 A common requirement of hyperconjugation is that the bonds donating and accepting electron density are antiperiplanar to each other, to allow for maximum orbital overlap.The Cieplak effect uses hyperconjugation to explain the face-selective addition of nucleophiles to carbonyl carbons.Specifically, donation into the low-lying σ*C-Nuc bond by antiperiplanar electron-donating substituents is the stabilizing interaction which lowers the transition state energy of one stereospecific reaction pathway and thus increases the rate of attack from one side.