Most of the reactions in the organic chemistry there is short lived formation of radicals or carbonium ions. They are the highly reactive. Element Carbon, the element around which the whole branch of organic chemistry revolves, has 4 valency and thus forms 4 single covalent bonds. It has got 4 unpaired electrons in the sp3 configuration which means that 4 equivalent hybrid orbitals are formed by combination of 1s and three 3p orbitals in the second shell.
During many reactions, one unpaired electrons remains when 1 atom attached to carbon is ripped off. This is of the form R3C where R are alkyl groups. This is a radical and is extremely reactive and seeks to stabilize itself by combining with another atom with 1 unpaired electron. These radicals thus live for very short time during the steps of organic reactions. Sometimes their existence seems to be a fable.
But then, there was a very brilliant Russian organic chemist. His name was Gomberg. Still a student, he wanted to substitute all the hydrogen atoms on the methane and ethane with bulky C6H5 phenyl groups. No one had done this before. He did many experiments and after excluding oxygen from his experiments he was able to prepare the first stable radicals ever found in organic chemistry.
This was tri-phenyl methyl radical. Its solution when oxygen was blanketed have slightly yellow color which intensified upon warming. As we have said that radicals try to combine with other atoms with unpaired electrons. The trick is to become stabilized and have lowest energy. They are sitting on the top of potential energy high called transition state in chemistry. Here come the phenyl groups which share their electrons to stabilize the radical. Phenyl rings have a cloud of 6 pi electrons and they begin to disperse and stabilize the unpaired electron on the carbon be transporting the electron into the clouds especially on ortho and para positions. Since there are 3 phenyl rings, the dispersal of the unpaired electron is very large and thus the radical becomes stabilized. Also such molecules and radicals absorb light in the visible spectrum of light. Since it looks yellow in color, it means it absorbs the light in the lower part of spectrum leaving the yellow light for transmission.
When these solutions are exposed to oxygen which is famous for breaking into free radicals then the color begins to disappear and stable compounds are formed by reaction of 2 tri-phenyl methyl radical with one oxygen atom and is colorless.
We must appreciate the fact that in those times when not many of the analytical instruments were available, these chemists with their logic and classical compositional analysis were able to achieve so much insight. This is why the chemistry is called a fundamental science. Without it no field like geology, physics, biology, genetics and other modern science can progress. We must also give the due credit to Russian scientists whose works remained obscure in the era of communism when even the science discoveries were under the whip of communism.