In the aldol reaction of acetaldehyde under basic conditions, which species acts as electrophile?

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In a base-catalyzed aldol reaction, one molecule is turned into a nucleophile (the enolate) and it attacks the carbonyl carbon of another molecule, which is the electrophile. The electrophilic center is the carbonyl carbon of acetaldehyde because the carbon in a C=O bond carries partial positive charge and is susceptible to nucleophilic attack. The enolate carbon provides the nucleophilic attack, forming a new C–C bond to that carbonyl carbon. The hydroxide ion’s role is to remove a proton to generate the enolate; it is not the species that accepts the carbonyl carbon in the bond-forming step. Water, if involved, is typically just a proton source during workup, not the electrophile in the aldol step. So the acetaldehyde carbonyl carbon is the electrophile, and the enolate carbon is the nucleophile. The product, after protonation, is a beta-hydroxyaldehyde.