Which reaction, intermediate is formed during the condensation reaction between acetaldehyde and formaldehyde ?

Step-by-step solution

The -hydrogen atoms of acetaldehyde due to -E effect of > C = O group is slightly acidic in nature. In crossed aldol condensation between formaldehyde and acetaldehdye in the first step OH ⁻ ion (from the base added) abstracts one of these acidic -hydrogens to form carbanion or enolate ion which is stabilised by resonance.