Note: Distinct from hemiacetals and hemiketals, acetals and ketals are not easily returned to their carbonyl form, because the OR bond is not easily broken to allow a return of the oxygen to a double bond with carbon. This makes acetals and ketals good protecting groups when there is a need to prevent an aldehyde or ketone from reacting. In other words, an aldehyde or ketone can be temporarily changed into an acetal or ketal to prevent reaction with a nucleophile. This is often necessary, as aldehydes and ketones are more reactive than anhydrides, carboxylic acids, esters and amides, and will be attacked first by nucleophiles when left vulnerable to attack. Acid-catalyzed hydrolysis is most often used to remove protection once the desired reaction is complete.