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) \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{2} \mathrm{CD}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}^{3}\) (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 (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH} \mathrm{(OD)} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\).

Step 1: Identify the Reactants and Reaction Type

We have 3-hexanone, a ketone, being treated with sodium borohydride \(\text{NaBH}_4\) and then with deuterium oxide \(\text{D}_2\text{O}\). This reaction is a reduction followed by hydrolysis.

Step 2: Reduction of 3-Hexanone

The \(\text{NaBH}_4\) will reduce the carbonyl group (\text{C}=\text{O}) of 3-hexanone to a secondary alcohol by adding a hydrogen (\text{H}) to the carbonyl carbon, converting \text{C}=\text{O} to \text{-CH(OH)-} at the third position in the carbon chain.

Step 3: Determine the Initial Alcohol Product

After the reduction, the product is \(\text{CH}_3\text{CH}_2\text{CH(OH)CH}_2\text{CH}_2\text{CH}_3\), a hexanol where the carbonyl carbon has become a chiral alcohol center.

Step 4: Influence of Deuterium Oxide (\(\text{D}_2\text{O}\))

In \(\text{D}_2\text{O}\), the \text{OH} group of the alcohol can exchange its \text{H} with \text{D} (deuterium) from \(\text{D}_2\text{O}\), resulting in \text{OD}. Therefore, the product will be \(\text{CH}_3\text{CH}_2\text{CH(OD)CH}_2\text{CH}_2\text{CH}_3\).

Key Concepts

Ketone Reduction

Ketone reduction is a key reaction in organic chemistry, where a ketone is converted into an alcohol. This is achieved by adding hydrogen to the carbonyl group of the ketone. The process reduces the double-bonded oxygen in the ketone by introducing hydrogen atoms to it. This transforms the carbon-oxygen double bond (\(\text{C}=\text{O}\)) into a single bond hydroxyl group (\(-\text{CH(OH)}-\)). Reduction of ketones is a significant synthetic process, allowing the conversion into secondary alcohols. The reduction process decreases the oxidation state of the molecule, which is central to altering its chemical properties. - Converts ketone to alcohol- Adds hydrogen to the carbonyl group- Introduces a hydroxyl group in place of the carbonyl oxygen

Sodium Borohydride

Sodium borohydride (\(\text{NaBH}_4\)) is a powerful reducing agent commonly used in organic chemistry. It specializes in reducing aldehydes and ketones to their corresponding alcohols. This compound contains boron and hydrogen, and it donates hydride ions (\(\text{H}^-\)) during reactions. - Commonly used in ketone and aldehyde reductions- Effective at mild temperatures- Simple to handle and relatively safe compared to other reducing agentsThe hydride ions from \(\text{NaBH}_4\)facilitate the addition of hydrogen to the carbonyl carbon, which results in the formation of an alcohol from the ketone. This makes \(\text{NaBH}_4\)highly valuable in synthetic organic reactions for creating desired alcohol products from ketones.

Deuterium Oxide

Deuterium oxide, or heavy water (\(\text{D}_2\text{O}\)), is water in which both hydrogen atoms are replaced with deuterium, an isotope of hydrogen. Used often in organic chemistry for isotope exchange reactions, deuterium oxide can replace the hydrogen in hydroxyl groups with deuterium. - Heavy water contains deuterium instead of regular hydrogen- Used in isotopic labeling studies- Useful in exchange reactions, where it can convert an OH group to an OD groupDuring reactions involving alcohols, like the reduction of ketones, deuterium oxide can replace the hydrogen in an alcohol (OH group) with deuterium. This results in the formation of an OD group, providing insights into the mechanism and tracking of the chemical reaction.

Carbonyl Group

The carbonyl group (\(\text{C}=\text{O}\)) is an essential functional group in organic chemistry, found in many different chemical classes such as ketones, aldehydes, carboxylic acids, and esters. It consists of a carbon atom double-bonded to an oxygen atom. - Key functional group in organic molecules- Carbon atom is sp2 hybridized- Reacts in reductions to form hydroxyl groups (OH)In ketone reduction, the carbonyl group of the ketone is transformed. The oxygen atom in the carbonyl group interacts with reducing agents like \(\text{NaBH}_4\), which helps in reducing the double-bond nature of the carbonyl, converting it to an alcohol.

Secondary Alcohol

Secondary alcohols are alcohols where the hydroxyl group (\(-\text{OH}\)) is connected to a secondary carbon atom. This means the carbon with the hydroxyl group is attached to two other carbon atoms, which typically arises when a ketone is reduced. - Formed from ketone reduction- Characterized by the hydroxyl group on a secondary carbon- Central to many biological processes and industrial applicationsThe reduction of 3-hexanone with \(\text{NaBH}_4\)produces a secondary alcohol because the original carbonyl carbon (carbon 3 in the hexanone) gains an OH group, changing the ketone structure into a more stable alcohol format. This transformation is valuable in forming various synthetic chemicals owing to the unique properties of alcohols.