Question

The Haller-Bauer cleavage of } 2,2 \text { -dimethyl-1-phenyl-1-propanone with sodium amide forms }\end{array}$ benzenecarboxamide and 2-methylpropane. Write a mechanism for the Haller-Bauer reaction analogous to the haloform cleavage reaction.

Short answer

The mechanism includes deprotonation, nucleophilic attack, bond cleavage, and product formation.

Step-by-step solution

Understanding the Reaction

The Haller-Bauer reaction involves a cleavage of an α-diketone with a strong base, similar to the haloform reaction. In this case, sodium amide acts as the strong base and cleaves 2,2-dimethyl-1-phenyl-1-propanone into benzenecarboxamide and 2-methylpropane.

Deprotonation of the Keto Group

Sodium amide (\[\text{NaNH}_2\]) deprotonates the α-hydrogen of 2,2-dimethyl-1-phenyl-1-propanone, forming a carbanion at the α-carbon adjacent to the keto group. This step is crucial as it makes the carbon atom nucleophilic.

Nucleophilic Attack

The negatively charged carbanion then attacks the carbonyl carbon, generating an alkoxide intermediate. The alkoxide is a tetrahedral intermediate common in nucleophilic addition reactions involving carbonyl groups.

Cleavage and Elimination

The alkoxide intermediate undergoes a rearrangement which leads to the cleavage of the carbon-carbon bond adjacent to the carbonyl. This results in the expulsion of 2-methylpropane and the formation of a carboxylate ion connected to a ring structure of an aromatic compound.

Protonation and Amide Formation

The carboxylate ion is then protonated to form benzenecarboxylic acid, which further reacts with the amide ion from sodium amide to form benzenecarboxamide as the final product.

Key concepts

α-Diketone Cleavage

During the Haller-Bauer reaction, one of the pivotal steps is the cleavage of an α-diketone. An α-diketone is a compound containing two ketone groups separated by a single carbon atom. In this reaction, the structure of the α-diketone plays a crucial role. The diketone first undergoes deprotonation at the α-carbon atom adjacent to the keto group. This step creates a stabilized carbanion. This carbanion formation is key for the reaction to proceed, as it makes the carbon adjacent to the ketone ready for further transformation. Understanding where the cleavage occurs helps to distinguish which parts of the molecule will undergo transformations. In the Haller-Bauer reaction, this cleavage eventually leads to the breakdown of the α-diketone into two simpler molecules.

• One part of the molecule transforms to form a carboxamide.
• The other part forms a simple hydrocarbon fragment.

Sodium Amide Base

Sodium amide (\(\text{NaNH}_2\)) is a potent base used in organic reactions for its ability to deprotonate even weakly acidic hydrogen atoms. In the context of the Haller-Bauer reaction, it plays a critical role by abstracting a proton from the α-hydrogen of the diketone.
This deprotonation step is essential as it changes the initially neutral molecule into a reactive carbanion. Sodium amide's strong base property allows it to drive the reaction forward, creating conditions conducive for further nucleophilic attack.

• Sodium amide as a base is often preferred in reactions requiring strong deprotonation abilities.
• Its use is typical in reactions where stabilization of a carbanion intermediate is necessary.

Nucleophilic Attack

After deprotonation by sodium amide, a carbanion is formed at the α-position of the diketone. This carbanion exhibits nucleophilic characteristics, making it eager to attack positively charged or electrophilic centers.
In the Haller-Bauer reaction, the carbanion targets the carbonyl carbon of the ketone group within the molecule. This interaction is a classic example of a nucleophilic attack, where the electron-rich carbanion donates electrons to the electron-deficient carbonyl carbon.
This attack leads to the formation of a tetrahedral alkoxide intermediate, which is a common structure in reactions involving nucleophilic additions to carbonyl compounds.

• Nucleophilic attack is a frequent reaction mechanism in organic chemistry.
• It's vital for forming new carbon-oxygen or carbon-carbon bonds.

Carbanion Formation

The creation of a carbanion is a critical step driving the Haller-Bauer reaction forward. A carbanion is a species with a negatively charged carbon atom, stabilized by adjacent electronegative atoms or groups.
In the case of the Haller-Bauer mechanism, this carbanion is generated at the α-carbon upon deprotonation by sodium amide.
This negatively charged species is highly reactive and plays a vital role in subsequent reaction steps, primarily the nucleophilic attack.

• Carbanions are crucial intermediates in many organic reactions.
• Their stability often dictates the feasibility and direction of a reaction.