nucleophilic addition

Meaning: The carbonyl carbon is partially positive because oxygen is more electronegative. A nucleophile donates an electron pair to this carbon, the C=O π electrons move onto oxygen, and the carbon changes from sp² (trigonal planar) to sp³ (tetrahedral). The alkoxide is then protonated, often by solvent or added acid, to give an alcohol. Under acidic conditions, protonating the carbonyl oxygen first makes the carbon more electrophilic.

Equation: R₂C=O + Nu⁻ → R₂C(Nu)–O⁻ → R₂C(Nu)–OH (after protonation).

On the MCAT: Expect questions that ask you to rank carbonyl reactivity or predict products. Examples are Grignard or hydride (NaBH₄, LiAlH₄) reagents giving alcohols, cyanide giving cyanohydrins, alcohols adding to aldehydes to give hemiacetals and acetals, and amines giving imines. Aldehydes are generally more reactive than ketones because they have less steric hindrance and less electron donation to the carbon. Electron-withdrawing groups next to the carbonyl increase reactivity. You may also need to identify the tetrahedral intermediate or the stereochemical result. Attack on a planar carbonyl from either face can give a racemic mixture if a new stereocenter forms.

Common confusion: Nucleophilic addition is not the same as nucleophilic acyl substitution. In addition, the carbon has no good leaving group, so it stays tetrahedral. In carboxylic acid derivatives, the tetrahedral intermediate can expel a leaving group and re-form the C=O, giving substitution.