91Ó°ÊÓ

The two carbonyl compounds undergo a reaction in the presence of a base to form a $\mathrm{β}$-hydroxy carbonyl $\left(C=O\right)$ compound.

The process completes in 3 steps. In the first step, the base abstracts the $\mathit{Î\pm }$-hydrogen from the carbonyl compound(s) to form resonance stabilized enolate(s).

In the next step, the nucleophilic enolate compound thus formed attacks the electrophilic carbon center of another carbonyl $\left(C=O\right)$compound. Thus, a new C-C bond is formed between the carbonyl carbon of one compound and the -carbon of the other.

In the last step, the alkoxide ion is protonated to form the -hydroxy carbonyl $\left(C=O\right)$ compound.

The tetravalent black balls must be carbon, the white balls must be representing hydrogen, and the divalent red balls must be representing oxygen.