Name the reaction when a mixture of acetaldehyde and propanal is treated with aqueoous alkali solution.

Cross Aldol Condensation

The correct answer is Option (1) → Cross Aldol Condensation

Core Concept:

Aldol reaction occurs between aldehydes/ketones having α–hydrogen in presence of dilute base.

When two different carbonyl compounds react → Cross Aldol Condensation.

Detailed Explanation of Each Option:

Option 1: Cross Aldol Condensation

Acetaldehyde (CH₃CHO) and Propanal (CH₃CH₂CHO):

• Both have α–hydrogen
• Both form enolate ions in base

In aqueous alkali:

Enolate of one aldehyde attacks the carbonyl carbon of the other.

Since two different aldehydes are reacting → Cross Aldol Condensation occurs.

Option 2: Self Aldol Condensation

Self aldol happens when:

Only one aldehyde reacts with itself

Example:

2 CH₃CHO reacting together

But here we have two different aldehydes → not self aldol.

Option 3: Intramolecular Aldol Condensation

Intramolecular aldol occurs:

Within a single molecule containing two carbonyl groups

Example:

Dialdehydes or diketones

But acetaldehyde and propanal are separate molecules → not intramolecular.

Option 4: Cannizzaro Reaction

Cannizzaro occurs only when:

Aldehydes do not have α–hydrogen

Example:

Formaldehyde, Benzaldehyde

But both acetaldehyde and propanal contain α–hydrogen → so they undergo aldol instead.