 In the Cieplak model, electrons from a neighboring bond delocalize into the forming carbon–nucleophile (C–Nuc) bond, lowering the energy of the transition state and accelerating the rate of reaction.Whichever bond can best donate its electrons into the C–Nuc bond determines which face of the carbonyl the nucleophile will add to.The nucleophile may be a number of reagents, most commonly organometallic or reducing agents.