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The reaction begins when a nucleophile donates an electron pair to the carbonyl carbon and the C=O π bond shifts to oxygen, forming a tetrahedral alkoxide. Protonation then gives an addition product. Aldehydes generally react more readily than ketones because they have less steric hindrance and weaker electron donation from alkyl groups. Acid catalysis can increase carbonyl electrophilicity, while base catalysis can strengthen the nucleophile. The lesson distinguishes strong nucleophiles, which commonly give effectively irreversible additions, from weaker neutral nucleophiles that may add reversibly. Examples include reductions, carbon–carbon bond formation, cyanohydrin formation, and reactions with alcohols or amines. Because the carbonyl is planar, attack on either face can produce a racemic mixture unless chiral control is introduced.