Question

Acetaldehyde when heated with \(\mathrm{NaOH}\) gives (a) but-2-enal (b) but- 3 -enal (c) propenal (d) butanal

Short answer

The product formed when Acetaldehyde is heated in the presence of NaOH is but-2-enal.

Step-by-step solution

Identify the structure of Acetaldehyde

Acetaldehyde is an organic compound with the formula \(CH_{3}CHO\). Its structure can be represented as: H H O | | || C-C=O

Understand the reaction with NaOH

On heating with \(\mathrm{NaOH}\), Acetaldehyde undergoes Aldol condensation. This reaction involves the formation of a new carbon-carbon bond between the carbonyl group and the alpha carbon atom of another carbonyl compound. In this case, the other carbonyl compound is also an Acetaldehyde molecule.

Write the reaction for Aldol condensation

In the first step of Aldol condensation, the alpha carbon atom (C-H) is deprotonated by the base (\(\mathrm{OH^{-}}\) from \(\mathrm{NaOH}\)), leading to the formation of an enolate ion: H H O | | || C-C=O + OH^- -> C=C-O- + H2O In the second step, the enolate ion attacks the carbonyl group of another Acetaldehyde molecule, forming a carbon-carbon bond, and gives an aldol intermediate: O^- H H O || | | || C-C C-C=O -> H H O H | | | | C-C-C-C=O

Identify the final product

The last step of the reaction involves the dehydration of the aldol intermediate to form the final product. This involves the elimination of a water molecule from the aldol to form a double bond: H H O H | | | | C-C-C-C=O -> C-C=C-C=O + H2O The final product has the following structure: H H O | | || C-C=C-C=O Now, we can compare the obtained product's structure with the given options to find the correct answer.

Choose the correct option

The correct structure obtained in Step 4 stands for but-2-enal, which corresponds to option (a). Therefore, the final product of the reaction between Acetaldehyde and \(\mathrm{NaOH}\) is but-2-enal. The correct answer is (a) but-2-enal.

Key concepts

Acetaldehyde

Acetaldehyde is a simple yet fundamental molecule in organic chemistry. It has the chemical formula \( \text{CH}_3\text{CHO} \). Structurally, it consists of a methyl group (CH₃) connected to a carbonyl group (C=O). The carbonyl carbon is flanked by hydrogen atoms, making it a reactive aldehyde. This small molecule plays a significant role in various chemical reactions, including the Aldol condensation.
Acetaldehyde is a versatile reagent. It is known for its ability to undergo condensation reactions, which are vital in creating more complex organic compounds. In this context, it acts as both a nucleophile and an electrophile.
Understanding Acetaldehyde's structure and reactivity helps in predicting its behavior in chemical reactions and its role as an intermediate in organic synthesis.

Enolate Ion

Enolate ions are critical intermediates in many organic reactions, especially Aldol condensation. They are formed from the deprotonation of aldehydes or ketones. In the case of Acetaldehyde, - The alpha carbon, which is the carbon adjacent to the carbonyl group, has hydrogen atoms that can be readily removed.- When Acetaldehyde is treated with a base like \( \text{NaOH} \), the alpha hydrogen is deprotonated to form an enolate ion.This enolate ion is a resonance-stabilized intermediate, meaning it has several forms that share the same electron configuration. The enolate ion can act as a nucleophile, ready to attack electrophilic centers, such as the carbonyl carbon of another Acetaldehyde molecule. This reactivity is what drives the formation of new carbon-carbon bonds in Aldol condensation, leading to the formation of larger organic compounds.

Dehydration Reaction

Dehydration reactions are processes where water molecules are expelled from a compound. In the context of Aldol condensation, this step follows the initial formation of the aldol intermediate.
Once the aldol intermediate is formed, it undergoes dehydration: - A hydroxyl group and a hydrogen atom eliminate a molecule of water. - This results in a double bond, forming an α,β-unsaturated carbonyl compound. This dehydration step is crucial as it stabilizes the product and completes the Aldol condensation process. It's through dehydration that our initial reactants, such as Acetaldehyde, transform into more complex structures like but-2-enal, showcasing the synthesis power of these reactions.

Carbonyl Group

The carbonyl group is a defining feature in organic chemistry, characterized by a carbon atom double-bonded to an oxygen atom (C=O). This functional group is highly reactive due to the polarity of the carbon-oxygen bond.
In Aldol condensation- The carbonyl group serves as both the electrophile and a key reactive site.- The electron-deficient carbon atom is susceptible to nucleophilic attack, such as from the enolate ion, during the reaction.Its ability to participate in various transformations, like bond formation in Aldol condensation, makes the carbonyl group central to organic chemical reactions. In the context of Acetaldehyde reacting with \( \text{NaOH} \), the carbonyl group is instrumental in forming new carbon-carbon bonds, leading to more complex molecules. This showcases its importance in building-block reactions that form the backbone of organic synthesis.