Question

The reagent used to carry out following conversion followed by hydrolysis is : CCOC(=O)CCC(=O)CC OC1CCC(O)OC1 (A) \(\mathrm{LiAlH}_{4}\) (B) \(\mathrm{NaBH}_{4}\) (C) DIBAL-H (D) \(\mathrm{H}_{2}-\mathrm{Pd}\)

Short answer

The correct reagent for the conversion of the given ester into a cyclic hemi-acetal followed by hydrolysis is (C) DIBAL-H, as it selectively reduces esters to aldehydes without further reduction, allowing the target product to form.

Step-by-step solution

Analyze the given reactant and target product.

The reactant is a compound with two ester functional groups. The target product is a cyclic hemi-acetal, which forms from the reaction between an aldehyde and a hydroxyl group.

Analyze the reactivity of the given reagents.

(A) \(\mathrm{LiAlH}_{4}\) is a strong reducing agent that can reduce esters to primary alcohols, as well as aldehydes to primary alcohols. (B) \(\mathrm{NaBH}_{4}\) is a milder reducing agent that can reduce aldehydes and ketones to alcohols, but cannot reduce esters. (C) DIBAL-H is a selective reducing agent that can reduce esters to aldehydes, while not reducing other carbonyl functional groups such as aldehydes and ketones. (D) \(\mathrm{H}_{2}-\mathrm{Pd}\) will catalytically reduce the C=C double bonds and some carbonyl groups, which is not required for this reaction.

Determine the appropriate reagent for the conversion.

Based on the reactivity of the given reagents, we need a reagent that can selectively reduce esters to form the desired aldehyde, without further reduction to the alcohol. This will allow the target cyclic hemi-acetal to be formed through hydrolysis. The only reagent that fulfills this requirement is DIBAL-H (C). Therefore, the correct answer is (C) DIBAL-H.

Key concepts

Reducing Agents in Organic Chemistry

Understanding the role of reducing agents is critical in mastering organic chemistry, especially when preparing for exams like JEE Main and Advanced. Reducing agents are compounds that donate electrons to another substance and are thus oxidized in the process. In organic chemistry, common reducing agents like Lithium Aluminum Hydride (\textbf{LiAlH}\(_4\)) and Sodium Borohydride (\textbf{NaBH}\(_4\)) are frequently used to convert carbonyl compounds to alcohols.

\textbf{LiAlH}\(_4\) is highly reactive and can reduce a wide range of carbonyl compounds, including esters, ketones, aldehydes, and acid chlorides to their corresponding alcohols. On the other hand, \textbf{NaBH}\(_4\) is milder and typically reduces only aldehydes and ketones. An important aspect in organic chemistry is the selectivity of a reducing agent. For instance, DIBAL-H (Diisobutylaluminium Hydride) is a selective reducing agent used specifically for the reduction of esters to aldehydes without affecting other functional groups.

When confronted with a complex reduction task, the choice of reducing agent becomes critical as it dictates the product obtained. For example, if one intends to convert an ester into an alcohol, \textbf{LiAlH}\(_4\) would be the appropriate choice. However, to get an aldehyde from an ester, a more selective agent like DIBAL-H should be used. This precision allows chemists to manipulate molecules and create specific products tailored for further reactions or applications.

Conversion of Esters

The conversion of esters into other functionalities is a common transformation in organic synthesis. Esters, characterized by their RCOOR' structure, where R and R' can be alkyl or aryl groups, are often transformed into alcohols, aldehydes, or acids. This conversion is typically performed using reducing or hydrolyzing agents.

Hydrolysis converts esters into carboxylic acids and alcohols in the presence of an acid or base. In contrast, reducing agents can transform esters to alcohols and, with careful selection, to aldehydes. DIBAL-H is an agent that can selectively stop at the aldehyde stage when used in controlled conditions, typically at low temperatures. This conversion is pivotal in multi-step synthesis pathways where the aldehyde might be an intermediate en route to more complex molecules.

The choice of reagent and reaction conditions (like temperature and solvent) heavily influences the outcome of ester conversions. For example, a reaction involving DIBAL-H may yield different products at various temperatures, emphasizing the importance of understanding the mechanism and properties of both the reagent and the functional group being targeted.

Formation of Cyclic Hemi-acetals

Cyclic hemi-acetals are formed through the reaction between an aldehyde and an alcohol. The process can take place intramolecularly when both functional groups are present in the same molecule. These structures are important because they serve as intermediates in the formation of cyclic acetals and they can be found in carbohydrate chemistry as structural elements of sugars.

An aldehyde can react with an alcohol in an acid-catalyzed reaction to form a hemi-acetal. When the reaction occurs intramolecularly, and both the aldehyde and the alcohol are part of the same molecule, a cyclic hemi-acetal is formed. This reaction is reversible, and the equilibrium position can be influenced by the reaction conditions.

Cyclic hemi-acetals are significant in carbohydrate chemistry. For example, sugars in their ring form are hemi-acetals containing a cyclic ether and a hydroxyl group. This reactivity is exploited in various biochemical pathways and can also be used in synthetic chemistry to design complex organic molecules with specific desired properties.