The weaker the base, the better the leaving group. Electron withdrawing groups make the carbonyl more electrophilic. Electron donors make the carbonyl *less* electrophilic. Carboxylic acid derivatives always protonate on carbonyl first. Steric bulk slows down reactions.Read more
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So given identical reactlon conditions (e.g. temperature, solvent, and electrophile) the conjugate base is always a better nucleophile.Read more