Problem 110 - A Complete Resource Book in Chemistry for JEE Main

Question

If 3 -hexanone is reacted with \(\mathrm{NaBH}_{4}\) followed by hydrolysis with \(\mathrm{D}_{2} \mathrm{O}\), the product will be (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CD}(\mathrm{OD}) \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{OD}) \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CD}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

The product is (a) \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CD(OD)CH}_2 \mathrm{CH}_2 \mathrm{CH}_3 \).

Step 1: Understand the Reaction

3-hexanone is a ketone with the structure \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{COCH}_2 \mathrm{CH}_2 \mathrm{CH}_3 \). When reacted with \( \mathrm{NaBH}_4 \), the carbonyl group (C=O) is reduced to an alcohol (C-OH). This occurs because \( \mathrm{NaBH}_4 \) is a reducing agent that adds hydrogen (H) to the carbonyl group.

Step 2: Reduction with \( \mathrm{NaBH}_4 \)

During the reduction, the \( \mathrm{C=O} \) group in 3-hexanone receives a hydrogen from \( \mathrm{NaBH}_4 \) and turns into an alcohol group (\( \mathrm{-OH} \)). The new intermediate structure becomes \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CHOHCH}_2 \mathrm{CH}_2 \mathrm{CH}_3 \). At this stage, the \( \mathrm{OH} \) includes a normal hydrogen atom.

Step 3: Hydrolysis with \( \mathrm{D}_2\mathrm{O} \)

When the alcohol formed during reduction undergoes hydrolysis with \( \mathrm{D}_2\mathrm{O} \), the deuterium (D) from D2O exchanges with the hydrogen (H) in the alcohol group. Therefore, the structure becomes \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CHD(OD)CH}_2 \mathrm{CH}_2 \mathrm{CH}_3 \). This is a result of the hydrogen-deuterium exchange process that occurs in the presence of \( \mathrm{D}_2\mathrm{O} \), turning \( \mathrm{OH} \) to \( \mathrm{OD} \) and \( \mathrm{H} \) to \( \mathrm{D} \) where the exchange occurs.

Step 4: Conclusion: Identify the Product

The final product, after reduction with \( \mathrm{NaBH}_4 \) and hydrolysis with \( \mathrm{D}_2\mathrm{O} \), is \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CD(OD)CH}_2 \mathrm{CH}_2 \mathrm{CH}_3 \). This matches option (a).

Key Concepts

Reduction Reactions

Reduction reactions are a vital part of organic chemistry, involving the addition of electrons or hydrogen to a molecule, thereby reducing its oxidation state. In the context of organic compounds, these reactions often involve the conversion of carbonyl groups to alcohols. This is typically accomplished using a reducing agent, which donates hydrogen atoms to the molecule.
Reduction reactions play a crucial role in transforming various functional groups. For instance, in our exercise with 3-hexanone reacting with sodium borohydride (NaBH_4), the ketone is reduced to an alcohol. NaBH_4 is a mild reducing agent, widely used in laboratories due to its effectiveness at selectively targeting carbonyl groups without affecting other functional groups such as olefins or esters.
This reduction works by sodium borohydride delivering a hydride ion (an anion of hydrogen with two electrons) to the carbonyl carbon. This nucleophilic attack results in the conversion of the carbonyl (C=O) group into an alcohol (C-OH), reducing the molecule and altering its chemical properties. Understanding reduction reactions is essential in fields such as pharmaceuticals where they synthesize alcohols from ketones.

Hydrogen-Deuterium Exchange

Hydrogen-deuterium exchange is an interesting chemical process where a hydrogen atom in a molecule is replaced by deuterium. Deuterium is a stable isotope of hydrogen with an additional neutron, giving it a different mass. This process is often used in chemical research to label compounds, helping scientists track chemical reactions and the fate of specific atoms.
In the given exercise, after 3-hexanone is reduced by sodium borohydride, it undergoes a hydrogen-deuterium exchange when it is treated with deuterium oxide (D_2O). The initial alcohol formed during the reduction phase (C-OH) exchanges its hydrogen in the presence of D_2O, resulting in deuterated alcohol (C-OD).

• This exchange is possible due to the reversible nature of proton transfer in the hydroxy group.
• It is commonly used in nuclear magnetic resonance (NMR) studies to understand more about the molecular structure and dynamics.

This process exemplifies the dynamic nature of organic reactions, demonstrating how isotopic labeling can provide valuable insights into complex chemical behaviors.

Ketone Reduction

Ketone reduction is a specific type of reduction reaction where a ketone is transformed into an alcohol. In the exercise, 3-hexanone is the ketone undergoing reduction, illustrating a common type of reaction in organic chemistry.
The reduction of ketones generally occurs under mild conditions using agents like sodium borohydride (NaBH_4) or lithium aluminum hydride (LiAlH_4). For 3-hexanone, NaBH_4 is preferred because it specifically targets the ketone group without affecting other parts of the molecule. During the process:

• The hydride ion from the reducing agent attacks the electrophilic carbonyl carbon.
• As a result, the carbonyl (C=O) group is converted to an alcohol (C-OH).

This transformation reduces the ketone to a secondary alcohol, making it an essential reaction for creating alcohols from carbonyl-containing precursors. Understanding this process is crucial for industries that manufacture fragrances, flavors, and pharmaceuticals where alcohol derivatives are necessary.

Nucleophilic Addition

Nucleophilic addition reactions are fundamental transformations in organic chemistry where a nucleophile, a molecule or ion with an electron pair to donate, attacks an electrophilic carbon atom in a multiple bond (like a carbonyl group). This attack leads to the addition of the nucleophile to the carbon and results in the transformation of the double-bonded system.
In the case of ketone reduction discussed in the exercise, the nucleophilic addition occurs when the hydride ion from sodium borohydride (NaBH_4) attacks the electrophilic carbon atom of the carbonyl group in 3-hexanone. This reaction sequence involves:

• The nucleophile (hydride ion) donates an electron pair to the carbonyl carbon, breaking the C=O double bond and converting it into a single bond.
• The result is the formation of an alkoxide ion intermediate, which is then protonated (in this case through deuteration) to yield an alcohol.

Nucleophilic addition to carbonyls is critical because it forms the basis of many other reactions, such as aldol reactions, and is part of the backbone of organic synthesis strategies involving the creation of complex molecules from simpler ones.