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9. Functional groups react the same in different molecules. Chemists classify groups of atoms that take part in characteristic reactions as functional groups such as –OH groups, carbonyls, haloalkanes or enolates.  Although functional groups can influence each other in some complex molecules, functional groups serve as the most generally useful organizing principle in organic chemistry because they mostly react the same in different molecules.  Recognizing functional groups and understanding their characteristic reactions are key to being able to predict reactions in molecules you have never seen before.