Question

\(\alpha, \beta\)-unsaturated carboxylic acid is obtained as a product in the reaction: (A) Claisen condensation (B) Perkin reaction (C) Benzoin condensation (D) Aldol reaction

Short answer

The correct answer is the \(\textbf{Perkin Reaction}\) (option B), which is an organic reaction that leads to the formation of α, β-unsaturated carboxylic acids. The other reactions, including Claisen condensation (A), Benzoin condensation (C), and Aldol reaction (D), do not form α, β-unsaturated carboxylic acids as products.

Step-by-step solution

Understanding the Claisen Condensation

Claisen condensation is a carbon-carbon bond-forming reaction that typically involves two esters or one ester and a carbonyl compound to form a β-keto ester or a β-diketone. The key points to note about this reaction are that it requires heating and a base catalyst to move the reaction to completion. At the end of the Claisen condensation, the product formed is a β-keto ester, not an α, β -unsaturated carboxylic acid. Therefore option (A) is not correct.

Understanding the Perkin Reaction

The Perkin reaction is an organic reaction, named after its discoverer William Henry Perkin. It involves the alkylation of acetic anhydride with an aromatic aldehyde using an alkali. This reaction leads to the formation of an α, β-unsaturated carboxylic acid following an aldol condensation. So option (B) is correct.

Understanding the Benzoin Condensation

The benzoin condensation is a reaction often defined as the dimerization of aldehydes. It is a reaction that forms benzoin by the cyanide-catalyzed dimerization of benzaldehyde. It does not form an α, β-unsaturated carboxylic acid. Therefore Option (C) is not correct.

Understanding the Aldol Reaction

The Aldol reaction involves the addition of a nucleophilic enolate ion to a carbonyl group. It is specifically considered an Aldol addition if it generates a β-hydroxy aldehyde or ketone. However, the product of an Aldol reaction can proceed through an elimination reaction (loss of water) to form an α, β-unsaturated carbonyl (Aldol condensation), but not an α, β-unsaturated carboxylic acid. Therefore option (D) is not correct. To conclude, in the context of the given options, the correct answer is the Perkin reaction which is an organic reaction leading to the formation of α, β-unsaturated carboxylic acids.

Key concepts

Claisen Condensation

Claisen condensation is an organic reaction that plays a critical role in forming carbon-carbon bonds, primarily useful in building the molecular frameworks of many organic compounds, including complex natural products. The reaction specifically involves the combination of two ester molecules in the presence of a strong base, such as an alkoxide ion. During this process, one molecule acts as a nucleophile, attacking the carbonyl carbon of the other, leading to the formation of a new carbon-carbon bond and ultimately, a β-keto ester.

A key aspect of Claisen condensation is the requirement of a base strong enough to deprotonate the alpha hydrogen of the ester, as this step generates the necessary enolate ion. The enolate then attacks the electrophile in a nucleophilic acyl substitution, culminating in the condensation product once the elements of the leaving group are eliminated.

Analyze and Optimize

It's beneficial for students to compare the products of Claisen condensation with other related reactions: for example, recognizing the differences between a β-keto ester and an α,β-unsaturated ester can provide deeper understanding of why Claisen condensation would not be the answer to the exercise problem.

Benzoin Condensation

Benzoin condensation is a fascinating reaction for students to explore, specifically because it introduces a type of catalysis that isn't as widely encountered in general chemistry courses: cyanide ion catalysis. This reaction typically involves the dimerization of aromatic aldehydes, particularly benzaldehyde, resulting in a more complex structure known as benzoin.

In this reaction, cyanide ions facilitate the formation of an intermediate radical anion from the aldehyde. The radical anion properties of this intermediate allow for the unique carbon-carbon bond formation between two aldehyde molecules. The end product, benzoin, is a hydroxyl-containing compound with ketone functionality.

Concept Exploration

Understanding the role of the cyanide catalyst can be a significant learning point. Cyanide facilitates the creation of the intermediate necessary for the transformation but is not consumed in the reaction, which is a hallmark characteristic of a catalyst. Emphasizing this mechanism can help students grasp the subtleties of catalytic reaction pathways.

Aldol Reaction

The Aldol reaction is a cornerstone of organic chemistry, serving as one of the most versatile methods of forming carbon-carbon bonds. This process involves the reaction between an enol or enolate ion of one carbonyl compound with another carbonyl compound, leading to the creation of a β-hydroxy aldehyde or ketone - the Aldol addition product.

However, the Aldol reaction can further proceed to form an α,β-unsaturated carbonyl compound through a subsequent elimination step, known as Aldol condensation. This transformation exemplifies how reaction conditions, such as temperature and the presence of an acid or base catalyst, can dramatically affect the outcome, steering the reaction towards either the Aldol addition or the Aldol condensation product.

Practical Application and Insights

To aid student understanding, it is helpful to present graphic representations of this bifurcation in reaction pathways. Additionally, the Aldol reaction can link to discussions on the topic of thermodynamic vs. kinetic control, as conditions can favor the formation of one product over the other. This understanding can illuminate why, when comparing the Aldol reaction to the problem in the exercise, an Aldol condensation process cannot lead to the formation of an α,β-unsaturated carboxylic acid.