On this page
Free radicals turn up all over the syllabus: in the chlorination of methane, in the peroxide effect, and in polymer chemistry. In 1900 most chemists did not believe a carbon free radical could exist as a real, bottled substance. Moses Gomberg, working at the University of Michigan, made one by accident while trying to make something else. This post covers his life, how the discovery happened, a worked example using his actual analysis figures, common mistakes about radicals, and what you can learn from his care.
Who was Moses Gomberg?
The American Chemical Society's National Historic Chemical Landmark page gives his story. He was born on 8 February 1866 in Elizavetgrad in the Russian Empire, a town now in Ukraine. His Jewish family faced persecution after 1881 and fled to Chicago, where young Moses, who spoke no English at first, worked in the stockyards. He entered the University of Michigan in 1886, earned his PhD in 1894 and stayed there for his whole career, chairing the chemistry department from 1927 until he retired in 1936. He died on 12 February 1947.
His National Academy of Sciences biographical memoir by John C. Bailar Jr. adds that on leave in Germany he spent two terms in Adolf von Baeyer's laboratory in Munich and one with Victor Meyer in Heidelberg. There he made tetraphenylmethane, a compound others, Meyer included, had failed to make.
How did Gomberg find triphenylmethyl?
Back in Michigan he aimed for the next fully phenylated hydrocarbon, hexaphenylethane, (C6H5)3C–C(C6H5)3. The memoir describes the steps:
Treating triphenylmethyl chloride with sodium did nothing.
With finely divided silver, a reaction happened and he got a white crystalline solid. But its carbon and hydrogen analysis did not match hexaphenylethane.
Silver free of oxide, then zinc and mercury, gave the same product, so the metal was not to blame. He concluded the extra element was oxygen from the air.
He rebuilt the experiment to exclude air completely, using ground-glass joints rather than cork or rubber. Now he obtained a hydrocarbon that reacted readily with oxygen and with halogens such as iodine.
He wrote, in words quoted by both the ACS and the memoir: "The experimental evidence ... forces me to the conclusion that we have to deal here with a free radical, triphenylmethyl". The paper, An Instance of Trivalent Carbon: Triphenylmethyl, appeared in the Journal of the American Chemical Society in November 1900. The ACS notes he ended it by writing: "This work will be continued and I wish to reserve the field for myself."
Worked example: what Gomberg's numbers told him
The memoir gives his figures. For hexaphenylethane, C38H30, he calculated 93.83% carbon and 6.17% hydrogen. His air-exposed product gave only 87.93% carbon and 6.04% hydrogen. Check these with C = 12, H = 1, O = 16:
Molar mass of C38H30 = 38 × 12 + 30 × 1 = 456 + 30 = 486 g/mol
%C = 456 ÷ 486 × 100 ≈ 93.83%; %H = 30 ÷ 486 × 100 ≈ 6.17%. These match his calculated values.
The found values add up to 87.93 + 6.04 = 93.97%. About 6% of the mass is something else.
Try the peroxide, (C6H5)3C–O–O–C(C6H5)3 = C38H30O2: molar mass = 486 + 32 = 518 g/mol, giving %C = 456 ÷ 518 × 100 ≈ 88.0% and %O ≈ 6.2%.
That is close to what he found, and the memoir records that later work confirmed the air product was triphenylmethyl peroxide.
Now count electrons. The radical (C6H5)3C is C19H15, with 19 × 4 + 15 × 1 = 91 valence electrons, an odd number. The dimer C38H30 has 182, an even number. An odd count means at least one electron must be unpaired.
What mistakes do students make with free radicals?
Confusing radicals with ions. A radical is neutral with an unpaired electron; a carbocation is positive with an empty orbital.
Thinking all radicals are fleeting. Most are, but triphenylmethyl survived for weeks in the absence of air.
Mixing up homolytic and heterolytic cleavage. Homolysis gives one electron to each fragment, making radicals; heterolysis gives both to one fragment, making ions.
Forgetting oxygen. Many radicals react fast with oxygen, which is why Gomberg had to exclude air.
For how radicals compare with carbocations and carbanions, see our post on reaction intermediates. If you get stuck on a mechanism step, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
Why did the idea take time to be accepted?
The ACS page says the existence of triphenylmethyl remained disputed for nearly a decade, and free radicals did not enter mainstream organic chemistry until the 1930s. Part of the reason, the memoir explains, is that triphenylmethyl in benzene is mostly paired up as a dimer. Today free radicals appear in NCERT's Class 11 Hydrocarbons unit, in the chain mechanism for chlorinating methane. The parent hydrocarbon, triphenylmethane, can be made from benzene and chloroform by the method in our post on Friedel and Crafts.
What can a student learn from how Gomberg worked?
When his analysis came out wrong, Gomberg did not round it away. He changed one thing at a time (the metal, then the air) until the discrepancy had a cause. Treat a "wrong" answer in a numerical problem the same way: find exactly how big the gap is and what could account for it.
Calculate the percentage of carbon in triphenylmethyl chloride, C19H15Cl, using Cl = 35.5, and compare it with the radical.
