Question

\(\mathrm{R}-\mathrm{COOH} \longrightarrow \mathrm{R}-\mathrm{CH}_{2} \mathrm{OH}\) Which of the following reagent(s) can be used? (A) \(\operatorname{LiAlH}_{4}\) (B) \(\mathrm{NaBH}_{4}\) (C) \(\mathrm{B}_{2} \mathrm{H}_{6}-\mathrm{THF}\) (D) Both (A) and (C)

Short answer

The correct answer is option (D), as both Lithium Aluminum Hydride \(\operatorname{LiAlH}_{4}\) and Diborane-THF \(\mathrm{B}_{2}\mathrm{H}_{6}-\mathrm{THF}\) can effectively reduce the carboxylic acid group (\(\mathrm{R}-\mathrm{COOH}\)) to an alcohol group (\(\mathrm{R}-\mathrm{CH}_{2}\mathrm{OH}\)). Sodium Borohydride \(\mathrm{NaBH}_{4}\) is not strong enough for this reaction.

Step-by-step solution

Examine Lithium Aluminum Hydride (LiAlH4)

First, let's consider option (A) which is \(\operatorname{LiAlH}_{4}\). Lithium Aluminum Hydride is a strong reducing agent that can effectively reduce the carboxylic acid group (\(\mathrm{COOH}\)) to an alcohol group (\(\mathrm{CH}_{2}\mathrm{OH}\)). Therefore, this reagent can be used for the given reaction.

Examine Sodium Borohydride (NaBH4)

Next, let's look at option (B) which is \(\mathrm{NaBH}_{4}\). Sodium borohydride is a mild reducing agent that can reduce aldehydes and ketones to their respective alcohols. However, it is not strong enough to reduce carboxylic acids like \(\mathrm{R}-\mathrm{COOH}\). So, this reagent cannot be used for the given reaction.

Examine Diborane-THF (B2H6-THF)

Now, let's examine option (C) which is \(\mathrm{B}_{2}\mathrm{H}_{6}-\mathrm{THF}\). Diborane in Tetrahydrofuran (THF) is a strong reducing agent that can effectively reduce carboxylic acid groups (\(\mathrm{COOH}\)) to alcohol groups (\(\mathrm{CH}_{2}\mathrm{OH}\)). Thus, this reagent can be used for the given reaction.

Identify the correct option(s)

Based on our individual examination of the given reagents, we can conclude the following: (A) \(\operatorname{LiAlH}_{4}\) can reduce the carboxylic acid to the alcohol. (B) \(\mathrm{NaBH}_{4}\) cannot reduce the carboxylic acid to the alcohol. (C) \(\mathrm{B}_{2}\mathrm{H}_{6}-\mathrm{THF}\) can reduce the carboxylic acid to the alcohol. So, the correct answer is option (D), which includes both (A) and (C) as the suitable reagents for the given reaction.

Key concepts

Lithium Aluminum Hydride (LiAlH4)

Lithium Aluminum Hydride (LiAlH4) is widely recognized as a powerful reducing agent in organic chemistry. It is particularly effective in the reduction of carboxylic acids to primary alcohols, a transformation crucial in various synthetic pathways.

Its mechanism involves the donation of hydride ions (H-) to the electrophilic carbon in the carbonyl group of the carboxylic acid. This process produces an alkoxy intermediate, which upon hydrolysis yields the desired alcohol. Due to its high reactivity, LiAlH4 can reduce most carbonyl-containing compounds, including esters, amides, and even some nitro compounds, providing a versatile tool for chemists.

However, handling LiAlH4 requires caution due to its aggressive reaction profile, particularly with water, necessitating anhydrous conditions and an inert atmosphere for safe use.

Sodium Borohydride (NaBH4)

Sodium Borohydride (NaBH4) finds its place in the chemist's toolkit as a milder alternative to LiAlH4. Being less reactive, NaBH4 specializes in reducing aldehydes and ketones to their corresponding alcohols.

Due to its selectivity, NaBH4 is favored when the reduction of more sensitive or less reactive functional groups is required. Unlike LiAlH4, NaBH4 cannot reduce carboxylic acids or esters under standard conditions. This selective reactivity provides chemists with the ability to fine-tune the reductions in complex molecules without affecting other functional groups.

NaBH4 is also safer to handle and can be used in protic solvents such as alcohols or water, which broadens the scope of its applicability and simplifies its use in laboratory settings.

Diborane-THF (B2H6-THF)

Diborane (B2H6) complexed with Tetrahydrofuran (THF) is another potent reducing agent and can be viewed as a heavy hitter, much like LiAlH4, in the domain of carboxylic acid reduction.

Its unique reactivity profile allows B2H6-THF to reduce carboxylic acids to alcohols. This ability stems from its structure, characterized by boron atoms bearing a strong partial positive charge that facilitates the addition of hydrogen atoms to the carboxylic substrate.

Despite its efficiency, diborane carries some inherent challenges including its gaseous form at room temperature and sensitivity to moisture. Working with its THF complex helps stabilize the reagent, making it more handleable for chemists when reducing carboxylic acids to alcohols.

Organic Chemistry Reactions

Organic chemistry is replete with reactions that transform molecules from one functional group to another. Reduction of carboxylic acids to alcohols is a cornerstone reaction, illustrating the conceptual art of electrons and bonds dancing to the tune of a chemist's design.

In the context of organic transformations, the choice of a reducing agent is critical. It must be powerful enough to cleave the resilient bonds found in functional groups like carboxylic acids, yet selective enough to leave other parts of the molecule unscathed. This delicate balance is what makes agents like LiAlH4 and B2H6-THF invaluable.

Overall, understanding the nuances of each reducing agent and the types of reactions they catalyze allows chemists to harness the true potential of organic chemistry, whether in academic research or industrial pharmaceutical development.