CHEMISTRY: G. N. LEWIS 
591 
:C1 : CI: = :C1- + • CI: , H: 6 : 6 : H = H: 6. + .0:H, 
HH 
H:N:N:H = H:N- + •N:H, H:C:C:H = H: C* + 'C: H' 
H H H H 
The symbols in heavy type represent the atomic kernels, while the 
dots represent the electrons of the outer shells. It is the odd electrons 
that gives the free radicals color and high reactivity. It will be under- 
stood that the particular substances used for illustration are dissoci- 
ated only to an extremely small degree; only upon substitution of 
heavier radicals will the dissociation be appreciable at ordinary tem- 
peratures. 
In emphasizing the effect of mass and thus of centrifugal force in 
breaking a chemical bond we must not forget, however, that this is 
only one of the two main factors that determine the stability of a 
molecule. The other factor is the strength of the constraints which con- 
stitute the bond itself. (Electrochemical influences which are so fre- 
quently important do not concern us here since we are considering a 
bond symmetrically placed between two identical radicals.) I have 
pointed out, in the paper already cited, that the bond between two car- 
bon atoms is weakened when either atom has a double bond or is at- 
tached directly to an atom with a double bond. Thus the great change 
produced by the substitution of phenyl for hydrogen or alkyl is due to 
both increasing mass and a weakened bond. If the phenyl groups in 
hexaphenylethane are replaced by the somewhat heavier benzyl groups 
the dissociation will nevertheless be diminished, since the benzyl will 
not aid in weakening the central bond. On the other hand there are 
radicals which cause a still greater weakening of the central bond than 
the phenyl group. Thus Gomberg has shown that the linking of two 
of the phenyl groups of triphenylmethyl by an atom of oxygen in the 
ortho positions greatly increases the percentage of free radical. 
The odd molecules produced by the thermal dissociation of the 
substances here considered may combine not only with their like but 
with other free radicals to produce mixed types. Thus the reaction 
(Ar)2N-N(Ar)2 + (Ar)3C-C(Ar)3 = 2(Ar)3C-N(Ar)2 is analogous to the 
reaction (Ar)3C-C(Ar)3+l2 = 2(Ar)3C-I. Wieland^ has found a very 
interesting reversible reaction (C6H5)3C-C(C6H5)3 + (C6H5)2N-N(C6 
H5)2 = 2(C6H5)3C-N(C6H5)2. 
