Question

What is the compound formed when acetaldehyde is reacted with \(\operatorname{LiAlH}_{4} ?\) (a) \(\mathrm{CH}_{3} \mathrm{COOH}\) (b) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (d) \(\mathrm{C} 2 \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{5}\)

Short answer

The compound formed is ethanol (c).

Step-by-step solution

Identify the Process

The reaction involves acetaldehyde and lithium aluminum hydride (LiAlH₄). LiAlH₄ is known as a reducing agent, meaning it will donate hydrogen atoms to the compound it reacts with.

Understand the Product Possibilities

The product of a reduction depends on the starting compound. Acetaldehyde (\( ext{CH}_3 ext{CHO} \)) is an aldehyde, and when aldehydes are reduced, they are typically converted into primary alcohols.

Predict the Reaction Outcome

Given that acetaldehyde is reduced, the likely primary alcohol formed would be ethanol (\( ext{C}_2 ext{H}_5 ext{OH} \)). This is because adding hydrogen to acetaldehyde converts the aldehyde group (\( ext{-CHO} \)) into an alcohol group (\( ext{-CH}_2 ext{OH} \)).

Choose the Correct Option

Review the options given: (a) \( ext{CH}_3 ext{COOH} \) is acetic acid, (b) \( ext{CH}_3 ext{COCH}_3 \) is acetone, (c) \( ext{C}_2 ext{H}_5 ext{OH} \) is ethanol, (d) \( ext{C}_2 ext{H}_5 ext{OC}_2 ext{H}_5 \) is diethyl ether. The correct product of the reduction of acetaldehyde by LiAlH₄ is ethanol (\( ext{C}_2 ext{H}_5 ext{OH} \)), so the correct option is (c).

Key concepts

Acetaldehyde

Acetaldehyde is a simple aldehyde with the chemical formula \( \text{CH}_3\text{CHO} \), where \(\text{CHO} \) signifies the aldehyde group connected to a methyl group \( \text{CH}_3 \). Aldehydes are organic compounds containing a formyl group, which is pivotal to their chemical behavior.
Acetaldehyde is a significant intermediate in the production of many chemicals and plays an essential role in biological systems. It's often encountered in the human body as a byproduct of alcohol metabolism.

• The formyl group in acetaldehyde is prone to undergo specific reactions, such as reduction, due to the polar nature of the carbon-oxygen double bond.
• As a reactive molecule, acetaldehyde acts as a starting point for synthesizing various chemicals including plastics, perfumes, and flavors.

In organic chemistry, understanding the properties of aldehydes like acetaldehyde helps predict the types of reactions these compounds can undergo.

LiAlH₄ Reduction

Lithium aluminum hydride, abbreviated as LiAlH₄, is a powerful reducing agent renowned in organic chemistry for its ability to donate hydride ions \(\text{H}^- \) to various compounds. Reduction by LiAlH₄ typically involves the conversion of carbonyl groups (such as aldehydes and ketones) into alcohols.
This process begins with the donation of hydride ions from LiAlH₄ to the carbon atom in the carbonyl group \(\text{C=O}\). This interaction results in
the conversion of the double-bonded oxygen into an OH group, effectively transforming the carbonyl compound to an alcohol.

• LiAlH₄ reduction is very efficient and is used extensively for reducing not only aldehydes but also ketones, esters, and carboxylic acids.
• It requires careful handling as LiAlH₄ is highly reactive, especially in the presence of moisture.

This reduction mechanism is useful because it allows chemists to selectively synthesize alcohols from more oxidized compounds like aldehydes and ketones.

Ethanol Formation

Ethanol, a simple two-carbon alcohol, has the chemical formula \( \text{C}_2\text{H}_5\text{OH} \). It derives its structure from the reduction of acetaldehyde, which involves the conversion of the aldehyde group into an alcohol group.
When acetaldehyde \( \text{CH}_3\text{CHO} \) is reduced by LiAlH₄, it gains a hydrogen atom at the aldehyde carbon, forming ethanol.
This occurs because the \( -\text{CHO} \) group in acetaldehyde changes to a \( -\text{CH}_2\text{OH} \) group, characteristic of ethanol:

• Ethanol is a versatile organic solvent commonly used in industry and laboratories.
• It is also the type of alcohol found in alcoholic beverages, produced biologically through the fermentation of sugars by yeast.

Understanding this transformation highlights the significance of reduction reactions in organic synthesis and their capability to yield valuable products like ethanol from simple starting materials.