Question

In aldol condensation, $H^{\oplus }$ is abstracted from the $C$ atom ______ places away from carbonyl $C$.

Answer 1

In aldol condensation, aldehydes and ketones containing at least one alpha hydrogen atom, on treatment with dilute base (such as NaOH, barium hydroxide or potassium carbonate etc.) or dilute acid undergo condensation to form aldol.
The aldol thus formed on heating, loses a molecule of water to form alpha-beta unsaturated carbonyl compound.
For example, Two moles of acetaldehyde condense in presence of dil. alkali to form acetaldol.
Acetaldol on heating loses a molecule of water to form crotonaldehyde.