Question

Ethyl dimethylacetoacetate reacts instantly at room temperature when treated with ethoxide ion to yield two products, ethyl acetate and ethyl 2 -methylpropanoate. Propose a mechanism for this cleavage reaction.

Short answer

The ethoxide ion initiates enolate formation, which cleaves the molecule into ethyl acetate and ethyl 2-methylpropanoate through nucleophilic acyl substitution.

Step-by-step solution

Identify the Enolate Formation Site

Ethyl dimethylacetoacetate has a keto group, which can form an enolate ion at the α-carbon position when treated with a strong base like the ethoxide ion. This enolate formation is the first step in the reaction mechanism.

Enolate Attacks the Ester Carbonyl

The formed enolate ion attacks the carbonyl carbon of the same molecule's ester group, leading to a nucleophilic acyl substitution reaction. This creates a tetrahedral intermediate.

Breakdown of the Tetrahedral Intermediate

The tetrahedral intermediate rearranges, leading to the expulsion of the ethoxide ion as a leaving group, breaking the C–C bond between the α-carbon and the carbonyl carbon. This results in the formation of ethyl acetate.

Formation of Ethyl 2-Methylpropanoate

The expelled ethoxide ion can now deprotonate the central carbon of the remaining part of the ethyl dimethylacetoacetate. Proton transfer leads to the stabilization of the molecule as ethyl 2-methylpropanoate.

Key concepts

Ethyl Dimethylacetoacetate

Ethyl dimethylacetoacetate is an organic compound that plays a key role in various chemical reactions. It features a unique structure with both ester and keto groups, making it highly reactive under certain conditions. The ester group consists of an alkoxy carbonyl chain, which is typical in many organic molecules, while the keto group includes a carbonyl (C=O) linked to an alkyl chain.
These structural elements enable ethyl dimethylacetoacetate to undergo specific reactions, such as nucleophilic acyl substitution, particularly when influenced by bases such as ethoxide ions. The presence of the α-carbon, which lies adjacent to the carbonyl group of the keto function, is crucial. This position is where enolate ion formation easily occurs, initiating the reaction process.
This compound is often used in the synthesis of various chemical products, owing to its ability to participate in reactions that form useful derivatives like ethyl acetate and ethyl 2-methylpropanoate.

Enolate Ion

The enolate ion is a fundamental concept in organic chemistry, especially in reactions involving compounds like ethyl dimethylacetoacetate. An enolate ion forms when a base, such as the ethoxide ion, removes a proton from the α-carbon adjacent to a carbonyl group. The ion itself is resonance-stabilized, meaning it shares the extra electron density between the oxygen of the keto group and the α-carbon.
This ion's formation is essential in the reaction mechanism as it acts as a powerful nucleophile. The enolate ion's ability to attack electrophilic centers, like the carbonyl carbon in the ester group, enables further progress in reactions.

• Formation occurs with strong bases.
• Acts as a nucleophile.
• Essential for initiating nucleophilic acyl substitution reactions.

Understanding enolate ions is crucial for predicting reaction outcomes and mechanisms in synthetic organic chemistry.

Tetrahedral Intermediate

The tetrahedral intermediate is a transient state occurring during the nucleophilic acyl substitution reaction. When the enolate ion attacks the carbonyl carbon of the ester group in ethyl dimethylacetoacetate, the result is the formation of a new tetrahedral structure.
This structure is characterized by a central carbon atom with four different groups attached, hence the name 'tetrahedral'. It is a key step because it disrupts the usual planar structure of the carbonyl group, facilitating the breakdown and rearrangement of bonds.
The stability of the tetrahedral intermediate determines the success and direction of the reaction. It soon collapses, leading to the departure of the ethoxide ion as a leaving group and subsequent product formation. This step is pivotal in the cleavage reaction, manifesting in the generation of ethyl acetate as one of the products.

Nucleophilic Acyl Substitution

Nucleophilic acyl substitution is a vital mechanism in organic chemistry reactions, particularly involving esters or acyl compounds. It occurs when a nucleophile, such as the enolate ion, substitutes the acyl group by replacing the ester linkage.
This process is observed in the reaction of ethyl dimethylacetoacetate with ethoxide ions. The nucleophile attacks the electrophilic carbonyl carbon, leading to the formation of the tetrahedral intermediate.

• The nucleophile forges a bond with the carbonyl carbon.
• The tetrahedral intermediate forms, rearranges, and expels a leaving group.
• Substitution results in new product formation (e.g., ethyl acetate).

This substitution is beneficial for constructing diverse organic compounds and is frequently applied in both laboratory and industrial settings.