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 reaction produces \((\mathrm{CH}_{3})_{3} \mathrm{CD}\). (Option a)

Step 1: Identify the Compound

We are working with tert-butyl magnesium chloride, which can be written as \((\mathrm{CH}_{3})_{3} \mathrm{C}^{-}\) with the \(\mathrm{Mg}^+\) cation. This compound is a type of Grignard reagent.

Step 2: Determine the Reaction Type

Grignard reagents react with water through a proton exchange process, where the acidic hydrogen from water replaces the magnesium chloride group. In this case, water is replaced by heavy water \(\mathrm{D}_{2} \mathrm{O}\), hence deuterium \((\mathrm{D})\) ions are exchanged instead of hydrogen \((\mathrm{H})\) ions.

Step 3: Exchange Magnesium Chloride Group

The \(\mathrm{MgCl}\) in the Grignard reagent \((\mathrm{CH}_{3})_{3} \mathrm{CMgCl}\) is replaced by a deuterium ion \((\mathrm{D})\) from the heavy water, leading to the formation of \((\mathrm{CH}_{3})_{3} \mathrm{CD}\).

Step 4: Write the Final Product

After the exchange, the final product of the reaction is \((\mathrm{CH}_{3})_{3} \mathrm{CD}\). This shows that the \(\mathrm{D}_{2} \mathrm{O}\) provides \(\mathrm{D}^-\) ions which bond with the carbon atom previously connected to the Grignard reagent.

Key Concepts

Tert-butyl magnesium chloride

Tert-butyl magnesium chloride, denoted as \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CMgCl}\), is a specific type of Grignard reagent. This compound features a carbon atom lightly bound to magnesium chloride. Grignard reagents are notable for their reactivity with various substrates, particularly in forming carbon-carbon bonds. The carbon in the group, tert-butyl, refers to a tertiary butane structure, which is connected to the magnesium chloride.

• "Tert-butyl" indicates a carbon chain where the carbon atom connected to magnesium is bound to three methyl groups (\(\mathrm{CH}_3\)).
• The magnesium atom in Grignard reagents acts as a bridge between the carbon ion and other atoms during a reaction.

The importance of tert-butyl magnesium chloride in reactions stem from its ability to act as a nucleophile.This allows it to donate electrons to other species, making it very reactive in processes like the proton exchange we will talk about next.

Proton exchange process

The proton exchange process is central to the reaction when a Grignard reagent interacts with water. This process involves the transfer of a proton, which is essentially a hydrogen ion (\(\mathrm{H}^+\)), from water to the organic component of the Grignard reagent. This exchange converts the reagent's magnesium-bound carbon into a hydrocarbon.

Role in the Reaction

• Grignard reagents like tert-butyl magnesium chloride react swiftly because the carbon-metal bond is highly polarized.
• The negative charge on the carbon makes it keen to bind with positive ions, such as protons from water molecules.
• However, in our reaction, heavy water \(\mathrm{D}_{2}\mathrm{O}\) replaces regular water, facilitating a deuterium ion exchange instead of just proton exchange.

These reactions are highly useful in synthetic chemistry for creating a variety of carbon-containing compounds.

Heavy water (D2O)

Heavy water, with the chemical formula \(\mathrm{D}_{2}\mathrm{O}\), is a form of water where each hydrogen atom is replaced by deuterium. Deuterium, a stable isotope of hydrogen, features an additional neutron compared to regular hydrogen.This additional neutron gives deuterium greater mass and alters the bonding characteristics of heavy water.

• Heavy water reacts similarly to regular water in many chemical processes but can yield different products due to the presence of deuterium.
• In reactions, the exchange of \(\mathrm{D}^+\) ions instead of \(\mathrm{H}^+\) ions influences the properties of the end product, such as molecular weight and stability.

Sampling from specific reactions, such as the one involving tert-butyl magnesium chloride, can display noticeable differences when heavy water is applied, resulting in the formation of deuterium-containing compounds.

Deuterium ion exchange

Deuterium ion exchange is a pivotal step in the reaction when a Grignard reagent reacts with \(\mathrm{D}_{2}\mathrm{O}\). This exchange involves the replacement of \(\mathrm{MgCl}\) in the reagent with \(\mathrm{D}^-\) from heavy water, creating a deuterium-substituted hydrocarbon.

Incorporation of Deuterium

• During the reaction with tert-butyl magnesium chloride, \(\mathrm{D}^-\) ions replace the \(\mathrm{Cl}^-\) associated with magnesium. This yields \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CD}\) as the final product.
• Deuterium adds unique physical properties to the compound, including altered spectral characteristics and slight increases in stability.
• This process demonstrates how deuterium can label compounds, which is valuable for tracing chemical pathways or studying reaction mechanisms.

By applying deuterium exchange, chemists can synthesize compounds with properties tailored for specific research applications or industrial needs.