One common reaction carbonyl groups undergo is attack by a nucleophile on the partially positively charged C atom with the addition of a proton to the O atom. Note the possibility of forming a stereogenic carbon at the point of attack depending on what groups are attached to the carbonyl carbon. The carbocation formed will then be susceptible to nucleophilic attack on the C atom. Addition of Alcohols Across Carbonyls Alcohols are nucleophiles due to the lone pair of electrons on the O atom, but they are relatively weak nucleophiles and require an acid catalyst in order to add to a carbonyl. This reaction occurs in three reversible steps.
Hemiacetals can further react in the presence of excess alcohol to form acetals.
This is equivalent to the hydroxyl group on the hemiacetal being replaced by the alkoxide portion of the alcohol. In summary, aldehydes and ketones can react with alcohols to form hemiacetals, which can react with more alcohol to form acetals. The hemiacetals are not normally isolable compounds. These compounds may exist in equilibrium with their cyclic hemiacetal form. Addition of Water to Carbonyls In much the same fashion as alcohols add to carbonyls, so can water.