Hydroxide ions also are good catalysts for many organic reactions. The mechanism for base-catalyzed hydrolysis of ethyl acetate is shown at the right. A negatively charged hydroxide ion is attracted to the slight positive charge on the carbon atom of a C=O bond, and a C-O bond is formed with the hydroxyl group. This pushes electrons in the C=O double bond toward the other O atom, thereby giving it a negative charge (b). This excess of electrons can be shifted back either to the hydroxyl oxygen (a) or to the bridge oxygen (c). As electrons in that C-O bridge bond are pushed toward the O, the C₂H₅-O^- group can fall away. The steps from b to c and b to a are identical except that they involve different C-O bonds. Once the C₂H₅-O^- group has departed, it can react with water to regenerate a hydroxide ion:

As with acid catalysis, the OH^- ion is involved in a lower-energy pathway, but is regenerated intact at the end of the process. Acids and bases are effective catalysts because they contribute charged ions that can push and pull electrons around in a molecule. We will see in Chapter 24 that some of the catalytic mechanisms used by enzymes are very similar to mechanisms of acid-base catalysis.