Acetaldehyde forms white crystalline precipitate on mixing with a ________solution of __________.

(A) Acidic Zn, Hg

(B) Alcoholic Na2 S3

(C) Saturated, aqueous NaHSO3

(D) Aqueous NaCl

Acetaldehyde, a simple aldehyde, can react with certain reagents to form precipitates. In this case, the correct answer to your question is (C) Saturated, aqueous NaHSO3. Let’s delve into why this is the case and how the reaction works.

Understanding the Reaction

When acetaldehyde is mixed with saturated aqueous sodium bisulfite (NaHSO3), it undergoes a reaction that leads to the formation of a white crystalline precipitate. This occurs because acetaldehyde can react with bisulfite ions to form a bisulfite adduct.

The Chemistry Behind It

Acetaldehyde has a carbonyl group (C=O), which is highly reactive. The bisulfite ion (HSO3-) acts as a nucleophile and attacks the carbonyl carbon of acetaldehyde. This reaction can be summarized in the following steps:

• Nucleophilic Attack: The bisulfite ion attacks the carbonyl carbon, forming a tetrahedral intermediate.
• Proton Transfer: A proton transfer occurs, stabilizing the intermediate.
• Formation of the Adduct: The final product is a stable bisulfite adduct, which is typically a white crystalline solid.

Why Other Options Don’t Work

Let’s briefly consider why the other options do not lead to the formation of a precipitate with acetaldehyde:

• (A) Acidic Zn, Hg: These metals are often used in reduction reactions, but they do not form a precipitate with acetaldehyde.
• (B) Alcoholic Na2S3: Sodium thiosulfate can react with aldehydes, but it does not typically form a precipitate in this context.
• (D) Aqueous NaCl: Sodium chloride is a neutral salt and does not react with acetaldehyde to form any precipitate.

Practical Implications

This reaction is not just a theoretical exercise; it has practical applications in organic chemistry. The formation of bisulfite adducts can be used to purify aldehydes and ketones from mixtures, as the adducts can often be crystallized and then decomposed back to the original carbonyl compound.

In summary, when acetaldehyde is mixed with saturated aqueous NaHSO3, it forms a white crystalline precipitate due to the formation of a bisulfite adduct. This reaction is a classic example of how aldehydes can interact with nucleophiles, showcasing the reactivity of carbonyl compounds in organic chemistry.