step 5: a molecule can contain several reactive functional groups. in a synthesis, it may be desirable to mask the functionality of one group before undergoing a key reaction with the second group. the masked group can then be unmasked to reform the functional group. the masked group is called a protecting group in organic chemistry. acetals can be used to mask the reactive carbonyl group and then easily removed by hydrolysis. which reactions do not need to be protected using an acetal?