The reactivity and other chemical properties of an organic molecule are primarily dependent upon of the molecule.

The reactivity and other chemical properties of an organic molecule are primarily dependent upon the attached functional group of the molecule.

For example hydroxyl group (-OH) in alcohols or carboxyl group in carboxylic acid are poplar groups and that is why this compounds are polar.
The atoms of functional groups are linked to each other and to the rest of the molecule with covalent bonds.