=\mathrm{CHCH}_{3}\). This can potentially form two carbonyl compounds upon ozonolysis.

Step 3: Predict the Ozonolysis Products

Treatment of the alkene with ozone \(\mathrm{O}_{3}\) breaks the double bond and forms an ozonide, this breaks down to two aldehyde or ketone molecules upon further reductive workup with zinc \(\mathrm{Zn}\) and water \(\mathrm{H}_{2}\mathrm{O}\).

Step 4: Identify the Products of Ozonolysis

When but-2-ene undergoes ozonolysis followed by zinc workup, it forms two molecules of acetaldehyde (ethanal), \(\mathrm{CH}_{3}\mathrm{CHO}\). This is compound (A).

Step 5: Determine Compound B

Compound (B) is \(\mathrm{CH}_{3}\mathrm{CHO}\) after ozonolysis of but-2-ene and reaction with \(\mathrm{Zn}\) and \(\mathrm{H}_{2}\mathrm{O}\).

Step 6: Choose the Correct Option

Based on the product identified, compound (B) is acetaldehyde, matching option (d), \(\mathrm{CH}_{3}\mathrm{CHO}\).

Key Concepts

Ozonolysis

Ozonolysis is a fascinating reaction in organic chemistry where an alkene reacts with ozone (_3). This procedure is utilized to cleave double bonds in alkenes, transforming them into simpler carbonyl compounds. The process initiates by adding ozone to the alkene double bond, creating an ozonide, an unstable intermediate.
The ozonide can then be split into more stable carbonyl compounds, such as aldehydes or ketones, depending on the structure of the alkene.

• It is typically part of synthetic strategies to break down larger molecules into smaller, more functionalized units.


• The exact carbonyl compounds formed depend on the structure of the original alkene.

The use of ozonolysis in structural elucidation of organic compounds is a classic approach, shedding light on the exact nature of the carbon framework by pinpointing where the alkene bonds were cleaved. Understanding this reaction provides insights into vast possibilities within synthetic and analytical chemistry.

Alkenes

Alkenes, sometimes referred to as olefins, are hydrocarbons characterized by carbon-carbon double bonds (C=C). This double bond is key to their chemical reactivity and is the feature that differentiates them from alkanes, which only contain single bonds. The presence of a double bond makes alkenes more reactive than their alkane counterparts. This reactivity becomes the basis for a wide array of important reactions, like halogenation, hydrogenation, and of course, ozonolysis.
What makes alkenes special is

• their ability to easily participate in addition reactions, whereby the double bond can open up and bond with another atom or molecule,


• because of cis and trans isomerism, alkenes exhibit interesting properties worth studying further for their impact on the geometry and, subsequently, the reactivity of compounds.

Analyzing but-2-ene, the alkene from the original exercise, it shows that both carbons on either end of the double bond hold methyl groups, making ozonolysis straightforward since both possible carbonyl products are identical.

Carbonyl Compounds

When a reaction like ozonolysis occurs, one product result is a class of compounds known as carbonyl compounds, characterized by a carbon atom double-bonded to an oxygen atom (C=O). This bond is highly polar, making carbonyl compounds generally reactive, allowing them to undergo various nucleophilic addition and oxidation reactions. Carbonyl compounds are divided into two main categories:

• Aldehydes: have the general form RCHO. In the case of ozonolysis of but-2-ene, acetaldehyde is the product formed when each end of the alkene transforms into such a compound.


• Ketones: have the structure RCOR', typically appearing when the original alkene structure cannot simply break apart to yield terminal carbon atoms.

These compounds serve as building blocks in organic synthesis and are essential in manufacturing, including plastics, fragrances, and pharmaceuticals.

Reductive Workup

Reductive workup is the critical final step in ozonolysis, enabling us to convert the ozonide intermediate into stable carbonyl compounds. Following the generation of the ozonide from the alkene and ozone reaction, we need to cleave this unstable and potentially explosive intermediate safely.
By using zinc (Zn) and water (H_2O), we perform a reductive workup, transforming the ozonide into carbonyl compounds. In our example, two molecules of acetaldehyde are produced. Here's why this step is necessary:

• Zinc acts as a reducing agent, helping to break apart the ozonide in a controlled fashion without forming hazardous peroxides.


• This reduction step ensures that the double bond cleavage results in fully formed aldehydes or ketones without leaving hazardous residues from ozonolysis.

For students, understanding the importance of this workup helps in grasping not just the completion of ozonolysis, but also how to safely perform and manage this reaction in laboratory settings.