To make sense of the endless variety of chemical reactions that can occur, chemists have devised many ways to categorize reactions that seemed similar. One early attempt to explain reactions related to combustion or the burning of various substances involved speculation that a fire-like substance called phlogiston was released. One big problem was the phlogiston could not be detected, and an even bigger problem was that substances that underwent combustion actually gained weight!
Phlogiston Theory is No More
The phlogiston theory was debunked by English clegyman/scientist Joseph Priestley and French chemist Antoine-Lauent Lavoisier in the late 18th century with the discovery of oxygen and proof that combustions involved an element combining with oxygen and releasing heat energy. This process was named oxidation for the chemical process of an element combining with oxygen. The word reduction originally referred to the reverse process, the loss of weight upon heating a metallic ore such as a metal oxide to extract the metal. In other words, ore was “reduced” to elemental metal. Antoine Lavoisier showed that this loss of weight was due to the loss of oxygen as a gas.
Oxidation and Reduction
So, originally the terms oxidation had to do with either combining with oxygen (oxidation) or losing oxygen (reduction). But soon chemists realize that when other non-metals such as nitrogen, sulfur or phosphorous combined with metals, the process was similar. The essential element in each process was the metal element lost electrons and the non-metal gained electrons.
As a result, oxidation was redefined as any reaction where a substance gives up electronsand reduction was redefined as any reaction where a substance gains electrons.
However, when applied to organic substances the process is less clear and needed to be clarified and simplified, because in organic compounds electrons are typically shared in covalent bonds, not gained or lost.
An Organic Definition
A more specialized definition for organic molecules was developed. Organic oxidation is a process by which a carbon atom gains bonds to more electronegative elements, most commonly oxygen. Reduction is a process by which a carbon atom gains bonds to less electronegative elements, most commonly hydrogen. If a carbon atom bonds with a more electronegative atom in a shared pair, it is like it has sort of lost electrons, or at least doesn’t share them as equally as before. And if it bonds with a less electronegative atom it has the shared electrons a little bit more to themselves.
With all that in mind, we can turn our attention to the excellent infographic from the April 2017 edition of ChemMatters magazine. The graphic details several organic reduction reactions and explains the reducing agents used for each reaction pathway. The reactions illustrated in this graphic show some of the ways organic chemists can synthesize new compounds from various starting reagents.
The formulas at the top of the infographic are called functional groups and they need a bit of explaining. An organic functional group is a set of atoms and bonds that are part of organic molecules and are responsible for the characteristic reactions of the molecule. Let’s look at one example, carboxylic acids. An organic molecule that ends in a carbon atom with one double bonded oxygen and one hydroxyl (OH) group is what we define as a carboxylic acid. You might be wondering what the ‘R’ stands for in these examples. The R-group usually stands for any combination of carbon and hydrogen atoms that might be attached to the functional group. Think of it as the ‘Rest of the molecule’. It is sort of like the variable X in mathematics. The R-group can be as simple as a CH3 or as complex as multiple carbons and hydrogens. The key idea is that any molecule that contains a specific functional group will react similarly. That is what the infographic is showing.
A Winning Graphic
This graphic is a winning entry in the inaugural ChemClub Infographic Contest. ACS ChemClubs were challenged to take a chemistry topic and turn it into an original informational graphic. Entries were judged on originality, and the ability to convey accurate science details clearly and creatively. This infographic was conceived by Maksim Fomich of Belarus, our Teacher/Chem Enthusiast category winner.