Question

What do the functional groups determine?

Answer 1

A functional group is like a family. It is a predictor of sorts. Just like looking at fruit, you know what an apple is going to taste like by looking at it. In chemistry you can see a certain grouping of atoms together and you would be able to predict its reactivity.

For example, with C=C, this is an alkene and I already can tell that it will react with HBr