Problem 23 - A Complete Resource Book in Chemistry for JEE Main

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 formed is \\( (CH_3)_3CD \\, which corresponds to option (a).

Step 1: Understanding the Reagent

The reagent given is tert-butyl magnesium chloride, \((CH_3)_3CMgCl\), which is a Grignard reagent. Grignard reagents are nucleophilic organometallic compounds that are commonly used to introduce alkyl groups into synthesis reactions.

Step 2: Recognizing the Reaction Type

Grignard reagents react with deuterium oxide (\(D_2O\)) in a way that replaces the metal-bound carbon's \(H\) or \(C\) with a \(D\) atom, resulting in deuterated hydrocarbons. Here, the Grignard reagent \(CH_3)_3CMgCl\) will abstract a \(D\) using the deuterium from \(D_2O\).

Step 3: Predicting the Product

In this reaction, the \(Mg-Cl\) complex will leave and accept the negatively charged \(D\) from \(D_2O\), converting \(MgCl\) into the by-product \(Mg(OD)Cl\), while the \(D\) is added to the carbon center. As a result, the formed product is \( (CH_3)_3CD \).

Step 4: Conclusion

The reaction between \( (CH_3)_3CMgCl \) and \( D_2O\) results in the formation of \( (CH_3)_3CD \) as \(D\) replaces the hydrogen in the \( (CH_3)_3C-H \) bond.

Key Concepts

Organometallic Chemistry

Organometallic chemistry is a fascinating area that explores compounds containing bonds between carbon and metal atoms. One key player in this field is the Grignard reagent. Named after Victor Grignard, who received the Nobel Prize for its discovery, Grignard reagents are typically composed of an alkyl or aryl group bonded to a metal, such as magnesium in the form of magnesium halides, like \((CH_3)_3CMgCl\). These reagents act as nucleophiles, meaning they are electron-rich and seek out electron-deficient centers, usually carbon atoms.

• Grignard reagents are highly versatile in synthetic chemistry.
• They are used to form carbon-carbon bonds.

This versatility comes from their ability to react with various electrophiles, substances that accept electron pairs. The reactivity of Grignard reagents is primarily due to the polarized carbon-metal bond, where carbon carries a partial negative charge.

Deuterium Oxide Reactions

Deuterium oxide, known commonly as heavy water and symbolized as \({D_2O}\), is an isotopic form of water, where both hydrogen atoms are replaced by deuterium — an isotope of hydrogen. In reactions involving Grignard reagents, deuterium oxide provides an invaluable tool for the incorporation of deuterium into organic molecules.

• Deuterium is a key tool for studying reaction mechanisms.
• It alters reaction pathways subtly due to its mass difference from hydrogen.

When a Grignard reagent reacts with deuterium oxide, the magnesium metal serves as a bridge that facilitates the transfer of the deuterium atom to the carbon backbone of the organic molecule. Furthermore, because deuterium is heavier than hydrogen, it allows for the study of kinetic isotope effects in reactions and can be detected easily by nuclear magnetic resonance (NMR) spectroscopy, providing insights into reaction dynamics and pathways.

Nucleophilic Substitution

Nucleophilic substitution is an essential concept in organic chemistry, where a nucleophile — a molecule or ion with a lone pair of electrons — replaces a leaving group in a molecule. It plays a critical role in the behavior of Grignard reagents.

• Nucleophiles are electron-rich species that form bonds with positive or neutral electrophile centers.
• Common leaving groups in such reactions can be halides or other electronegative entities.

In the context of Grignard reagents reacting with deuterium oxide, the nucleophilic carbon (from the Grignard reagent) is ready to accept a deuterium atom. During this process, the magnesium halide part of the Grignard compound effectively "leaves" the organic molecule, creating a unique transformation: the abstraction of a deuterium atom in place of the existing hydrogen. This step illustrates how nucleophilic substitution can lead to the formation of deuterated compounds, which are extremely useful in both research and synthetic contexts.