Give aldol condensation reaction of acetaldehyde and explain why formaldehyde does not give this reaction ?

Explanation

Aldol Condensation. Two molecules of an aldehyde or a ketone having atleast one `alpha`-hydrogen atom condense in the presence of dilute alkali to form a `beta`-hydroxy aldehyde or `beta`-hydroxy ketone. This on heating gives `alpha, beta`-unsaturated aldehyde or ketone. This reaction is called aldol condensation. This reaction involve the formation of a bond between carbonyl carbon of one molecule and the `alpha`-carbon atom of the other moelcule. For example,
`underset("Ethanal or Acetaldehyde (Two moelcule)")(CH_(3)-underset(H)underset(|)overset(O)overset(||)(C)+H)-CH_(2).CHO overset("Dil. NaOH")hArr underset("3-Hydroxybutanal (Aldol)")(overset(4)(C)H_(3)-underset(H)underset(|)overset(OH)overset(|3)(C)-overset(2)(C)H_(2)overset(1)(C)HO)underset(-H_(2)O)overset(Delta)rarr underset("But-2-en-1-al")(CH_(3)-CH=CH-CHO)`
This reaction is due to acidic nature of `alpha`-hydrogen in aldehydes and ketones. Formaldehyde does not give this reaction because formadelhyde does not contain `alpha`-H atom.