Question

Write structural formulas and names of four possible aldol condensation products from propanal and butanal. In each case, indicate which aldehyde acts as nucleophile and which as electrophile.

Short answer

Four aldol products: two β-hydroxy aldehydes and two enones, with propanal and butanal both serving as nucleophiles and electrophiles.

Step-by-step solution

Understand Aldol Condensation

Aldol condensation involves the formation of a β-hydroxy aldehyde or ketone, followed by dehydration to form a conjugated enone. It requires at least one hydrogen atom on the α-carbon of the carbonyl compound.

Examine Propanal and Butanal

Propanal is an aldehyde with three carbon atoms (CH_3CH_2CHO")). Butanal is an aldehyde with four carbon atoms (CH_3CH_2CH_2CHO"). Each can act either as a nucleophile or electrophile.

Structural Formula for Propanal as Electrophile

When propanal acts as an electrophile, its carbonyl carbon is attacked by the nucleophilic enolate form of butanal. The resulting aldol is butanal-2-methyl-3-hydroxybutanal. Its structure is: CH_3CH_2CH_2CH(OH)CH(CH_3)CHO.

Structural Formula for Butanal as Electrophile

When butanal acts as an electrophile, its carbonyl carbon is attacked by the nucleophilic enolate form of propanal. The resulting aldol is butanal-2-hydroxy-3-methylpentanal. Its structure is: CH_3CH(OH)CH_2CH_2CH(CH_3)CHO.

Dehydration to Form Conjugated Enones

Dehydration of the β-hydroxy aldehydes formed in steps 3 and 4 gives conjugated enones. From Step 3 product, the enone is CH_3CH_2CH_2CH=CH(CH_3)CHO. From Step 4 product, the enone is CH_3CH=CHCH_2CH=CH(CH_3)CHO.

Identify all Four Products

1. Propanal as electrophile forms (a) β-hydroxy product CH_3CH_2CH_2CH(OH)CH(CH_3)CHO and (b) enone CH_3CH_2CH_2CH=CH(CH_3)CHO. 2. Butanal as electrophile forms (a) β-hydroxy product CH_3CH(OH)CH_2CH_2CH(CH_3)CHO and (b) enone CH_3CH=CHCH_2CH=CH(CH_3)CHO.

Key concepts

Propanal

Propanal is a type of aldehyde, which is an important class of organic compounds characterized by the presence of a carbonyl group bonded to a hydrogen atom and an alkyl or aryl group. In the case of propanal, it has three carbon atoms, giving it the molecular formula of \( C_3H_6O \). The structural formula can be represented as \( CH_3CH_2CHO \).

This compound is quite reactive due to the polar nature of the carbonyl group. The carbon atom of the carbonyl group carries a partial positive charge, making it susceptible to attacks by nucleophiles. In aldol condensation reactions, propanal can play the role of either a nucleophile, through the formation of enolate ions, or an electrophile, depending on the reaction mechanism.

• As a nucleophile, propanal can form enolate ions that attack other carbonyl compounds.
• As an electrophile, the carbonyl carbon of propanal can be targeted by nucleophilic enolates from other aldehydes or ketones.

Butanal

Butanal, also known as butyraldehyde, is another type of aldehyde that consists of four carbon atoms, with a molecular formula of \( C_4H_8O \). Its structural formula is \( CH_3CH_2CH_2CHO \).

Just like propanal, butanal contains a reactive carbonyl group, which makes it an important participant in aldol condensation reactions. Here is how it works:

• The carbonyl carbon of butanal is electrophilic, making it open to attack by nucleophiles.
• The α-hydrogens adjacent to the carbonyl carbon can be removed to form an enolate ion, which can act as a nucleophile in attacking another carbonyl compound.

In reactions like aldol condensations with propanal, butanal can switch roles as needed, functioning as either the nucleophile or electrophile, depending on which specific reaction mechanism is at play.

Nucleophile

A nucleophile is a chemical species that donates an electron pair to form a chemical bond. The term 'nucleophile' literally means "nucleus-loving," and these species are typically rich in electrons. This character allows them to attack electrophiles, which are electron-deficient species.

In the context of aldol condensation involving propanal and butanal:

• The enolate ion generated from either propanal or butanal acts as a nucleophile because it can donate an electron pair to attack the carbonyl carbon of the other molecule.
• The ability to form an enolate ion is determined by the presence of α-hydrogens. These hydrogens can be deprotonated to form the nucleophilic species.

Nucleophiles are crucial in many organic reactions, particularly those that involve the formation of new carbon-carbon bonds, such as in aldol condensation.

Electrophile

An electrophile is a chemical species that accepts an electron pair to form a chemical bond. These species are generally electron-deficient, with a partial positive charge, making them highly receptive to attacks by nucleophiles.

In aldehydes, like propanal and butanal, the carbonyl carbon is typically electrophilic due to its partial positive charge that arises from the polarity of the carbonyl bond. Here's how they function:

• The carbonyl carbon of propanal or butanal can act as an electrophile in an aldol condensation when attacked by the nucleophilic enolate ion of another molecule.
• When acting as an electrophile, it facilitates the formation of new carbon-carbon bonds in the resulting aldol products.

Understanding the dual roles of propanal and butanal as nucleophiles and electrophiles is key to predicting the products of their aldol condensation reaction.