Nucleophilic and electrophilic center:
Because of the polarity of the carbonyl group, aldehydes and ketones comprises anucleophilic oxygen center and an electrophilic carbon as displayed for centers propanal. Hence, nucleophiles react with aldehydes and ketones at the carbon center, and electrophiles react at the oxygen center.
Figure: Nucleophilic and electrophilic centers of the carbonyl group.