Question

Explain why decarboxylation of \(a, \alpha\) -dimethylacetoacetic acid, \(\mathrm{CH}_{3} \mathrm{COC}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CO}_{2} \mathrm{H}\), in the presence of bromine gives \(\alpha\) -bromoisopropyl methy 1 ketone, \(\mathrm{CH}_{3} \mathrm{COC}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{Br}\).

Short answer

The decarboxylation of α, α-dimethylacetoacetic acid in the presence of bromine gives α-bromoisopropyl methyl ketone as the final product due to two sequential reactions: First, the decarboxylation process takes place with heat and removes the carboxyl group, releasing carbon dioxide and forming an intermediate molecule (\(CH3COCH(CH3)2\)). Second, this intermediate molecule reacts with bromine, which undergoes electrophilic addition to the α carbon (adjacent to the carbonyl group), resulting in the formation of α-bromoisopropyl methyl ketone (\(CH3COC(CH3)2Br\)).

Step-by-step solution

Understanding Decarboxylation

Decarboxylation is a chemical reaction where a carboxyl group (COOH) is removed from a molecule, resulting in the release of carbon dioxide (CO2). In this exercise, we have α, α-dimethylacetoacetic acid (CH3COC(CH3)2CO2H) undergoing decarboxylation.

Mechanism of Decarboxylation

The decarboxylation of α, α-dimethylacetoacetic acid occurs when heat is applied, causing the carboxyl group to be removed, and the carbon dioxide is released. The resulting intermediate molecule after the decarboxylation is \(CH3COCH(CH3)2\).

Bromination Reaction

Now that we have the intermediate molecule \(CH3COCH(CH3)2\), it reacts with the bromine (Br2) present in the reaction mixture. Bromination occurs on the α carbon (adjacent to the carbonyl group).

Formation of the Final Product

The intermediate with the bromine attached to the α carbon is our final product, which is α-bromoisopropyl methyl ketone (\(CH3COC(CH3)2Br\)). The reaction involves the electrophilic addition of bromine to the α carbon, subsequently forming the final product. In summary, the decarboxylation of α, α-dimethylacetoacetic acid in the presence of bromine results in the formation of α-bromoisopropyl methyl ketone due to the sequential reactions of decarboxylation and electrophilic bromine addition to the α carbon.

Key concepts

Alpha-Dimethylacetoacetic Acid

Alpha-dimethylacetoacetic acid is an organic compound with the formula \( \mathrm{CH}_3 \mathrm{COC}\left(\mathrm{CH}_3\right)_2 \mathrm{CO}_2 \mathrm{H} \). It features both ketone and carboxylic acid functional groups, making it an important reactant in various organic reactions.

This compound is known for its propensity to undergo decarboxylation when heated, which involves the loss of its carboxyl group \( \mathrm{CO}_2 \mathrm{H} \) and leads to the formation of a ketone moiety. Additionally, the alpha-carbon, adjacent to the carbonyl (CO) group, is a site of high reactivity – making the compound a prime candidate for bromination at this position.

Bromination Reaction

Bromination reaction in organic chemistry involves the addition of bromine \( \mathrm{Br}_2 \) to organic compounds. For alpha-dimethylacetoacetic acid, this reaction typically occurs after decarboxylation has taken root.

The mechanism of bromination is electrophilic in nature. It involves the attack of a bromine molecule on an electron-rich site, such as the alpha-carbon next to a carbonyl group.

Electrophilic Addition

In our case, the electron-rich alpha-carbon attracts the slightly positive end of the bromine molecule, resulting in the substitution of a hydrogen atom for a bromine atom and forming an alpha-brominated ketone.

Alpha-Bromoisopropyl Methyl Ketone

Alpha-bromoisopropyl methyl ketone \( \mathrm{CH}_3 \mathrm{COC}\left(\mathrm{CH}_3\right)_2 \mathrm{Br} \) is the product formed from the bromination of alpha-dimethylacetoacetic acid post-decarboxylation.

This compound is an alpha-brominated ketone with significant applications in organic synthesis. The presence of the bromine atom at the alpha position makes this compound highly reactive and useful for further chemical transformations, such as nucleophilic substitution reactions, where the bromine can be replaced by various other groups.

Organic Chemistry Mechanisms

Mechanisms in organic chemistry are step-by-step descriptions of the processes through which chemical reactions occur. They depict the movement of electrons leading to the breaking and formation of bonds.

In the decarboxylation and subsequent bromination of alpha-dimethylacetoacetic acid, the mechanism involves multiple steps.

Key Steps:

• Initially, heat causes the carbonyl group to weaken and lose the carboxyl group as CO2, forming a stabilized ketone intermediate.
• The ketone's alpha carbon, now electron-rich, is vulnerable to electrophilic attack by bromine, forming a carbon-bromine bond.
• The resulting product is an alpha-brominated ketone, obtained from a two-stage mechanism combining decarboxylation and electrophilic addition.

Understanding these mechanisms is crucial for students to predict the outcomes of chemical reactions and perform successful organic syntheses.