Problem 156 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following are examples of aldol condensation? \(12 \mathrm{CH}_{3} \mathrm{CHO} \stackrel{\text { dil. } \mathrm{NaOH}}{\longrightarrow} \mathrm{CH}_{3} \mathrm{CHOHCH}_{2} \mathrm{CHO}\) \(22 \mathrm{CH}_{3} \mathrm{COCH}_{3} \stackrel{\text { dil. } \mathrm{NaOH}}{\longrightarrow}\) \(\mathrm{CH}_{3} \mathrm{COH}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{COCH}_{3}\) 3\. \(2 \mathrm{HCHO} \stackrel{\text { dil. } \mathrm{NaOH}}{\longrightarrow} \mathrm{CH}_{3} \mathrm{OH}+\mathrm{HCOOH}\) 4\. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}+\mathrm{HCHO} \frac{\text { dil. } \mathrm{NaOH}}{\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{2} \mathrm{OH}}\) (a) 2 and 3 (b) 1 and 3 (c) 1 and 2 (d) 1,2 and 3

Step-by-Step Solution

Answer

The examples of aldol condensation are reactions 1 and 2.

Step 1: Understand the Reaction Context - Aldol Condensation

Aldol condensation involves the reaction of aldehydes or ketones with at least one containing an α-hydrogen in the presence of a base like dilute NaOH. The typical product is a β-hydroxy aldehyde or ketone, which may dehydrate to form an α,β-unsaturated carbonyl compound.

Step 2: Analyze the First Reaction

The first reaction, \(12 \mathrm{CH}_{3}\mathrm{CHO} \stackrel{\text { dil. } \mathrm{NaOH}}{\longrightarrow} \mathrm{CH}_{3} \mathrm{CHOHCH}_{2} \mathrm{CHO}\), is an aldol condensation. Here, acetaldehyde acts as both donor and acceptor, forming a β-hydroxy aldehyde.

Step 3: Analyze the Second Reaction

The second reaction, \(2 \mathrm{CH}_{3} \mathrm{COCH}_{3} \stackrel{\text { dil. } \mathrm{NaOH}}{\longrightarrow} \mathrm{CH}_{3} \mathrm{COH}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{COCH}_{3}\), involves acetone and leads to a β-hydroxy ketone. This is another example of aldol condensation.

Step 4: Evaluate the Third Reaction - Cannizzaro Reaction

The third reaction, \(2 \mathrm{HCHO} \stackrel{\text { dil. } \mathrm{NaOH}}{\longrightarrow} \mathrm{CH}_{3} \mathrm{OH}+\mathrm{HCOOH}\), involves formaldehyde which has no α-hydrogen. This is a Cannizzaro reaction, not aldol condensation.

Step 5: Analyze the Fourth Reaction

The fourth reaction, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}+\mathrm{HCHO} \frac{\text { dil. } \mathrm{NaOH}}{\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{2} \mathrm{OH}}\), does not fit the typical aldol reaction pathway. It represents a crossed Cannizzaro reaction.

Step 6: Choose Correct Options Based on Analysis

Since reactions 1 and 2 are confirmed as aldol condensations, the correct option is (c) 1 and 2.

Key Concepts

β-hydroxy aldehyde

Aldol condensation, a cornerstone reaction in organic chemistry, can produce β-hydroxy aldehydes. This type of reaction involves two aldehyde molecules, or a combination of an aldehyde and a ketone, provided that at least one of them contains an α-hydrogen. The presence of a base, such as dilute NaOH, facilitates the reaction.

The initial step includes the abstraction of an α-hydrogen to form an enolate ion. This enolate acts as a nucleophile and attacks the carbonyl carbon of another aldehyde molecule, resulting in the formation of a β-hydroxy aldehyde. The "β" indicates that the hydroxyl group is located at the β-position relative to the aldehyde group, hence the name β-hydroxy aldehyde.

These compounds are significant because they can further undergo dehydration to give α,β-unsaturated carbonyl compounds, which are important in various synthesis processes. The dehydration step involves the loss of water, resulting in a double bond between the α and β carbons. This could be advantageous for creating more complex molecules in synthetic organic chemistry.

β-hydroxy ketone

In the realm of aldol condensations, the formation of β-hydroxy ketones is another notable outcome. These reactions typically involve ketone molecules, with one serving as the donor via its enolate form. The presence of a weak base like NaOH initiates the process by abstracting an α-hydrogen from the ketone, generating an enolate ion.

The next step involves the nucleophilic attack of this enolate on the carbonyl carbon of another ketone or aldehyde, leading to the formation of a β-hydroxy ketone. The hydroxyl group adds to the β-carbon relative to the ketone's carbonyl group.

β-hydroxy ketones are often intermediates in organic synthesis, acting as precursors for further transformations. Just like β-hydroxy aldehydes, β-hydroxy ketones can dehydrate, forming α,β-unsaturated ketones which possess more significant chemical reactivity and stability, making them versatile building blocks in chemical synthesis.

Cannizzaro reaction

The Cannizzaro reaction provides an alternative pathway for aldehydes that lack α-hydrogens to engage in redox reactions. Such aldehydes, like formaldehyde, cannot undergo aldol condensation due to the absence of an α-hydrogen. This reaction is also facilitated by a base such as sodium hydroxide (NaOH).

In this reaction, one aldehyde molecule is oxidized to a carboxylic acid, while the other is reduced to an alcohol. This disproportionation reaction is unique because it involves no external oxidizing or reducing agents.

The Cannizzaro reaction produces two distinct products: one molecule of an alcohol and one of a carboxylic acid for every two molecules of the aldehyde used. Crossed Cannizzaro reactions expand this concept further by using two different aldehydes, yielding varied alcohol and acid products. This reaction is particularly valuable when handling specific aldehydes that cannot participate in more traditional carbon-carbon bond-forming reactions due to their lack of an α-hydrogen.