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Oxidation And Reduction

Here are two classes of reactions that are going to be very important going forward. In the first, we’re breaking C–H and forming C–O (actually a C-O Pi bond, hence the pi symbol).  In the second, we’re breaking C–O and forming C–H.

  • A reaction where we’re forming C–O at the expense of C–H  is called an oxidation. A reactant that performs this reaction is an oxidizing agent. 
  • A reaction where we’re forming C–H at the expense of C–O  is called a reduction. A reactant that performs this reaction is a reducing agent. 

You’ll often hear it said that we oxidize “up” and reduce “down”. That’s because each of these reactions affects the “oxidation state” of carbon (oxidation increases, reduction decreases).
When you think about it, oxidation and reduction are an essential part of life on earth. Plants take CO2 and use sunlight to reduce it “down” to sugars (forming C-H (and C-C), breaking C-O) while we humans take sugars and oxidize them “up” to CO2, releasing energy in the process. You could say the same thing for automobiles burning hydrocarbons.

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*Although these are the most common examples, “oxidation” technically also applies to reactions that break C–H (or C–C) and replace them with a more electronegative element such as Cl, N, Br, etc. More here: Oxidation and Reduction in Organic Chemistry