Question

The reduction of a carboxylic acid using lithium aluminum hydride will yield what final product? (A) An aldehyde (B) An ester (C) A ketone (D) An alcohol

Short answer

(D) An alcohol

Step-by-step solution

- Understand the Reduction Agent

Lithium aluminum hydride (LiAlH4) is known as a very strong reducing agent that is capable of reducing a variety of functional groups. Specifically, it can reduce carboxylic acids.

- Determine the Functional Group

When lithium aluminum hydride reacts with a carboxylic acid, it typically reduces the carboxylic acid. An understanding of what functional group a carboxylic acid is transformed into when fully reduced is necessary.

- Identify the Final Product

The full reduction of a carboxylic acid by lithium aluminum hydride results in the formation of an alcohol. This reaction occurs because the carboxylic acid group (which has a carbon double-bonded to an oxygen and single-bonded to a hydroxyl group) is reduced to a hydroxyl group.

- Choose the Correct Answer

Out of the given options, the final product of the reduction of a carboxylic acid using lithium aluminum hydride is an alcohol. Thus, the correct answer is (D) An alcohol.

Key concepts

Lithium Aluminum Hydride

Lithium aluminum hydride, often abbreviated as LiAlH4, is a powerful reducing agent used in organic chemistry. A reducing agent is a substance that donates electrons to another substance, thereby reducing it. LiAlH4 is particularly useful because it can reduce a wide range of functional groups. One of the most important applications of lithium aluminum hydride is in the reduction of carboxylic acids. This compound is highly reactive and potent, often stored and handled with care due to its reactivity with water and air. When used in reactions, LiAlH4 donates hydride ions (H^-) to the substrate, facilitating the reduction process. This strong reducing property makes it invaluable in synthetic organic chemistry.

Reducing Agents

Reducing agents are substances that cause a reduction in other substances by donating electrons. In the process, the reducing agent itself gets oxidized. Common reducing agents include lithium aluminum hydride (LiAlH4), sodium borohydride (NaBH4), and hydrogen gas (H2) in the presence of a catalyst. LiAlH4 is among the most potent reducing agents and can reduce even the most resistant functional groups, such as carboxylic acids. By donating hydride ions (H^-), these agents are able to convert more oxidized forms of carbon (like those in carbonyl groups) into less oxidized forms, such as alcohols. This makes reducing agents essential tools in the chemistry lab for transforming the structure of molecules.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. In organic chemistry, understanding functional groups is critical because they define the reactivity and properties of the molecule. Examples of functional groups include hydroxyl groups (-OH), carbonyl groups (C=O), carboxylic acid groups (-COOH), and amino groups (-NH2). These groups determine how molecules interact with each other. In the reduction of carboxylic acids, for instance, the carboxylic acid group (-COOH) is reduced to a hydroxyl group (-OH) through the action of a strong reducing agent like LiAlH4. Recognizing and understanding functional groups is key to predicting the outcome of chemical reactions in organic chemistry.

Organic Chemistry Reactions

Organic chemistry reactions involve the transformation of organic molecules through chemical processes. These reactions can include additions, eliminations, substitutions, and rearrangements. Understanding these reactions is crucial for anyone studying organic chemistry. In the context of reduction reactions, lithium aluminum hydride (LiAlH4) is especially relevant. The reduction process typically involves the gain of electrons or hydrogen, leading to a decrease in the oxidation state of the molecule. For instance, when LiAlH4 is used to reduce a carboxylic acid, the reaction leads to the formation of an alcohol. This is because the LiAlH4 donates hydride ions, transforming the carboxyl group into an alcohol group. Recognizing the types of reactions and the possible outcomes is essential for mastering organic chemistry.