An atom or group of atoms with one or more unshared electrons, which may enter into chemical-bond formation is callcd a free radical. (The same group in a molcculc is called a radical; for example the methyl radical in methyl cyanide or other molcculcs.) Free radicals are usually highly rcactivc and difficult to prepare in any except low concentration.
One way of making the methyl radical as a dilute gas is by heating mercury dimethyl, Hg(CH3):, which decomposes to give metallic mercury and methyl radical. Methyl radical can also be made conveniently by the decomposition of diacctyl, (CH3CO:):> by cither heat or ultraviolet light. The diacctyl molcculc liberates two molecules of carbon dioxide and two methyl radicals.
The American chcmist Moses Gombcrg (1866-1947) discovered in 1900 that some hydrocarbon free radicals are stable. He attempted to synthesize the substancc hcxaphcnylcthanc, (C6Hj)3C—C(C6H5)j, which he expected to be a stable, white ciystalline substancc. Instead, he obtained a strongly coloured solution, with the property of combining readily with oxygen. He concluded correctly that the solution did not contain the hexaphenyl derivative of ethane, but instead, the free radical tripheny- lmcthyl, with the formula (Ñ6Í5)çÑ. Many similar hydrocarbon free radicals have been made, and it has been shown that they arc paramagnetic, and accordingly contain unpaired electrons (the paramagnetism is due to the magnetic moment of the electron spin of the unpaired electron). The stability of the triphcnylmcthyl radical, which is responsible for the low bond energy of the carbon-carbon bond in the substituted ethane, is attributed to the resonance energy of the unpaired electron among the various carbon atoms of the molcculc.