This makes sense because reactions involve making and breaking bonds. Bonds are electrons, so making and breaking bonds will change the location of electrons. Functional groups are the places where changing the location of electrons can happen fairly easily, which means that the distribution of electrons in a functional group is a key to its reactivity. We need a specific example to make these ideas useful. We'll begin with the addition of water to a carbonyl group, specifically the aldehyde carbonyl group in acetaldehyde.
Addition reactions occur typically with functional groups which include pi bonds.
Functional groups which include pi bonds are called unsaturated functional groups because some of the atoms in them have fewer than the maximum number of sigma bonds. As we study more addition reactions of unsaturated functional groups during the semester, we'll notice that the pi bond in the reactant is typically broken. In the product we find that there are now two groups or atoms attached where the pi bond had been. In our instance, the pi bond between carbon and oxygen has disappeared and the hydrogen from water has added to the oxygen while the OH group from water has added to the carbon. We can also notice that nothing happened to the CH 3 group.