Single bonds between carbon and carbon or hydrogen atoms are not very reactive. Organic molecules are therefore rather reacting at sites activated by double bonds or atoms like oxygen, nitrogen, sulfur, phosphorus.

Common groups for example are ether-, alcohol-, aldehyde-, keto-, carbonic acid-, ester-, thiol- and amino groups. In the case of nearby double bonds, reactivities can be changed, (for example in the case of enoles or enamines). Further special groups are formed by the successive sequence of double bonds and single bonds (conjugated double bonds).