Triarylmethyl and Related Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 3

either because both Gomberg and Schlenk were not nominated in the same year, or because too much
time had elapsed since the initial discovery. Even in 1940 Gomberg was still being nominated, but without
success. Gomberg's discovery was a clearly momentous discovery by a single individual, and although
well recognized he did not receive the ultimate accolade he deserved. Paradoxically, Gerhard Herzberg
was awarded the 1971 prize in chemistry “for his contributions to the knowledge of electronic structure
and geometry of molecules, particularly free radicals”, studies which had occupied him for 30 years.
By 1968 the study of stable free radicals had advanced to the stage that the topic Organic Chemistry
of Stable Free Radicals appeared, 4 but in Chapter 2 entitled “Triarylmethyls and Other Carbon Radicals”
the introductory paragraph describing the discovery of triphenylmethyl in 1900 ended with the dispiriting
words “The behaviour of such radicals was elucidated during the following twenty years, mainly by
the work of Gomberg, Schlenk and Wieland, since then little new chemistry has come to light although
numerous triarylmethyls have been prepared and more physical data are available.” However six pages
later, under “Dimerisation” there was the alarming statement “Much of what has been said in this section
may require revision in light of the recent communication by Lankamp, Nauta and MacLean”, which
described the surprising but in retrospect completely predictable finding 3d,e that the triphenylmethyl dimer
had the head-to-tail structure 5 . The rapid development in triarylmethyl radical chemistry since 1968
also belies the tacit assumption of the authors noted above that such studies had become an intellectual
backwater. Much of the rather extensive chemistry of triarylmethyl radicals described in this earlier review
is not repeated here. The period from 1968 has been a new golden age for free radical chemistry, and this
was given great impetus by the widespread use of electron paramagnetic resonance (EPR) spectroscopy,
which led to a rapid development of free radical chemistry, and includes many advances in the study of
triarylmethyl radicals.

1.1.2 Bis(triphenylmethyl) peroxide
The reaction of the triphenylmethyl radical with oxygen to form the peroxide discovered by Gomberg in
1900 (Equation 1.1) was a strong piece of evidence for the radical structure 1 . 5,6 The affinity of carbon-
centered radicals for oxygen remains one of their defining characteristics, and was a striking chemical
property that provided strong evidence for the proposed free radical character. The addition of oxygen to
1 had been shown to discharge the color, 5a and this formation of an initial peroxy radical was later shown

to be reversible. 5b
Just as for the triphenylmethyl radical dimer, there were three conceivable structures for the peroxide
corresponding to the dimer structural models, namely head-to-head ( 2 ), head-to-tail ( 7 ), and tail-to-tail ( 8 ).
The head-to-head structure 2 proved to be correct, but substituted derivatives of 7 and 8 were later found. 6

H

O

O

O

O

H

H

8

7

The tris(2,6-dimethoxyphenyl)methyl radical 9 was crystallized and found to have a structure with two
of the aryl rings twisted out the plane of the central carbon by 61 ◦ , while the third was almost coplanar
with an 11 ◦ twist angle. 6

Reaction of 9 , and of the bis(2,6-dimethoxyphenyl)phenylmethyl radical 10 ,