Question

Cross aldol between formaldehyde and acetaldehyde .

Answer 1

Aldol condensation reaction is one of the important reactions of carbonyl compounds (i.e. aldehydes and ketones)

Condensation between two molecules of aldehydes or ketones having at least one α – hydrogen atom in presence of dilute alkali to form β-hydroxy aldehyde or β-hydroxy ketone is known as an aldol condensation reaction. Examples:

Formaldehyde will not undergo simple aldol condensation as it does not have alpha hydrogen. Only acetaldehyde will undergo simple aldol with another molecule of acetaldehyde.
The result will be a molecule with four carbon atoms:

Formaldehyde and acetaldehyde do undergo crossed aldol condensation and form 3-Hydroxypropanal.