Question

Which of the following reagents may be used to convert propanoic acid to propan-1-ol? (A) \(\mathrm{NaBH}_{4}\) (B) \(\mathrm{LiAlH}_{4}\) (C) \(\left(\mathrm{Me}_{\mathrm{CHO}}\right)_{2} \mathrm{Al}, \mathrm{Me}_{2} \mathrm{CHOH}\) (D) \(\mathrm{NaOH}, \mathrm{CaO} \Delta\)

Short answer

The reagent capable of converting propanoic acid to propan-1-ol is option (B) \(\mathrm{LiAlH}_{4}\). This is because it is a strong reducing agent that can reduce the carboxyl group of the propanoic acid to a hydroxyl group, thus resulting in propan-1-ol. The other reagents either cannot reduce carboxylic acids directly or result in different reaction products.

Step-by-step solution

Understand the structure of propanoic acid and propan-1-ol

Propanoic acid has the formula \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{COOH}\), which contains a carboxyl group (-COOH). Propan-1-ol has the formula \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{OH}\), which has a hydroxyl group (-OH) at the 1st carbon atom. We need to find a reagent that can convert the carboxyl group into a hydroxyl group.

Analyze the reaction of propanoic acid with option (A) - \(\mathrm{NaBH}_{4}\)

Sodium borohydride (\(\mathrm{NaBH}_{4}\)) is a mild reducing agent that can reduce aldehydes and ketones to their corresponding alcohols but cannot reduce carboxylic acids directly. Therefore, it will not convert propanoic acid to propan-1-ol.

Analyze the reaction of propanoic acid with option (B) - \(\mathrm{LiAlH}_{4}\)

Lithium aluminum hydride (\(\mathrm{LiAlH}_{4}\)) is a strong reducing agent that can reduce carbonyl compounds such as aldehydes, ketones, and carboxylic acids. When reacted with propanoic acid, it is capable of reducing the carboxyl group to a hydroxyl group, converting propanoic acid to propan-1-ol.

Analyze the reaction of propanoic acid with option (C) - \((\mathrm{Me}_{\mathrm{CHO}})_{2} \mathrm{Al}, \mathrm{Me}_{2}\mathrm{CHOH}\)

Option (C) represents Meerwein-Ponndorf-Verley (MPV) reduction conditions, which are typically used to reduce aldehydes and ketones to alcohols. This reagent system is not effective in reducing carboxylic acids directly. Therefore, it will not convert propanoic acid to propan-1-ol.

Analyze the reaction of propanoic acid with option (D) - \(\mathrm{NaOH}, \mathrm{CaO} \Delta\)

Sodium hydroxide (\(\mathrm{NaOH}\)) and calcium oxide (\(\mathrm{CaO}\)) with heat (\(\Delta\)) are used for the decarboxylation of carboxylic acids, which typically results in the formation of an alkene, not an alcohol. Therefore, this option will not convert propanoic acid to propan-1-ol.

Conclusion

Based on the analysis of each reagent's reducing capabilities and reaction mechanisms, only option (B) \(\mathrm{LiAlH}_{4}\) is capable of converting propanoic acid to propan-1-ol.

Key concepts

Carboxylic Acids

Carboxylic acids are a class of organic compounds characterized by the presence of a carboxyl group (-COOH). This group consists of a carbon atom double-bonded to an oxygen atom and singly bonded to a hydroxyl group (-OH). Carboxylic acids are typically identified by their sour taste and pungent smell, common in substances like vinegar (which contains acetic acid).
Carboxylic acids are important in organic chemistry due to their acidic properties. They can donate a proton (H⁺) to bases, thus behaving as acids. The acidity of carboxylic acids arises from the ability of the carboxylate ion, which forms when the acid loses a proton, to stabilize by resonance.
Understanding carboxylic acids is crucial as they serve as starting materials for many chemical reactions. For example, these acids can be reduced to form alcohols, which is a topic we will delve into further below.

Reducing Agents

In organic chemistry, reducing agents play a pivotal role in transformation reactions where compounds are often converted from one functional group to another. A reducing agent is a substance that donates electrons to another substance, effectively "reducing" that compound by adding electrons.
When converting carboxylic acids into alcohols, the choice of reducing agents is critical. Sodium borohydride \(\mathrm{NaBH}_{4}\) is classified as a mild reducing agent. It can effectively reduce aldehydes and ketones but lacks the strength to tackle carboxylic acids.
On the other hand, lithium aluminum hydride \(\mathrm{LiAlH}_{4}\) is known for its strength. It is capable of reducing a wide range of carbonyl-containing compounds, including carboxylic acids. Thus, \(\mathrm{LiAlH}_{4}\) is effective for the conversion of carboxylic acids to alcohols, which involves breaking strong bonds and forming alcohol groups.

Alcohol Formation

Alcohols are organic compounds characterized by the presence of one or more hydroxyl groups (-OH) attached to a carbon atom. In the conversion process from carboxylic acids to alcohols, a crucial change occurs: the carboxyl group (-COOH) is transformed into a hydroxyl group (-OH) on an aliphatic chain.
This conversion requires the breaking of the carbon-oxygen double bond in the carboxyl group. Reducing agents facilitate this transformation by donating electrons, leading to the formation of a single-bonded -OH group, typical of an alcohol.
The reduction of carboxylic acids to primary alcohols, such as converting propanoic acid to propan-1-ol, is significant in organic synthesis. The alcohols formed are versatile and can serve as intermediates in further chemical reactions, expanding the possibilities in synthetic chemistry and pharmaceutical applications.