Question

Treatment of 1 -phenyl-2-propenone with a strong base such as LDA does not yield an anion, even though it contains a hydrogen on the carbon atom next to the carbonyl group. Explain.

Short answer

1-Phenyl-2-propenone is stabilized by resonance with the phenyl group, making proton removal by LDA unfavorable.

Step-by-step solution

Identify the Components

1-Phenyl-2-propenone is a molecule composed of a phenyl group attached to a propenone structure. The propenone is essentially a carbonyl group adjacent to a double-bonded carbon-carbon structure.

Resonance Understanding

The alpha-hydrogen next to the carbonyl group in propenone is generally considered to be acidic and can form an enolate ion when deprotonated by a strong base. However, this may not occur if other factors stabilize the parent compound more than the enolate.

Analyze the Electronic Effects

In 1-phenyl-2-propenone, the phenyl group can participate in conjugation with the carbonyl group, offering extra resonance stability. This conjugation delocalizes the electrons, making the molecule more stable even without forming an anion.

Consider the Base's Function

LDA (Lithium diisopropylamide) is a strong, hindered, non-nucleophilic base usually employed to remove acidic protons. However, in this case, removing the hydrogen from the carbon next to the carbonyl would disrupt the resonance stability with the phenyl group.

Conclusion and Explanation

Deprotonating the alpha-hydrogen next to the carbonyl group would lead to loss of resonance stabilization provided by the phenyl group. Therefore, the stability from resonance outweighs the formation of an enolate anion, making the removal of the hydrogen less favorable.

Key concepts

Enolate Ion

An enolate ion is formed when an alpha-hydrogen, which is located next to a carbonyl group, is removed by a base. This leaves the carbon atom with a negative charge, making it highly reactive. The structure can exhibit resonance, meaning the electrons can spread between the oxygen and carbon atoms. This resonance provides stability to the enolate ion.

Enolate ions are crucial intermediates in organic reactions. Their formation depends on the acidity of the alpha-hydrogen and the base's strength used to deprotonate the molecule. Without favorable conditions, enolate formation may not occur, especially if the original molecule has other stabilizing factors, such as resonance with a phenyl group.

Strong Base

A strong base like Lithium Diisopropylamide (LDA) is essential to remove protons from a molecule, especially those alpha-hydrogens that are less acidic in nature.

Why Use LDA?

• LDA is used because of its strong basic properties.
• It is hindered and non-nucleophilic, which means it is less likely to cause side reactions or attacks on multiple sites.

However, in the context of 1-phenyl-2-propenone, the application of LDA is not straightforward. The phenyl group interferes with the base’s normal action by providing resonance stability to the molecule. When the phenyl group stabilizes the molecule more effectively, the base's tendency to remove the alpha-hydrogen is reduced.

Alpha-Hydrogen

The term "alpha-hydrogen" refers to hydrogen atoms attached to the carbon atom adjacent to a carbonyl group. These hydrogens are often acidic because they experience the electron-withdrawing effect from the oxygen in the carbonyl group.

Acidity of Alpha-Hydrogens

• The alpha-hydrogens are crucial for forming reactive enolate ions through deprotonation.
• The acidity depends on surrounding groups and their ability to stabilize a negative charge through resonance.

In 1-phenyl-2-propenone, the alpha-hydrogen's potential removal by a strong base like LDA is not favored. The resonance stability provided by the phenyl group outweighs the need to form an enolate ion. The stable delocalization of electrons keeps the alpha-hydrogens intact.

Phenyl Group

A phenyl group is a ring-like structure derived from benzene, well known for its ability to stabilize adjacent functional groups through resonance. This electron delocalization enables the phenyl group to donate electron density, particularly when adjacent to a carbonyl group.

Role of Resonance

• Resonance from the phenyl group enhances the compound's stability.
• By engaging with the carbonyl group, the phenyl can transform the electronic structure, delocalizing electrons effectively.

In the case of 1-phenyl-2-propenone, it implies that the phenyl group's conjugation with the carbonyl group provides a stabilizing effect that discourages the formation of an enolate ion. The molecule maintains its structure without forming an anion, even in the presence of strong bases like LDA, due to this added stability.