No, its not essential that oxidation and reduction occur side by side in a chemical reaction, there are some simple reactions which involveĀ either transfer of electrons, atoms or ions in one direction or loss and gain of oxygen or hydrogen only, these reactions may be either oxidation or reduction reactions. while in many reactions orĀ vital processes like photosynthesis or respiration both oxidation and reduction occurs simultaneously such reactions are termed disproportionate reactions. all oxidation-reduction reactions are chemical reactions but all chemical reactions are not oxidation-reduction reactions.