Question

Using your roadmaps as a guide, show how to convert acetaldehyde into racemic 3-hydroxybutanal. You must use acetaldehyde as the source of all carbon atoms in the target molecule. Show all reagents and all molecules synthesized along the way.

Short answer

Question: Outline the steps to convert acetaldehyde into racemic 3-hydroxybutanal using acetaldehyde as the carbon source. Answer: 1. Perform an aldol condensation with acetaldehyde and NaOH to form 3-hydroxy-2-methyl-2-propenal. 2. Reduce the alpha,beta-unsaturated carbonyl group of 3-hydroxy-2-methyl-2-propenal using catalytic hydrogenation with Pd/C and H2 to obtain racemic 3-hydroxybutanal.

Step-by-step solution

Double the Carbon Chain Length with Acetaldehyde

First, we need to increase the carbon chain from two carbons to four. We can achieve this through an aldol condensation reaction. In this reaction, we'll use sodium hydroxide (NaOH) as a base to promote the formation of an enolate intermediate from acetaldehyde. The enolate will then react with another molecule of acetaldehyde, joining the carbon chains together. The final product will be 3-hydroxy-2-methyl-2-propenal. \[ \text{Acetaldehyde} + \mathrm{NaOH} \rightarrow \text{3-Hydroxy-2-methyl-2-propenal} \]

Reduce the alpha,beta-Unsaturated Carbonyl Group

Now, we need to reduce the alpha,beta-unsaturated carbonyl group in the 3-hydroxy-2-methyl-2-propenal molecule to form 3-hydroxybutanal. We can accomplish this via catalytic hydrogenation using a catalyst, such as palladium on carbon (Pd/C), and a hydrogen source, such as hydrogen gas (H2). This reaction will form the desired 3-hydroxybutanal product. \[ \text{3-Hydroxy-2-methyl-2-propenal} + \mathrm{H_2} + \mathrm{Pd/C} \rightarrow \text{racemic 3-Hydroxybutanal} \] To summarize the process of converting acetaldehyde into racemic 3-hydroxybutanal, follow these steps: 1. Perform an aldol condensation reaction with acetaldehyde and sodium hydroxide (NaOH) to form 3-hydroxy-2-methyl-2-propenal. 2. Reduce the alpha,beta-unsaturated carbonyl group of 3-hydroxy-2-methyl-2-propenal using catalytic hydrogenation with palladium on carbon (Pd/C) and hydrogen gas (H2) to obtain racemic 3-hydroxybutanal.

Key concepts

Acetaldehyde

Acetaldehyde, also known as ethanal, is a simple aldehyde with the chemical formula \( ext{CH}_3 ext{CHO} \). It is a two-carbon compound and is a key building block in organic synthesis.
Being a reactive compound, acetaldehyde readily participates in various chemical reactions, including aldol condensation.
This ability makes it highly valuable for creating more complex molecules by building longer carbon chains.
In this context, acetaldehyde is used to synthesize larger compounds by joining with itself through aldol condensation. In the exercise, we utilize two molecules of acetaldehyde to convert them into a longer, four-carbon molecule, 3-hydroxy-2-methyl-2-propenal. This transformation is achieved by forming an enolate ion intermediate, which then combines with another acetaldehyde molecule.
Acetaldehyde's tendency to undergo such reactions allows chemists to construct a wide variety of more complex organic structures from relatively simple starting materials.

Racemic Mixture

A racemic mixture, commonly referred to as a racemate, is a 50:50 mixture of two enantiomers of a chiral molecule. These enantiomers are non-superimposable mirror images of each other, much like right and left hands.
The presence of chiral centers in a molecule leads to such enantiomer formations. In the context of the given exercise, the product 3-hydroxybutanal is a chiral compound, containing a new stereogenic center. As a result of this synthesis, a racemic mixture is naturally formed.
The process involves using an intermediate (3-hydroxy-2-methyl-2-propenal), which, after catalytic hydrogenation, results in the chiral product. Since the reaction doesn't favor one enantiomer over the other, both are produced in equal amounts.
Racemic mixtures are important in chemistry and pharmaceuticals because the activity of each enantiomer in a racemic mixture can differ. They offer insights into understanding and designing techniques to isolate or convert these enantiomers for improved efficacy in applications.

Catalytic Hydrogenation

Catalytic hydrogenation is a chemical reaction that involves the addition of hydrogen molecules across unsaturated bonds, such as double bonds, in organic compounds.
Typically, a metal catalyst like palladium (Pd), often supported on carbon (C), is used to facilitate the reaction. The hydrogen gas (\( ext{H}_2 \)) splits and adds on either side of the unsaturated bond, resulting in a saturated molecule.
In the exercise, catalytic hydrogenation is utilized to convert the alpha, beta-unsaturated carbonyl group in 3-hydroxy-2-methyl-2-propenal to 3-hydroxybutanal.
The process requires a catalyst, which accelerates the reaction without being consumed. Hydrogen molecules interact with the surface of the catalyst, making them reactive. These reactive hydrogen atoms can then reduce the unsaturated bond in the molecule.
Catalytic hydrogenation is a foundational technique in organic chemistry, not only for producing more stable compounds from unsaturated ones but also for its applications in drug synthesis and various other industrial processes.
Its effectiveness and efficiency make it an invaluable tool in creating desired structural features in molecules, essential for a wide range of chemical reactions.