Problem 69 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following will undergo aldol condensation? (a) acetaldehyde (b) propanaldehyde (c) benzaldehyde (d) trideuteroacetaldehyde

Step-by-Step Solution

Answer

Acetaldehyde and propanaldehyde can undergo aldol condensation.

Step 1: Understand Aldol Condensation

Aldol condensation involves the formation of a β-hydroxy aldehyde or ketone followed by dehydration to give a conjugated enone. It requires a compound with α-hydrogen atoms.

Step 2: Analyze Each Compound

Examine each compound to determine if they have α-hydrogen atoms. - Acetaldehyde (CH₃CHO) has α-hydrogens attached to the carbon next to the carbonyl group. - Propanaldehyde (CH₃CH₂CHO) also has α-hydrogens. - Benzaldehyde (C₆H₅CHO) does not have any α-hydrogens as the carbonyl is directly attached to the phenyl group. - Trideuteroacetaldehyde (CD₃CHO) does not have α-hydrogens but instead has α-deuterium atoms.

Step 3: Identify Suitable Candidates for Aldol Condensation

Only compounds with α-hydrogens can undergo aldol condensation. Thus, acetaldehyde and propanaldehyde are suitable candidates because they possess α-hydrogens, whereas benzaldehyde and trideuteroacetaldehyde do not.

Key Concepts

α-hydrogen

The concept of α-hydrogen is critical in understanding aldol condensation. An α-hydrogen atom is located on the carbon directly adjacent to a carbonyl group (C=O) in a molecule. This specific position makes these hydrogen atoms essential for certain chemical reactions.

• The presence of α-hydrogen allows for the formation of an enolate ion in the presence of a base.
• This enolate ion can then participate in further reaction steps, like aldol condensation.

Without α-hydrogen, a molecule cannot undergo this process.
For instance, acetaldehyde and propanaldehyde each have such hydrogens, making them eligible for aldol reactions. In contrast, compounds like benzaldehyde lack these critical atoms, thereby making aldol condensation impossible for them.

β-hydroxy aldehyde

A β-hydroxy aldehyde is an intermediate compound formed during aldol condensation. It involves the reaction of an enolate ion with another carbonyl compound.

• The enolate ion attacks the carbonyl carbon, forming a new carbon-carbon bond.
• This results in a molecule where the hydroxyl group (OH) is attached to the β-carbon, which is two carbons away from the carbonyl group.

The formation of a β-hydroxy aldehyde marks an important step, indicating progress in the aldol reaction.
For example, when two acetaldehyde molecules undergo aldol condensation, the product is a β-hydroxy aldehyde. Understanding this intermediate is crucial as it helps predict the dehydration step that follows.

dehydration

Dehydration is a subsequent step that often follows the formation of a β-hydroxy aldehyde in an aldol condensation reaction. It involves the removal of a water molecule from the β-hydroxy aldehyde.

• The hydroxyl group and a hydrogen atom (often α-hydrogen) are removed as water (H₂O).
• This step is facilitated by the neighboring carbonyl group's electrophilic nature.

Dehydration leads to the formation of a double bond between the α and β carbon, converting the molecule into an enone.
This step increases the molecule's stability by creating a conjugated system, characterized by alternating single and double bonds, which has significant implications for the reactivity and properties of the compound.

conjugated enone

A conjugated enone is the final product of the aldol condensation process after dehydration of the β-hydroxy aldehyde. This molecule features a structure where a carbonyl group is conjugated with a carbon-carbon double bond.

• The alternating single and double bonds (C=C-C=O) give the conjugated enone its unique stability.
• This conjugation allows the electrons to delocalize, lowering the overall energy of the molecule.

Conjugated enones are particularly important because they exhibit a high degree of chemical stability and often participate in further chemical reactions.
This conjugated system can absorb light over various wavelengths, and its stability makes it a valuable structural motif in many organic compounds, including pharmaceuticals and fragrances.