Additions to other double bonds or equivalents such as α-keto amides and nitriles are commonly reversible because the electrons from the double or triple bond are placed in the products on a heteroatom (oxygen or nitrogen), eventually forming an amine or alcohol. Alcohol and amine protons are moderately acidic, and the barrier to their removal is low. To reverse the Michael addition, a proton must be removed from the carbon atom next to the carbonyl group, and this proton is significantly less acidic (in most cases) than an amine or alcohol. Cleavage of the C-H bond is in most cases slower than deprotonation of an amine or alcohol, slowing down the retro-Michael reaction and rendering the addition irreversible under physiological conditions. However, if the C-H bond is made more acidic by, such as, the substitution of a proton with a second carbonyl group or nitrile, the deprotonation may be made rapid enough to make the retro-Michael reaction reversible. Bond formation at the terminal alkene carbon is an important step in Michael addition; steric hindrance at that carbon lowers the rate of Michael addition and may prevent it.