Question

Which of the following reaction cannot produce hydrocarbon? (A) Clemmensen reduction (B) Wolff-Kishner reduction (C) Mozingo reaction (D) Rosenmund reaction

Short answer

The Rosenmund Reaction (D) cannot produce hydrocarbons, as it only reduces acid chlorides to aldehydes without converting the carbonyl group to a methylene group.

Step-by-step solution

Understanding Clemmensen Reduction

Clemmensen Reduction is a reaction that reduces a carbonyl group to a methylene group (CH2) using zinc amalgam (Zn[Hg]) in the presence of hydrochloric acid (HCl). Since this reaction removes an oxygen atom and replaces it with hydrogen atoms, it has the ability to produce hydrocarbons.

Understanding Wolff-Kishner Reduction

Wolff-Kishner Reduction is another carbonyl group reduction reaction that uses hydrazine (N2H4) and a strong alkali, such as potassium hydroxide (KOH), to convert a carbonyl group into a methylene group (CH2). Similar to Clemmensen Reduction, this reaction can also produce hydrocarbons, as it removes oxygen and replaces it with hydrogen atoms.

Understanding Mozingo Reaction

Mozingo Reaction is a reduction of aldehydes and ketones to alkanes using diisobutylaluminum hydride (DIBAL-H) or bis(trimethylsilyl)trichloroaluminate ([AlCl3^+] [(CH3)3Si-]^3). This reaction reduces the carbonyl group to a methylene group (CH2), which can lead to the formation of hydrocarbons.

Understanding Rosenmund Reaction

Rosenmund Reaction is the catalytic reduction of acid chlorides to aldehydes using palladium on barium sulfate (Pd/BaSO4) and hydrogen gas (H2). This reaction does not reduce the carbonyl group to a methylene group, and therefore, cannot produce hydrocarbons. Instead, it produces aldehydes, which still contain the carbonyl group (C=O).

Identifying the reaction that cannot produce hydrocarbons

Based on the understanding of each reaction, we can conclude that out of the four given reactions, Rosenmund Reaction (D) cannot produce hydrocarbons, as it only reduces acid chlorides to aldehydes without converting the carbonyl group to a methylene group.

Key concepts

Clemmensen Reduction

The Clemmensen reduction is a revered method within organic chemistry for converting a carbonyl compound into a hydrocarbon by replacing an oxygen atom with hydrogen. This process utilizes zinc amalgam, which is a blend of mercury and zinc, and the reaction takes place in an acidic environment, commonly involving hydrochloric acid (HCl). The reaction's brilliance lies in its simplicity and efficiency, as carbonyl groups, which are carbon-oxygen double bonds found in aldehydes and ketones, are transformed into methylene groups, represented as (CH2).

This conversion is particularly valuable when synthesize hydrocarbons from more complex molecules is required in a laboratory setting. It's essential to note that the Clemmensen reduction proves most effective with aromatic and cyclic aldehydes and ketones, where the robust conditions do not adversely affect the ring structure.

Wolff-Kishner Reduction

The Wolff-Kishner reduction stands out as an alternative to the Clemmensen reduction for eliminating carbonyl groups within a molecule. The process employs hydrazine (N2H4), a nitrogen-based compound, as the reducing agent and typically uses a strong base such as potassium hydroxide (KOH). The Wolff-Kishner reduction takes place under high-temperature conditions, often requiring a heated reaction environment.

The key advantage of this method is its ability to proceed under non-acidic conditions, which is particularly beneficial for acid-sensitive molecules. Once the carbonyl group is converted into a hydrazone intermediate, the application of heat causes nitrogen gas to be expelled, leaving behind a methylene group. This allows for the synthesis of hydrocarbons from ketones and aldehydes without affecting other potentially acid-labile functional groups in the molecule.

Mozingo Reaction

The Mozingo reaction is yet another technique employed to reduce carbonyl compounds to hydrocarbons, with a particular focus on aldehydes and ketones. This process uses specific reducing agents: diisobutylaluminum hydride (DIBAL-H) or the more complex reagent bis(trimethylsilyl)trichloroaluminate. Unlike the Clemmensen and Wolff-Kishner reductions, the Mozingo reaction takes advantage of milder, more controlled conditions.

During the reaction, the carbonyl group is replaced by a methylene group, resulting in the formation of hydrocarbons. Organic chemists favor the Mozingo reaction when finer control over the reaction conditions is necessary to protect sensitive functional groups in the molecule from harsh reaction environments.

Rosenmund Reaction

The Rosenmund reaction offers a unique approach when compared to the three aforementioned reductions, as it specifically targets the reduction of acid chlorides to aldehydes. Utilizing palladium on barium sulfate (Pd/BaSO4) as a catalyst along with hydrogen gas (H2), the reaction selectively stops at the aldehyde stage without further reducing the carbonyl group to a methylene group.

In this context, the Rosenmund reaction does not provide a pathway to hydrocarbons but rather to aldehydes, retaining the carbonyl group as an integral part of the product. This specificity makes it an essential tool for synthetic chemists who aim to stop the reduction process at the aldehyde level without risking over-reduction to a hydrocarbon, as would occur with other reduction reactions.