Problem 38 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following pairs is correctly matched? (a) \(\mathrm{R}-\mathrm{Cl} \frac{\mathrm{Nal}}{\text { Acetone }}\) Finkelstain reaction (b) \(\mathrm{R}-\mathrm{X} \stackrel{\mathrm{Na}}{\longrightarrow}\) Frankland reaction (c) \(\mathrm{R}-\mathrm{X} \stackrel{\mathrm{Zn} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}}{\longrightarrow}\) Wurtz reaction \(\mathrm{X}\) (d) \(\mathrm{CH}_{3}-\mathrm{CH}-\mathrm{CH}_{3} \stackrel{\text { Alc. KOH }}{\rightarrow}\) Grignard reaction

Step-by-Step Solution

Answer

Option (a) is correctly matched: Finkelstein reaction.

Step 1: Understanding each reaction

First, understand what each mentioned reaction does: - Finkelstein reaction involves the exchange of halogen atoms in an organic molecule using Nal in acetone. - The Frankland reaction involves the creation of an organometallic compound by the reaction of an alkyl halide with sodium. - The Wurtz reaction involves coupling two alkyl halides to form a higher alkane, typically using sodium. - The Grignard reaction involves the formation of a Grignard reagent (an organomagnesium compound) from an alkyl halide.

Step 2: Identifying correct matches

Now, evaluate each option to determine which is correctly matched: - Option (a) corresponds to the Finkelstein reaction, which is correctly described as the substitution of halogen using Nal in acetone. - Option (b) refers to the Frankland reaction, but it is incorrectly presented as a direct reaction with sodium without specifying that the group formed is an organometallic compound. - Option (c) mentions the Wurtz reaction, however, it involves the use of zinc and ethanol, which is incorrect for Wurtz. The correct reagent for Wurtz is sodium (Na). - Option (d) refers to the Grignard reaction but describes a dehydrohalogenation step, not the formation of a Grignard reagent, which is incorrect for the reaction description.

Key Concepts

Finkelstein Reaction

The Finkelstein Reaction is a classic organic chemistry reaction used to swap halogen atoms within an organic compound. This reaction is particularly useful for replacing a chlorine atom with an iodine atom. It employs sodium iodide (\( \text{NaI} \)) in acetone as the reagent. Here's how it works:

• The organic compound with a chloride group interacts with \( \text{NaI} \).
• Acetone acts as the solvent, facilitating the reaction without dissolving \( \text{NaCl} \), which precipitates out and drives the reaction forward.
• This process leads to the exchange of the chlorine atom for an iodine atom.

This method is an example of a nucleophilic substitution reaction (specifically \( \text{S}_\text{N}2 \)), where sodium iodide provides the nucleophile iodide ion (\( \text{I}^- \)). The reaction tends towards completion because sodium chloride (\( \text{NaCl} \)) is insoluble in acetone, and thus, it pushes the equilibrium towards the desired product.

Frankland Reaction

Named after the British chemist E. Frankland, the Frankland Reaction is an important technique for forming organometallic compounds. In this reaction, an alkyl halide reacts with metallic sodium to create an organometallic compound. Here are the key steps:

• An alkyl halide (\( \text{R}-\text{X} \)) is treated with sodium (Na).
• The reaction typically occurs in a dry ether solvent to avoid unwanted reactions with moisture.
• It results in the formation of an organosodium compound, a type of organometallic reagent.

Organometallic compounds like those formed in the Frankland Reaction have diverse applications, particularly in synthetic organic chemistry, due to their ability to enable further chemical transformations. It's crucial to maintain an inert atmosphere to prevent reactive metal from reacting with air or moisture. The reaction is typically performed under rigorous exclusion of water and air to ensure effective and desired outcomes.

Wurtz Reaction

The Wurtz Reaction is a classic method for synthesizing alkanes by coupling two alkyl halides. Typically, this reaction uses sodium as the metallic agent to facilitate the coupling. Here’s how it accomplishes this transformation:

• Two molecules of an alkyl halide are treated with sodium, typically in a dry ether solution.
• The sodium helps remove the halogen atoms from the alkyl halide molecules.
• The resulting radicals combine to form a new carbon-carbon bond, producing a longer-chain alkane. For instance, ethyl bromide can be transformed into butane.

The Wurtz Reaction is particularly useful for forming symmetrical alkanes, where both reacting alkyl groups are identical. However, it can lead to mixtures of products if different alkyl halides are used. This reaction demonstrates classic methods of chain short-cipation and length modification in hydrocarbon chemistry.

Grignard Reaction

The Grignard Reaction is a staple in organic synthesis, enabling the formation of carbon-carbon bonds with great versatility. It involves the creation of Grignard reagents, which are organomagnesium compounds formed from alkyl halides. Here's a breakdown:

• An alkyl halide (\( \text{R}-\text{X} \)) is reacted with magnesium metal in an anhydrous ether solution, typically diethyl ether or THF.
• This process generates a Grignard reagent (\( \text{R}-\text{MgX} \)).
• Grignard reagents are nucleophilic and can insert into various substrates, reacting with carbonyl compounds to form alcohols.

This reaction is central to many synthetic strategies, offering a route to form complex and multi-functional organic molecules. Special care must be taken to maintain anhydrous conditions, as Grignard reagents are highly sensitive to moisture and air, which can destroy their reactivity. Using the Grignard Reaction, chemists can expand molecular complexity and construct new structural motifs efficiently.