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) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

Short answer

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

Step-by-step solution

Identify reactants and process

We start with 3-hexanone and the reagents sodium borohydride (NaBH₄) followed by deuterated water (D₂O). The process involves reduction followed by hydrolysis.

Reduction with NaBH₄

Sodium borohydride (NaBH₄) is a reducing agent commonly used to reduce ketones to secondary alcohols. This process will reduce the carbonyl group in 3-hexanone (a ketone) to a secondary alcohol. The carbonyl group (C=O) in 3-hexanone is reduced to an alcohol group (C-OH), resulting in hexan-3-ol.

Hydrolysis with D₂O

In the presence of deuterated water (D₂O), the oxygen in the newly formed alcohol group (from the reduction step) will undergo hydrolysis, where the hydrogen replaces with deuterium. This results in the exchange of the hydrogen atom attached to the oxygen in the alcohol with a deuterium atom, forming an OD group in place of the OH group.

Determine correct product structure

After hydrolysis, the hexan-3-ol is converted to hexan-3-2-deuterium-ol (basically, hexan-3-ol but with D in place of H on the hydroxyl group). Thus, the final product is \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{OD}) \mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{3}\), corresponding to option (b).

Key concepts

Sodium Borohydride Reduction

Sodium borohydride, or NaBH₄, is a key reagent widely used in organic chemistry for the reduction of ketones. Ketones possess a carbonyl group, which is a carbon atom double-bonded to an oxygen atom. This carbonyl group is highly reactive and can be reduced to an alcohol group. Sodium borohydride provides the hydride ions needed to complete this reduction process.
Pertaining to the reaction, the hydride ions from NaBH₄ attack the electrophilic center at the carbon, bearing the double-bonded oxygen in the ketone group. As a result, the double bond breaks, and a hydrogen atom bonds to the carbon, transforming it into an alcohol. The oxygen now single-bonded to the carbon, bonds with a hydrogen to form an -OH group. This process is straightforward but highly effective, converting 3-hexanone into hexan-3-ol, a secondary alcohol.
NaBH₄ does not typically reduce other functional groups such as esters, amides, or carboxylic acids under standard conditions, but it is perfect for ketone reductions in mild conditions, maintaining the integrity of other parts of the molecule.

Hydrolysis with Deuterated Water

Hydrolysis using deuterated water (D₂O) is a fascinating step in the reaction sequence, enabling the exchange of hydrogen atoms with deuterium. Deuterium is a stable hydrogen isotope that contains an extra neutron, causing it to be heavier than regular hydrogen. When hexan-3-ol, the product of ketone reduction, comes into contact with D₂O, a specific interaction occurs at the -OH group.
In this hydrolysis, the hydrogen atom of the -OH group is replaced by a deuterium atom. Through this exchange process, the hydroxyl group ( -OH) becomes an -OD group. The reaction itself may involve acidic or basic catalysts, although in many cases, direct exchange occurs.
The value of using deuterated water lies in labeling functionalities within the molecule without altering their chemical behavior. This technique is especially useful for tracing and studying molecular pathways in chemical research due to deuterium’s unique nuclear properties.

Secondary Alcohol Formation

A secondary alcohol is characterized by the hydroxyl group (-OH) being attached to a carbon atom that is also connected to two other carbon atoms. In the reduction and subsequent hydrolysis processes, 3-hexanone is transformed into a secondary alcohol upon reduction with NaBH₄ to form hexan-3-ol.
The formation of secondary alcohols is crucial in many natural and synthetic processes. These molecules often serve as intermediates in various chemical reactions and are useful in the production of pharmaceuticals and perfumery.

• The structure of a secondary alcohol results from the position of the hydroxyl group on a carbon that is neither terminal nor only partly substituted by hydrogens.
• The transformation from ketone to secondary alcohol without altering other parts of the molecule showcases the selectivity and utility of the reduction and subsequent isotopic labeling via deuteration.

Understanding secondary alcohol formation, including the context of deuterated compounds, is key in tapping into the versatile applications of alcohols in both analytical and industrial chemistry sectors.