Problem 25 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

\(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CMgCl}\) on reaction with \(\mathrm{D}_{2} \mathrm{O}\) produces : (a) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CD}\) (b) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{OD}\) (c) \(\left(\mathrm{CD}_{3}\right)_{3} \mathrm{CD}\) (d) \(\left(\mathrm{CD}_{3}\right)_{3} \mathrm{OD}\)

Step-by-Step Solution

Answer

The product is \((\mathrm{CH}_{3})_{3} \mathrm{CD}\), option (a).

Step 1: Understanding the Grignard Reagent

The compound \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CMgCl}\) is a type of Grignard reagent. Grignard reagents contain a carbon-metal bond that is highly polarized, where the carbon has a partial negative charge. This makes the carbon an excellent nucleophile.

Step 2: Identify the Reactant Interaction

Determine how the Grignard reagent will react with \(\mathrm{D}_{2} \mathrm{O}\). The \(\mathrm{MgCl}\) group is bound to a carbon, and when it reacts with \(\mathrm{D}_{2} \mathrm{O}\), the \(\mathrm{MgCl}\) is removed, and a deuterium (from \(\mathrm{D}_{2} \mathrm{O}\)) is attached to the carbon.

Step 3: Substitution of Deuterium

When \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CMgCl}\) reacts with \(\mathrm{D}_{2} \mathrm{O}\), \(\mathrm{D}^-\) from the \(\mathrm{D}_{2} \mathrm{O}\) replaces the \(\mathrm{MgCl}\) group to form a C-D bond. The result is \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CD}\).

Step 4: Determine the Product

After considering the substitution reaction, the final compound formed is \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CD}\), which is the result of the original Grignard reagent losing \(\mathrm{MgCl}\) and gaining a deuterium atom instead.

Key Concepts

Nucleophilic Substitution

Nucleophilic substitution is a key reaction mechanism in organic chemistry. In these reactions, a nucleophile—an atom or molecule with a pair of electrons ready to form a bond—attacks and replaces a leaving group in a compound. The Grignard reaction involves such a nucleophilic substitution. Here, the Grignard reagent, represented as \(\left(\mathrm{CH}_3\right)_3 \mathrm{CMgCl}\), contains a carbon with a partial negative charge. This carbon acts as the nucleophile.

When the Grignard reagent comes in contact with \(\mathrm{D}_2 \mathrm{O}\) (deuterium oxide), the nucleophilic carbon attacks the deuterium atoms (\(\mathrm{D}\)), substituting them for the \(\mathrm{MgCl}\) group. This results in the formation of a C-D bond, since deuterium acts as the new part of the organic structure.

Understanding nucleophilic substitution allows you to predict how different reagents will interact and what products will result.

Deuterium Exchange

Deuterium exchange involves the replacement of hydrogen atoms in a molecule with deuterium atoms. Deuterium is a hydrogen isotope that has one neutron, making it heavier than the usual hydrogen atom. This exchange is crucial for labeling experiments that help trace molecular mechanisms.

In the context of the Grignard reaction with \(\mathrm{D}_2 \mathrm{O}\), a deuterium exchange occurs when the hydrogen from the Grignard reagent is replaced with deuterium from \(\mathrm{D}_2 \mathrm{O}\). The process leads to deuterium being incorporated into the organic structure, resulting in \(\left(\mathrm{CH}_3\right)_3 \mathrm{CD}\).

This exchange is efficiently facilitated due to the highly reactive nature of Grignard reagents, allowing them to react readily with isotopes like deuterium.

Organomagnesium Compounds

Organomagnesium compounds are central in the Grignard reaction. They are characterized by a direct carbon-magnesium bond. This bond is highly polarized, which makes the carbon atom act as a strong nucleophile eager to bond with electropositive atoms. The Grignard reagent \((\mathrm{CH}_3)_3 \mathrm{CMgCl}\) is a typical example of an organomagnesium compound.

These reagents are named after Victor Grignard, who won a Nobel Prize for discovering the reaction they undergo. The unique reactivity of organomagnesium compounds with water or other proton donors like \(\mathrm{D}_2 \mathrm{O}\) utilizes the difference in electronegativity between carbon and magnesium. During the reaction, the \(\mathrm{MgCl}\) group in the Grignard reagent is replaced with a deuterium atom, showing the versatility and importance of organomagnesium compounds in synthetic chemistry.

Grignard reagents are indispensable for forming carbon-carbon bonds, thus playing a crucial role in constructing complex molecules in laboratory syntheses.