Problem 61 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Compounds \(P\) and \(R\) upon ozonolysis produce \(Q\) and \(S\), respectively. The molecular formula of \(Q\) and \(S\) is \(\mathrm{C}_{8} \mathrm{H}_{8} \mathrm{O} . Q\) undergoes Cannizzaro reaction but not haloform reaction, whereas \(S\) undergoes haloform reaction but not Cannizzaro reaction

Step-by-Step Solution

Answer

\(Q\) is benzaldehyde; \(S\) is acetophenone; \(P\) and \(R\) are their precursor derivatives.

Step 1: Understand Ozonolysis

Ozonolysis is a reaction where ozone (\(O_3\)) breaks a double bond in alkenes and forms carbonyl compounds (aldehydes or ketones). We're given that compounds \(P\) and \(R\) produce \(Q\) and \(S\) after ozonolysis, both with the molecular formula \(\mathrm{C}_8\mathrm{H}_8\mathrm{O}\).

Step 2: Analyze Reaction Types

Compound \(Q\) undergoes a Cannizzaro reaction, which involves no α-hydrogen, indicating it is an aromatic aldehyde. Compound \(S\) undergoes a haloform reaction, indicating it contains a methyl ketone group since this reaction involves halogenation followed by a bisection of the methyl group.

Step 3: Determine Structures

Given \(Q\) is an aromatic aldehyde with no α-hydrogens, its likely structure is an aromatic ring with an aldehyde group, such as benzaldehyde (\(C_7H_5CHO\)). Compound \(S\) could be acetophenone (\(C_6H_5COCH_3\)), which is a methyl ketone that fits the haloform reaction criteria.

Step 4: Verify Ozonolysis Products

For \(S\) (acetophenone) to be a product of ozonolysis, \(R\) must have had a structure that cleaves to form \(C_6H_5COCH_3\). Similarly, \(Q\) (benzaldehyde) must result from an ozonolysis that forms \(C_7H_5CHO\), such as from a styrene derivative.

Step 5: Confirm Answer

After verifying, compound \(Q\), being benzaldehyde, and \(S\), being acetophenone, fit the provided reaction criteria and ozonolysis patterns. \(P\) could be a substituted styrene, and \(R\) a vinyl benzene derivative.

Key Concepts

Cannizzaro reaction

The Cannizzaro reaction is an interesting organic reaction involving aldehydes that lack an alpha hydrogen. This type of reaction takes place under basic conditions. When an aromatic aldehyde like benzaldehyde undergoes a Cannizzaro reaction, it does not have any alpha hydrogens. Here's what typically happens:

• The aldehyde is treated with a strong base.
• One molecule of the aldehyde is oxidized to a carboxylic acid.
• Another molecule of the aldehyde is reduced to an alcohol.

For example, benzaldehyde (\( C_6H_5CHO \)) can undergo this reaction to form benzoic acid (\( C_6H_5COOH \)) and benzyl alcohol (\( C_6H_5CH_2OH \)). This reaction highlights the importance of lacking alpha hydrogens and provides a unique way to convert aldehydes into two different types of compounds. Cannizzaro is especially relevant for aromatic aldehydes, due to their structural properties.

Haloform reaction

The haloform reaction is another fascinating chemical reaction that primarily involves methyl ketones. Interestingly, compound \( S \) in our scenario undergoes this reaction, indicating it has a methyl ketone group. The process of a haloform reaction includes several steps:

• The methyl ketone is treated with a halogen (like iodine, chlorine, or bromine).
• A base is present, typically hydroxide ions
• One of the halogens goes through full halogenation of the methyl group.

This is followed by the formation of a carboxylate ion. Finally, a haloform (\( CHX_3 \), where \( X \) is a halogen) is generated. So, for acetophenone (\( C_6H_5COCH_3 \)), which fits the description for compound \( S \), this reaction results in the release of chloroform or bromoform if chlorine or bromine is used, respectively.

Aromatic aldehyde

Aromatic aldehydes are aldehydes that contain an aromatic ring, which heavily influences their reactivity. An aromatic aldehyde, like benzaldehyde (\( C_6H_5CHO \)), combines the structural stability of the benzene ring with the reactive carbonyl group. Some key characteristics include:

• The aromatic ring provides resonance stability, affecting reactivity.
• They typically lack alpha hydrogens, which aligns with their inability to undergo reactions like the haloform reaction.
• These compounds can participate in a Cannizzaro reaction.

In reactions, aromatic aldehydes such as compound \( Q \) do not readily undergo processes where alpha hydrogens are involved. Instead, they favor reactions like Cannizzaro, where the aldehyde is transformed into a mix of carboxylic acids and alcohols. Understanding these characteristics helps to identify their behavior in chemical processes.

Methyl ketone

Methyl ketones are compounds featuring a methyl group directly bonded to a carbonyl group. Acetophenone (\( C_6H_5COCH_3 \)) is a typical example of a methyl ketone, aligning with compound \( S \). They are renowned for their distinct ability to partake in the haloform reaction. Key features of methyl ketones include:

• They consist of a carbonyl group (\( C=O \)) bonded to a methyl group.
• Highly reactive in halogenation reactions.
• Undergo haloform reaction to form carboxylates and haloforms.

Methyl ketones are pivotal in determining the presence of the methyl group through qualitative tests such as the iodoform test. This reaction provides insight into the structure of a compound and is a classic sign of the presence of methyl ketones in organic chemistry.

Ozonolysis mechanism

Ozonolysis is a significant reaction in organic chemistry, especially when dealing with alkenes. The process involves breaking down unsaturated bonds with ozone leading to the formation of carbonyl compounds. Here's how ozonolysis works:

• An alkene is treated with ozone (\( O_3 \)).
• The double bond breaks, forming a molozonide intermediate.
• This rearranges into an ozonide, which decomposes to form aldehydes or ketones.

In our exercise, compounds \( P \) and \( R \) go through this mechanism to form compounds \( Q \) and \( S \), respectively. Ozonolysis is useful for determining the position of the double bond in unknown alkenes, because the resulting aldehydes and ketones provide direct insights into its original placement. This mechanism not only helps synthesize key organic compounds but also aids in structural identification.