Problem 187 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following: List I (Reactants) 1\. \(\mathrm{HC} \equiv \mathrm{CH}+\mathrm{H}_{2} \mathrm{O} \stackrel{\mathrm{Hg}^{2 *}}{\longrightarrow}\) 2\. \(\mathrm{PhCH}_{3} \frac{1 . \mathrm{Cl}_{2}, \Delta}{2 . \mathrm{H}_{2} \mathrm{O} / \mathrm{OH}^{-}}\) 3\. \(\mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}_{2}+\mathrm{CO}+\mathrm{H}_{2} \stackrel{\left[\mathrm{Co}(\mathrm{CO})_{4}\right]_{2}}{\longrightarrow}\) 4\. \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}_{2}+\mathrm{O}_{2} \frac{\mathrm{PdCl}_{2}, \mathrm{CuCl}_{2}}{\mathrm{H}_{2} \mathrm{O}}\) List II (Products) (i) \(\mathrm{CH}_{3} \mathrm{CHO}\) (ii) \(\mathrm{PhCHO}\) (iii) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CHO}\) The correct matching is: \(\begin{array}{llll}1 & 2 & 3 & 4\end{array}\) (a) (iii) (ii) (i) (i) (b) (i) (i) (ii) (iii) (c) (ii) (iii) (i) (i) (d) (i) (ii) (iii) (i)

Step-by-Step Solution

Answer

The correct matching is 1-(i), 2-(ii), 3-(iii), 4-(i), which corresponds to option (d).

Step 1: Identify Reaction Type 1

For Reaction 1, \( \mathrm{HC} \equiv \mathrm{CH} + \mathrm{H}_{2}\mathrm{O} \longrightarrow \), in the presence of \( \mathrm{Hg}^{2+} \), this reaction is a hydration of alkynes resulting in a ketone or aldehyde. Usually, this converts terminal alkynes to aldehydes. The product is \( \mathrm{CH}_{3}\mathrm{CHO} \) (ethanal).

Step 2: Identify Reaction Type 2

For Reaction 2, \( \mathrm{PhCH}_3 \) with \( \mathrm{Cl}_2 \) under \( \Delta \) (heat) suggests free radical halogenation at the benzylic position, followed by hydrolysis. The chlorine is replaced by a hydroxyl group, leading to \( \mathrm{PhCHO} \) (benzaldehyde).

Step 3: Identify Reaction Type 3

For Reaction 3, \( \mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}_2+\mathrm{CO}+\mathrm{H}_2 \), catalyzed by \( \left[\mathrm{Co}(\mathrm{CO})_{4}\right]_{2} \), represents the hydroformylation (oxo process) of an alkene, generally forming an aldehyde with one more carbon than the original alkene. The product here is \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CHO} \) (butanal).

Step 4: Identify Reaction Type 4

For Reaction 4, the oxidation of \( \mathrm{H}_{2}\mathrm{C}=\mathrm{CH}_{2} \) using \( \mathrm{PdCl}_{2}, \mathrm{CuCl}_{2} \) in presence of \( \mathrm{H}_{2}\mathrm{O} \) is the Wacker process, resulting in the addition of water to form \( \mathrm{CH}_{3}\mathrm{CHO} \) (acetaldehyde).

Step 5: Matching the Products with Reactants

Based on previous steps, match as follows:1 - \( \mathrm{CH}_{3} \mathrm{CHO} \) (i)2 - \( \mathrm{PhCHO} \) (ii)3 - \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CHO} \) (iii)4 - \( \mathrm{CH}_{3} \mathrm{CHO} \) (i)

Key Concepts

Hydration of Alkynes

The process of hydrating alkynes involves the addition of water to the alkyne, typically resulting in a carbonyl compound like a ketone or aldehyde. This is catalyzed by an acid, often with the help of mercuric ions (\(\text{Hg}^{2+}\)).

In the given reaction, the terminal alkyne (\(\mathrm{HC} \equiv \mathrm{CH}\)) is hydrated to produce ethanal (\(\mathrm{CH}_{3}\mathrm{CHO}\)). This transformation generally follows Markovnikov's rule, where the hydroxyl group attaches to the more substituted carbon.

A key aspect of this reaction is the intermediate formation of an enol, which quickly tautomerizes to a more stable carbonyl compound. Tautomerization is an important concept where the molecule rearranges, shifting a hydrogen atom and forming a carbonyl group from an enol.

Free Radical Halogenation

Free radical halogenation is a reaction that typically involves alkanes and halogens, leading to the substitution of a hydrogen atom with a halogen. In this context, the reaction involves the use of chlorine (\(\mathrm{Cl}_2\)) and heat (\(\Delta\)) to initiate the reaction.

Initially, the chlorine molecules dissociate into radicals under the influence of heat. These radicals abstract hydrogen atoms from the alkyl group forming alkyl radicals that subsequently combine with chlorine, resulting in a halogenated compound.

• This is followed by hydrolysis, which replaces the chlorine with a hydroxyl group.
• The reaction is quite selective, often taking place at the benzylic or allylic positions, which are activated sites for such transformations.

Ultimately, this leads to the formation of benzaldehyde (\(\mathrm{PhCHO}\)), after the substitution at the benzylic position is completed and the halogen is replaced by a hydroxyl group.

Hydroformylation (Oxo process)

Hydroformylation is an important industrial process that entails converting alkenes into aldehydes by the addition of formyl groups (\(\mathrm{-CHO}\)), using carbon monoxide (\(\mathrm{CO}\)) and hydrogen (\(\mathrm{H}_2\)).

The catalyst commonly used for this reaction is cobalt carbonyl, specifically \(\left[\mathrm{Co}(\mathrm{CO})_{4}\right]_{2}\). This reaction is significant for producing aldehydes with one carbon atom more than the starting alkene.

• The resulting product in this context is butanal (\(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CHO}\)).
• This showcases the typical application of the oxo process—transforming a linear chain alkene into a linear aldehyde.

By adjusting the conditions and catalysts, the reaction selectivity can be tailored to produce different aldehydes or alcohols.

Wacker Process

The Wacker process is a chemical reaction used for the oxidation of ethylene (\(\mathrm{H}_{2}\mathrm{C}=\mathrm{CH}_{2}\)) to acetaldehyde (\(\mathrm{CH}_{3}\mathrm{CHO}\)) using a palladium (\(\mathrm{PdCl}_{2}\)) catalyst.

Combined with copper chloride (\(\mathrm{CuCl}_{2}\)), this reaction facilitates the formation of a carbon-carbon double bond into a carbonyl group in the presence of water. The reaction operates in an aqueous medium where water provides the necessary hydroxyl groups.

• The process exemplifies an application of the catalytic cycle where palladium alternates between different oxidation states, facilitating the introduction of oxygen into organic molecules.
• It's an efficient and controlled method for producing simple aldehydes like acetaldehyde from basic alkenes, utilizing mild conditions that reduce unwanted side reactions.

Being an industrially relevant reaction, the Wacker process is used extensively in the synthesis of various aldehydes and ketones.