Problem 21 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following gives aldol condensation reaction? (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{CO}-\mathrm{CH}_{3}\) (b) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{CO}-\mathrm{CH}_{3}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CO}-\mathrm{C}_{6} \mathrm{H}_{5}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\)

Step-by-Step Solution

Answer

Butan-2-one (option a) undergoes aldol condensation.

Step 1: Understanding Aldol Condensation

Aldol condensation is a reaction between an enolate ion and a carbonyl compound that requires at least one alpha hydrogen in order to form a β-hydroxy aldehyde or ketone, which can dehydrate to a conjugated enone.

Step 2: Identify Alpha Hydrogens

(a) \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{CO}-\mathrm{CH}_{3}\) - This molecule (butan-2-one) has alpha hydrogens available on both sides of the keto group.(b) \((\mathrm{CH}_{3})_{3} \mathrm{C}-\mathrm{CO}-\mathrm{CH}_{3}\) - The central carbon in tert-butyl methyl ketone has no alpha hydrogen because the three methyl groups provide no alpha hydrogen under the keto group.(c) \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{CO}-\mathrm{C}_{6} \mathrm{H}_{5}\) - Diphenyl ketone has no alpha hydrogen as the keto group is attached to two phenyl groups.(d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\) - Phenol \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\) does not have an alpha hydrogen nor a carbonyl group.

Step 3: Determine Capability for Aldol Condensation

Only option (a), butan-2-one, has alpha hydrogens that can participate in an aldol condensation reaction, forming an enolate ion that can attack a carbonyl carbon. Options (b), (c), and (d) lack alpha hydrogens necessary for this reaction.

Key Concepts

Enolate Ion

The enolate ion is a crucial player in aldol condensation reactions. Formed when an alpha hydrogen is removed from a compound by a base, it acts as a nucleophile and is characterized by its resonance structures.
Here's a quick look at how enolate ions are formed and how they participate in reactions:

• When a base such as hydroxide or alkoxide abstracts an alpha hydrogen, it generates an enolate ion. This is due to the high acidity of alpha hydrogens adjacent to a carbonyl group.
• The resulting enolate ion possesses a high electron density on the alpha carbon, allowing it to attack the electrophilic carbon of a carbonyl group.

The enolate ion's dual resonance structures, with negative charge delocalized between the alpha carbon and the oxygen, make it a stable yet reactive species. Aldol condensation begins when the enolate ion attacks a nearby carbonyl compound, initiating the formation of a β-hydroxy ketone or aldehyde.

Alpha Hydrogen

Alpha hydrogens are pivotal to the entire aldol condensation process. They are atoms that sit adjacent to a carbonyl group, such as a ketone or aldehyde. These hydrogens are slightly acidic due to the electron-withdrawing nature of the carbonyl group. This acidity allows them to be easily removed by a base, leading to the formation of an enolate ion.

• An alpha hydrogen's acidity makes it susceptible to deprotonation, resulting in an enolate ion that is ready to engage in reactions.
• In aldol condensation, compounds with accessible alpha hydrogens, like butan-2-one, can readily participate by offering these hydrogens for deprotonation.

This characteristic helps define which compounds can undergo aldol condensation. Without alpha hydrogens, as seen in certain compounds like tert-butyl methyl ketone and diphenyl ketone, the requisite enolate ion formation is impossible.

Carbonyl Compound

Carbonyl compounds, including aldehydes and ketones, form the backbone of aldol condensation reactions. Their unique structure, featuring a carbon-oxygen double bond, provides both acidity (through alpha hydrogens) and electrophilicity (at the carbon of the carbonyl group).

• The carbonyl carbon is electrophilic, making it a prime target for nucleophilic attack from enolate ions generated in the reaction.
• The carbonyl oxygen's partial negative charge helps stabilize the enolate ion via resonance, enhancing the reaction's feasibility and yield.

In the context of aldol condensation, not every carbonyl compound is suited for the reaction. The absence of alpha hydrogens, as seen in certain compounds, can prevent reaction participation, limiting activity to those like butan-2-one which can offer both the necessary reactivity and structural foundation necessary for a successful reaction.