Question

Which of the following reagents would convert both acetaldehyde and acetone to the corresponding alkanes? (A) \(\mathrm{NaBH}_{a}\) (B) \(\mathrm{LiAlH}_{4}\) (C) \(\mathrm{Na} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (D) \(\mathrm{Zn}-\mathrm{Hg} / \mathrm{conc} \cdot \mathrm{HCl}\)

Short answer

The correct reagent that will convert both acetaldehyde and acetone to the corresponding alkanes is (D) \(\mathrm{Zn-Hg} / \mathrm{conc. HCl}\), which is used for the Clemmensen reduction of carbonyl compounds.

Step-by-step solution

Understand the structure of acetaldehyde and acetone

Acetaldehyde is an aldehyde with the formula \(\mathrm{CH}_{3}\mathrm{CHO}\). Acetone is a ketone with the formula \(\mathrm{(CH}_{3})_{2}\mathrm{CO}\). Both compounds have a carbonyl group \(\mathrm{C=O}\).

Analyze the given reagents and their reactions with carbonyl compounds

We need to examine the reactions of the listed reagents with aldehydes and ketones, with a focus on whether these reagents can reduce the carbonyl group to form the corresponding alkanes. (A) Sodium borohydride \((\mathrm{NaBH}_{4})\) is a mild reducing agent that can reduce aldehydes and ketones to form the corresponding alcohols, not alkanes. (B) Lithium aluminum hydride \((\mathrm{LiAlH}_{4})\) is a stronger reducing agent and can also reduce aldehydes and ketones to the corresponding alcohols, but not alkanes. (C) Sodium and ethanol \((\mathrm{Na} \ /\ \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH})\) form an active metal-ethanol solution, which is used for the reduction of ketones and aldehydes to the corresponding alcohols via a process known as Bouveault-Blanc reduction. However, this reagent will not reduce the carbonyl compounds to alkanes. (D) Zinc and mercury with concentrated HCl, \(\mathrm{Zn-Hg}\) \(/\mathrm{conc. HCl}\), form the reagent known as the Clemmensen reduction reagent. This reagent is used to reduce aldehydes and ketones to the corresponding alkanes by reducing the carbonyl \(\mathrm{C=O}\) group.

Determine the reagent that will convert acetaldehyde and acetone to alkanes

Based on the analysis in step 2, we can conclude that only the Clemmensen reduction reagent \((\mathrm{Zn-Hg} / \mathrm{conc. HCl})\) can reduce both acetaldehyde and acetone to their corresponding alkanes. Therefore, the correct answer is: (D) \(\mathrm{Zn-Hg} / \mathrm{conc. HCl}\)

Key concepts

Reduction of Carbonyl Compounds

Reduction of carbonyl compounds is a crucial reaction in organic chemistry. Carbonyl groups are characterized by a carbon atom double-bonded to an oxygen atom (\(C=O\)). They are found in aldehydes and ketones, like acetaldehyde (\(CH_3CHO\)) and acetone (\((CH_3)_2CO\)). The goal of reduction reactions is to convert the carbonyl group into other functional groups, such as alcohols or alkanes. Different reagents have different capabilities. Some can stop the reduction at the alcohol level, while others can go further and form alkanes. The transformation of carbonyl groups to alkanes involves the complete removal of the oxygen atom and the formation of a single bond between carbon atoms.

Reagents in Organic Chemistry

In organic chemistry, selecting the appropriate reagent is essential for achieving the desired transformation. Reagents are substances or compounds that drive chemical reactions, and they can produce different outcomes based on their strength and nature.

• Sodium Borohydride (\(NaBH_4\)): A mild reducing agent, primarily effective for converting carbonyl groups to alcohols. It is not strong enough to convert carbonyls to alkanes.
• Lithium Aluminum Hydride (\(LiAlH_4\)): Stronger than \(NaBH_4\), capable of reducing carbonyls to alcohols, but cannot fully remove the oxygen to form alkanes.
• Sodium/Ethanol (\(Na \,/\, C_2H_5OH\)): Known for Bouveault-Blanc reduction, similarly stops at alcohol formation.
• Zinc-Mercury/Concentrated HCl (\(Zn-Hg / \text{conc. HCl}\)): Known as the Clemmensen reagent. It is suited for complete transformation of carbonyl groups into alkanes by removing the \(=O\) group entirely.

Understanding these reagents helps in choosing the correct one for specific synthetic pathways in organic chemistry.

Functional Group Transformation

Functional group transformation is a fundamental aspect of organic chemistry that involves changing the functional group of a molecule to another. This concept is vital because it allows chemists to modify the structure and thus the properties of organic compounds. In the context of carbonyl compounds like ketones and aldehydes, the transformation to alkanes is one of the many processes available.

• The Clemmensen reduction is a unique transformation specifically for reducing carbonyl groups to alkanes. It involves using \(Zn-Hg / \text{conc. HCl}\) to remove the oxygen atom and saturate the molecule with hydrogen.
• This method is especially useful when other functional groups that might react with more aggressive reducing agents are present.
• Such transformations allow for the diversification of compound libraries, which is crucial in fields like pharmaceuticals and materials science.