Problem 147 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following would, on ozonolysis, yields \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) and \(\mathrm{CH}_{2}(\mathrm{CHO})_{2}\) in the ratio \(1: 1 ?\) (a) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{C}\left(\mathrm{CH}_{3}\right)_{2}\) (d) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{C}\left(\mathrm{CH}_{3}\right)=\mathrm{CH}-\mathrm{CH}_{2}-\mathrm{CH}=\mathrm{CH}-\) \(\mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

The correct answer is option (c).

Step 1: Understand Ozonolysis

Ozonolysis is a chemical reaction where ozone (O_3) cleaves double bonds in alkenes, transforming them into carbonyl compounds. Thus, a given alkene will break at each double bond, forming corresponding aldehydes or ketones.

Step 2: Analyze Desired Products

The problem asks for a compound that will yield acetone \(\mathrm{CH}_{3}\mathrm{COCH}_{3}\) and glyoxal \(\mathrm{CH}_{2}(\mathrm{CHO})_{2}\) when subjected to ozonolysis. This suggests a structure where breaking double bonds at certain positions leads to these specific products.

Step 3: Consider Each Option

We need to evaluate which of the given molecules can break to form acetone and glyoxal. - (a) and (b) have too many double bonds or incorrect structures. - (c) seems promising due to its symmetry and position of double bonds. - (d) would not break to form the required mixture.

Step 4: Evaluate Option (c)

The structure in option (c) allows for the cleavage of bonds to form acetone and glyoxal. Breaking the central double bond and terminal alkene would give the products due to the arrangement of carbon atoms.

Step 5: Confirm Product Formation

By analyzing option (c), we find that it breaks into two molecules: one corresponding to acetone and the other to glyoxal, thus fitting the requirement of the question.

Key Concepts

Chemical Reactions

Chemical reactions are transformations where substances, known as reactants, are converted into new substances, termed products. These reactions involve the rearrangement of atoms and breaking/forming of bonds. Ozonolysis is a specific type of chemical reaction involving ozone and alkenes. In this process, ozone reacts with the carbon-carbon double bonds (\( C=C \)) in alkenes, resulting in the cleavage of these bonds. This transforms the molecule into carbonyl compounds, which include aldehydes, ketones, or acids depending on the starting material. Understanding how chemical reactions occur is fundamental to predicting the products that will form.

Alkenes

Alkenes are hydrocarbons that contain one or more carbon-carbon double bonds (\( C=C \)). They follow the general formula \( C_nH_{2n} \). These unsaturated compounds are highly reactive due to the presence of the double bond, which is a site of electron density. This electron-rich area makes alkenes reactive towards electrophiles, such as ozone, which is utilized in ozonolysis. In the context of ozonolysis, the double bonds in alkenes play a crucial role. By identifying where these double bonds are located on the molecule, we can determine how the compound will break down into smaller molecules, helping predict the resulting carbonyl compounds.

Carbonyl Compounds

Carbonyl compounds are characterized by the presence of a carbon-oxygen double bond (\( C=O \)). This class of compounds includes both aldehydes and ketones. During ozonolysis, alkenes are converted into these compounds. Aldehydes have the carbonyl group at the end of a carbon chain, while ketones feature the carbonyl group within the chain.

• Aldehydes: Typically end with -CHO.
• Ketones: Characterized by \( -CO- \) within the molecule.

The structure of the original alkene and the position of the double bonds dictate whether an aldehyde or a ketone will form. For example, breaking an internal double bond typically creates a ketone, while breaking a terminal double bond produces an aldehyde.

Acetone

Acetone is a simple ketone with the formula \( CH_3COCH_3 \). It consists of a central carbonyl group (a double bond between carbon and oxygen) flanked by two methyl groups. This compound is highly volatile and is frequently used as a solvent. In ozonolysis, acetone can form when specific double bonds in an alkene are fragmented correctly. It usually forms from alkenes with an appropriate carbon framework that allows for the creation of a ketone upon the cleavage of the central double bond.

Glyoxal

Glyoxal is the simplest dialdehyde, with the chemical formula \( CH_2(CHO)_2 \). It consists of two aldehyde groups adjacent to each other. This compound is an important intermediate in organic synthesis and can be formed via ozonolysis when an alkene's double bonds cleave in such a way that results in two terminal aldehyde groups. Successful prediction of glyoxal's formation through ozonolysis depends on recognizing an alkene structure that, upon reaction with ozone, creates a dialdehyde product. Hence, the arrangement and nature of the double bonds in the alkene securely influence the formation of glyoxal.