Question

Clemmensen reduction is carried with (a) \(\mathrm{LiAlH}_{4}\) in ether (b) \(\mathrm{Zn}-\mathrm{Hg}\) and \(\mathrm{HCl}\) (c) \(\mathrm{H}_{2}\) in the presence of \(\mathrm{Pd}\) (d) \(\mathrm{NH}_{2} \mathrm{NH}_{2} / \mathrm{glycol}\) and \(\mathrm{KOH}\)

Short answer

Clemmensen reduction is carried out by (b) \(\mathrm{Zn}-\mathrm{Hg}\) and \(\mathrm{HCl}\).

Step-by-step solution

Understanding Clemmensen Reduction

Clemmensen reduction specifically refers to the reduction of a ketone or aldehyde to a hydrocarbon using zinc amalgam (zinc treated with mercury) and hydrochloric acid. Therefore, we need to find which of the given options corresponds to this process.

Evaluate Option (a)

Option (a) involves \(\mathrm{LiAlH}_{4}\) in ether, which is a strong reducing agent commonly used to reduce esters, carboxylic acids, and amides to alcohols. However, it is not related to Clemmensen reduction, which converts carbonyl compounds to hydrocarbons without forming alcohols.

Evaluate Option (b)

Option (b) involves \(\mathrm{Zn}-\mathrm{Hg}\) and \(\mathrm{HCl}\), which is precisely the method used in Clemmensen reduction. The zinc amalgam and hydrochloric acid reduce the carbonyl group of aldehydes or ketones directly to the corresponding alkane.

Evaluate Option (c)

Option (c) involves \(\mathrm{H}_{2}\) in the presence of \(\mathrm{Pd}\), a process known as catalytic hydrogenation. This method is commonly used to reduce double or triple bonds, but it is not the specific procedure for Clemmensen reduction.

Evaluate Option (d)

Option (d) involves \(\mathrm{NH}_{2} \mathrm{NH}_{2} / \mathrm{glycol}\) and \(\mathrm{KOH}\), which is a description of Wolff-Kishner reduction. This process also reduces carbonyl groups to hydrocarbons, similar to Clemmensen reduction, but it operates under basic rather than acidic conditions.

Identify the Correct Answer

Based on the evaluations, Clemmensen reduction corresponds to option (b), which uses \(\mathrm{Zn}-\mathrm{Hg}\) and \(\mathrm{HCl}\). This matches the traditional approach for carrying out Clemmensen reduction under acidic conditions using zinc amalgam and hydrochloric acid.

Key concepts

Reduction of Ketones

The reduction of ketones is a common chemical reaction where a carbonyl group is converted into a hydrocarbon. In simpler terms, it involves transforming a molecule that has a C=O (carbon-oxygen double bond) into a molecule where the oxygen is replaced by hydrogen atoms. This process is significant because it allows for the conversion of more reactive and polar molecules (like ketones) into less reactive and nonpolar compounds (like alkanes).

• Ketones contain a carbonyl group (C=O) bonded to two alkyl groups.
• The reduction process replaces the double-bonded oxygen with hydrogen atoms.
• Converting ketones into hydrocarbons (alkanes) is beneficial in various chemical synthesis applications because it stabilizes the molecule.

Understanding the reduction of ketones is crucial in organic chemistry since it plays a key role in transforming wide-ranging carbonyl compounds into useful products, often using specific reducing agents.

Zinc Amalgam

Zinc amalgam is a pivotal component in the Clemmensen Reduction process. It is essentially zinc metal treated with mercury. The amalgamation process involves a reaction between zinc and mercury, which forms a surface layer of zinc amalgam. This altered surface boosts the reactivity of zinc, enabling it to act as an effective reducing agent.

• It works efficiently under acidic conditions, particularly with hydrochloric acid.
• The role of zinc amalgam is to donate electrons, which facilitate the reduction of the carbonyl group in ketones or aldehydes to alkanes.
• Handling zinc amalgam must be done with care due to mercury's toxicity.

Understanding the role and preparation of zinc amalgam is key for safely executing the Clemmensen reduction and ensuring successful conversion of the starting ketone.

Hydrochloric Acid

In Clemmensen Reduction, hydrochloric acid (HCl) serves a critical role. It is used to create the acidic environment necessary for the reaction between zinc amalgam and the ketone or aldehyde to occur smoothly. HCl acts not only as a catalyst but also facilitates the entire reduction process by ensuring the reactants are adequately protonated.

• Hydrochloric acid is a strong acid, providing the necessary protons for the reaction>
• The acidic environment it creates is crucial for the solubility and reactivity of zinc amalgam.
• Care must be taken when handling HCl due to its corrosive nature; it can pose safety hazards.

The role of HCl is vital in maintaining the reaction conditions and must be properly managed for a successful Clemmensen reduction.

Alkane Formation

The final goal of the Clemmensen Reduction is the formation of alkanes from ketones or aldehydes. An alkane is a hydrocarbon compound made entirely of single-bonded carbon and hydrogen atoms. This transformation is significant because alkanes are much less reactive than their predecessors, ketones and aldehydes.

• Alkanes serve as fundamental building blocks in organic chemistry.
• The conversion process during Clemmensen Reduction results in the removal of the carbonyl oxygen, yielding a simpler hydrocarbon structure.
• The resulting alkanes are saturated hydrocarbons, meaning they lack double or triple bonds, which increases their stability.

Ultimately, understanding alkane formation allows chemists to effectively utilize Clemmensen Reduction in creating diverse compounds in synthetic chemistry.