Question

\(\mathrm{CH}_{3}-\mathrm{CH}-\mathrm{COOH}\) can be converted into \(\mathrm{CH}_{3}-\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH}\) by the use of (a) \(\mathrm{H}_{2} / \mathrm{Pd}\) (b) LiAIH \(_{4}\) (c) \(\mathrm{NaBH}_{4}\) (d) \(\mathrm{CH}_{3} \mathrm{MgBr}\)

Short answer

Use LiAlH\(_4\) to convert the carboxylic acid to an alcohol.

Step-by-step solution

Understand the Reaction

The given compound, \(\mathrm{CH}_{3}-\mathrm{CH}-\mathrm{COOH}\), is a carboxylic acid. The target product, \(\mathrm{CH}_{3}-\mathrm{CH}-\mathrm{CH}_{2} \mathrm{OH}\), is an alcohol. The task is to determine which reagent can reduce the carboxylic acid to the alcohol by removing oxygen and giving hydrogen.

Consider the Reagents

Each reagent works differently: (a) \(\mathrm{H}_{2} / \mathrm{Pd}\) is typically used to hydrogenate alkenes or alkynes.(b) \(\text{LiAlH}_4\) (Lithium aluminium hydride) is a strong reducing agent capable of reducing carboxylic acids to alcohols.(c) \(\mathrm{NaBH}_{4}\) (Sodium borohydride) is a milder reducing agent, generally insufficient to reduce carboxylic acids.(d) \(\mathrm{CH}_{3} \mathrm{MgBr}\) (Grignard reagent) is used to elongate carbon chains rather than simply converting functional groups.

Identify the Appropriate Reagent

Based on the reagents' characteristics, \(\text{LiAlH}_4\) is the appropriate choice for reducing a carboxylic acid to an alcohol because it is strong enough to perform this transformation, unlike the other reagents listed.

Conclusion

Thus, the correct reagent to convert \(\mathrm{CH}_{3}-\mathrm{CH}-\mathrm{COOH}\) into \(\mathrm{CH}_{3}-\mathrm{CH}-\mathrm{CH}_{2}\mathrm{OH}\) is \(\text{LiAlH}_4\).

Key concepts

Carboxylic Acid Reduction

Carboxylic acids are a fundamental class of organic compounds characterized by the presence of a carboxyl group (-COOH).
They can undergo a specific type of chemical reaction known as reduction, where they are converted into primary alcohols. This transformation involves the replacement of the carbon-oxygen double bond in the carboxyl group with a carbon-hydrogen bond. The reduction of carboxylic acids is crucial because it allows the synthesis of alcohols, which are valuable as solvents, fuels, and intermediates in the production of various chemicals.
This process generally requires a strong reducing agent to break the stable carboxyl bond. The stoichiometry of the reduction reaction must ensure that the oxygen atoms are either completely removed or replaced by hydrogen atoms. This is often achieved through the addition of electrons or hydrogens. The choice of reducing agent is crucial, as the correct reagent can efficiently lead to the desired alcohol product. Proper understanding of the reagents and conditions required is key for success in carboxylic acid reduction.

Reducing Agents

Reducing agents are substances that donate electrons to another compound, reducing the oxidation state of a molecule.
In organic chemistry, they are essential for reactions that convert higher oxidation state compounds into lower ones. When reducing a carboxylic acid, we typically need a strong reducing agent capable of breaking the carboxyl group’s double bond. Lithium aluminium hydride (LiAlH extsubscript{4}) is a very powerful and widely used reducing agent for this purpose.
It is known for its ability to effectively turn carboxylic acids into alcohols under anhydrous conditions, meaning in the absence of water to prevent it from reacting with the reducing agent itself. Here are some other examples of reducing agents:

• **Hydrogen gas with a catalyst** (like Pd/C): Primarily reduces alkenes and alkynes rather than carboxylic acids.
• **Sodium borohydride (NaBH extsubscript{4})**: Milder than LiAlH extsubscript{4} and usually insufficient to reduce carboxylic acids.
• **Grignard reagents** (e.g., CH extsubscript{3}MgBr): Primarily used for carbon-chain elongation and other types of reactions.

The choice of reducing agent is vital for achieving the desired transformation from a carboxylic acid to an alcohol, and LiAlH extsubscript{4} is often the agent of choice for its effectiveness.

Alcohol Synthesis

Alcohol synthesis via reduction is a fundamental process in organic chemistry.
It involves converting more oxidized functional groups, such as carboxylic acids, into alcohols. This reduction typically requires strong conditions and precise reagent selection. The transition from carboxylic acid to alcohol requires a reducing agent to donate the necessary hydrogen atoms, converting the carboxyl group's carbonyl functionality into an alcohol. This process not only alters the functional group but also changes the compound's properties, such as boiling point and solubility. The typical steps in synthesizing an alcohol from a carboxylic acid include:

• **Selecting a strong reducing agent** like LiAlH extsubscript{4}, which is robust enough to handle complex reductions without affecting other parts of the molecule.


• **Performing the reaction under controlled conditions** to ensure that the reactants do not undergo side reactions. This can extend to temperature controls and the exclusion of water.


• **Isolating and purifying the resulting alcohol** to remove any unreacted starting material or byproducts, ensuring the primary alcohol is obtained in high purity.

The beauty of alcohol synthesis lies in its ability to transform simple carboxylic acids into versatile alcohols, which can further undergo a series of reactions to produce a wide range of organic compounds. This makes the understanding of basic reduction reactions indispensable for students pursuing organic chemistry.