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Chapter 19: Problem 436

An aldol condensation was carried out on equal amounts of propionaldehy de and butyraldehyde. The reaction gave rise to four distinct products. What were their structures, and how did each arise?

Short Answer

Expert verified
The four distinct products formed due to aldol condensation between propionaldehyde and butyraldehyde are: 1. Propionaldehyde self-aldol: CH3−CH2−CH(OH)−CH2−CHO 2. Butyraldehyde self-aldol: CH3−CH2−CH2−CH(OH)−CH2−CHO 3. Crossed-aldol (propionaldehyde enolate with butyraldehyde): CH3−CH2−CH(OH)−CH2−CH2−CHO 4. Crossed-aldol (butyraldehyde enolate with propionaldehyde): CH3−CH2−CH2−CH(OH)−CH2−CH2−CHO

Step by step solution

01

Formation of Enolate Ions

In the base-catalyzed aldol condensation reaction, there will be the formation of enolate ions from propionaldehyde and butyraldehyde. Let's denote the enolate ion of propionaldehyde as Enolate 1 and the enolate ion of butyraldehyde as Enolate 2. Enolate 1 (propionaldehyde): H2C=CH−CH2O− Enolate 2 (butyraldehyde): H2C=CH−CH2−CH2O−
02

Self-Aldol Condensation Products

Reacting each enolate with their corresponding aldehyde will give us the self-aldol product. Product 1 (propionaldehyde self-aldol): 2×CH3−CH2−CHO→CH3−CH2−CH(OH)−CH2−CHO Product 2 (butyraldehyde self-aldol): 2×CH3−CH2−CH2−CHO→CH3−CH2−CH2−CH(OH)−CH2−CHO
03

Crossed-Aldol Condensation Products

Mixing enolates 1 and 2 with the opposite aldehyde will yield the cross-aldol products. Product 3 (crossed-aldol, propionaldehyde enolate with butyraldehyde): CH3−CH2−CHO+H2C=CH−CH2O−→CH3−CH2−CH(OH)−CH2−CH2−CHO Product 4 (crossed-aldol, butyraldehyde enolate with propionaldehyde): CH3−CH2−CH2−CHO+H2C=CH−CH2−CH2O−→CH3−CH2−CH2−CH(OH)−CH2−CH2−CHO So, the four distinct products formed due to aldol condensation between propionaldehyde and butyraldehyde are, 1. Product 1 (propionaldehyde self-aldol): CH3−CH2−CH(OH)−CH2−CHO 2. Product 2 (butyraldehyde self-aldol): CH3−CH2−CH2−CH(OH)−CH2−CHO 3. Product 3 (crossed-aldol, propionaldehyde enolate with butyraldehyde): CH3−CH2−CH(OH)−CH2−CH2−CHO 4. Product 4 (crossed-aldol, butyraldehyde enolate with propionaldehyde): CH3−CH2−CH2−CH(OH)−CH2−CH2−CHO

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Enolate Ion Formation
Understanding enolate ion formation is crucial for aldol condensation reactions. Under basic conditions, aldehydes like propionaldehyde and butyraldehyde can undergo deprotonation at the alpha carbon (the carbon adjacent to the carbonyl group). This process is facilitated by a strong base, which abstracts an alpha hydrogen, leading to the formation of an enolate ion.

Here, the base acts by removing an H+ ion, creating a carbon-carbon double bond next to a negatively charged oxygen. This negative charge is delocalized, making the enolate ion a stable intermediate. The enolate ion can then act as a nucleophile in further reactions.

For example:
  • The enolate ion of propionaldehyde is H2C=CH−CH2O−
  • The enolate ion of butyraldehyde is H2C=CH−CH2−CH2O−
These enolate ions play a key role in both self-aldol and crossed-aldol condensations as they are responsible for attacking the carbonyl carbon of the aldehyde, driving the reaction forward.
Self-Aldol Condensation
Self-aldol condensation occurs when an enolate ion reacts with the same type of aldehyde from which it was formed. This reaction leads to the formation of a β-hydroxy aldehyde, which is a product containing both alcohol and aldehyde functionalities.

Here's the breakdown:
  • Propionaldehyde reacts with its enolate (Enolate 1) to form a β-hydroxy molecule: CH3−CH2−CH(OH)−CH2−CHO
  • Similarly, butyraldehyde reacts with its enolate (Enolate 2) resulting in CH3−CH2−CH2−CH(OH)−CH2−CHO
This type of reaction is quite illustrative because it forms the simplest products in an aldol reaction where both reactants are the same.

Upon heating or under acidic conditions, these β-hydroxy aldehydes can further lose water to form unsaturated carbonyl compounds via dehydration. However, in this scenario, these steps end with the β-hydroxy aldehydes.
Crossed-Aldol Condensation
Crossed-aldol condensation, also known as mixed aldol condensation, involves the reaction between two different aldehyde molecules or ketones. This reaction generates products with greater structural diversity.

During this process, the enolate ion derived from one aldehyde attacks the carbonyl carbon of a different aldehyde. As a result:
  • Propionaldehyde enolate reacts with butyraldehyde to yield: CH3−CH2−CH(OH)−CH2−CH2−CHO
  • Butyraldehyde enolate combines with propionaldehyde to produce: CH3−CH2−CH2−CH(OH)−CH2−CH2−CHO
These reactions underscore the ability to form complex molecules from simpler ones. Crossed-aldol reactions are significant because they allow the synthesis of a variety of useful compounds.

Choosing the right base, controlling temperature, and preventing unwanted dehydration are important for obtaining desired products in crossed-aldol condensations.

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Most popular questions from this chapter

2, 6 -Bicyclo [2.2.2] octanedione [1] exhibits no enolic properties. Explain.

1,2 Cyclopentanedione exists primarily as a monoenol whereas biacetyl exists primarily as the keto form. Remembering conformational variances between the two substances; (a) explain this occurrence, (b) To what degree would Bicyclo [2.2.1]heptan-2, 3-dione enolize? Explain.

(a) What prediction can be made about the acidity of the \Upsilon-hydrogens of α,β -unsaturated carbony1 compounds? (b) Suggest a pathway for the synthesis of 5-pheny1-2,4-pentadienal, C6H5CH=CH−CH=CH−CHO.

Write the mechanism for the aldol condensation of propionaldehyde, for which the net reaction is shown below.

Ninhydrin, a reagent used to detect amino acids by a color reaction, is the hydrate of hydrindantrione. Why is the hydrate stable? Why is this the most likely structure for the hydrate?
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