CHEMISTRY: G. N. LEWIS 
587 
the factors that determine the speed of a reaction and those that deter- 
mine the conditions of final equihbrium. In order to avoid such con- 
fusion it is necessary to distinguish between two possible types of steric 
influence : 
1. A group of atoms, owing to its size and to its position in a mole- 
cule, may stand in the way so as to impede a reaction in which that 
molecule is involved. In other words, it may act merely to diminish 
the speed of such a reaction. 
2. The space about a given atom may be so completely occupied by 
certain large groups of atoms, before the natural valence of the central 
atom is satisfied, that the entrance of another large group will be pre- 
vented; or if such a group occasionally enter, it will again be forced out 
through lack of room. 
I think it will be admitted that these ideas, plausible as they are, 
should not be introduced into the already very complex body of chemi- 
cal theory until phenomena are known which cannot be so well ex- 
plained in other ways. In this paper I wish to discuss the so-called 
free radicals of organic chemistry, in the interpretation of which the 
term ^steric hindrance' has so often been employed, and to show that, 
in this case at least, another explanation can be offered which is more 
satisfactory in that it correlates a larger number of known phenomena. 
After Gomberg, by treating triphenylmethyl chloride with metals, 
obtained a substance which he believed to contain triphenylmethyl, 
thus giving rise to one of the most interesting chapters of modern or- 
ganic chemistry, it was at first commonly believed that by some other 
method a hexaphenylethane might be produced which would be stable 
and unreactive, and which would be formed from triphenylmethyl ex- 
cept for the retardation of this reaction by the first type of steric hin- 
drance which I have just defined. We are now convinced that this is 
not the case. The later work of numerous investigators, which has 
been very completely reviewed by Gomberg^ in his interesting summary, 
^The Existence of Free Radicals,' has satisfactorily cleared an extremely 
intricate situation. A careful study of the work of these authors must, 
I believe, lead to the acceptance of the following perfectly definite con- 
clusions concerning the class of hexa-arylethanes, of which hexaphenyl- 
ethane is the simplest example. 
I. The hexa-arylethanes, aside from the irreversible processes of in- 
tramolecular rearrangement and autoxidation to which they are sub- 
ject and which we shall not further consider, exist in a state of tauto- 
meric equilibrium between quinoid forms and the benzoid form which 
represents the simple substituted ethane. As in most similar cases the 
