NATURE 
[SEPTEMBER 18, 1913 
take place, although susceptible of another explana- | Finally, he draws the conclusion that ‘excluding 
tion, 
So far as I can form a judgment, no case has been 
made out for the view that substitution of halogen 
for hydrogen under Volhard’s conditions differs in its 
“mechanism ” from substitution in the paraffins. This 
opinion finds support in the discovery just announced 
by Leuchs*® that, while the chief product of the 
bromination of dextro-8-carboxybenzyl-a-hydrindone 
Vaal 
CoH we DCH.CH,. CoH, CO,H > 
ye Ha 
Chie >CBr.CH5.C,H,:COse 
SEO” 
is the racemic compound, no less than 10 per cent. is 
the dextro-bromo-derivative; therefore, the inference 
is clear that in the formation of the latter compound, 
if not of both, substitution was effected by a process in 
which migration of the hydrogen atom did not occur. 
Attention may now be directed to the question of 
“direct substitution,” which, in its simplest form, is 
encountered in the paraffin series. As will be gathered 
from the following selection from among the various 
theories propounded to account for the mechanism of 
substitution, alternative explanations of the inter- 
mediate reactions leading up to substitution in these 
cases involve either elimination of the hydrogen atom 
before introduction of the halogen, or addition of the 
halogen in virtue of the supposed residual valency of 
both molecules, followed by disruption of the complex 
thus formed into the known products of the change. 
Dealing with these alternatives in the order given, 
Arrhenius adopts a view.of the process of substitution 
which, including as it does his explanation of optical 
inversion and racemisation, should perhaps be given 
in his own words :— 
“Every valency linking can be broken; this is true 
in all cases, since it is a necessary condition for every 
chemical reaction. An atom or an atomic complex 
is thereby removed from the molecule, and its place 
taken by another atom or atomic complex. One must 
therefore assume, as was first pointed out by William- 
son, that the atoms or complexes separate themselves 
from the molecule from time to time, even when they 
do not react with other molecules. Consider now a 
molecule in which four different atoms, A, B, C, and 
D, are bound to one carbon atom. The atoms A and 
B, which may possess equal charges, e.g. positive, 
are therefore separated at times from the molecule, 
and it may happen that they are both separated at one 
and the same time. It is therefore possible for them 
to change places on combining with the carbon atom 
again. This is synonymous with a transformation of 
the original molecules into its optical isomer.” *! 
Nef, making use of ‘‘the conception of dissociation 
in its broadest sense,”’ is of opinion that the decom- 
position of ethane into hydrogen and ethylene at 800° 
“proves that an extremely small per cent. of [its] 
molecules must exist at ordinary temperature in an 
active or dissociated condition, 
CH,CH; — CH;.CH,+H-” 
consequently, when “chlorine reacts with ethane to 
give the monochloro-substitution product, we have this 
reagent in the active molecular condition simply unit- 
ing by addition with the dissociated ethane particles, 
Cl=C€l 
Oe -GH, 1 
20 H. Leuchs, Ber., 1913, xlvi, 2435. 
*1 S, Arrhenius, ‘‘ Theories of Chemistry,” 
(Longmans, 1907), p. 76. 
22 J. U. Nef, “The Fundamental Conceptions underlying the Chemistry 
of the Carbon Atom,” J. Amer. Chem. Soc., 1904, xxvi, 1566. 
NO. 2290; VOL. 92] 
[> HCI+C,H,Cl.]” 
oll, 
edited by T. Slater Price 
reactions called ionic, a chemical reaction between 
two substances always first takes place by their union 
to form an additive compound.» 
Michael,?* in many published papers, has emphasised 
the view that in the substitution. of halogen for 
hydrogen in a saturated hydrocarbon or saturated acid 
the principal factors to be taken into account are the 
mutual chemical attraction of the two elements, on 
one hand, and that of the halogen and carbon, on 
the other. By applying his ‘ positive-negative "’ hypo- 
thesis to the directing influence of ‘‘relatively- 
positive’ methyl, and ‘relatively-negative ’’ carboxyl, 
he draws conclusions about the degree of firmness or 
looseness with which particular hydrogen atoms are 
bound to carbon in the molecule, and is thereby able 
to forecast with some success the position or positions 
in which replacement of hydrogen by halogen will 
occur. Fliirscheim, in the discussion of the relation 
between the strength of acids and bases, and the 
quantitative distribution of affinity in the molecule, 
also makes use of the idea that the relative degree 
of firmness or looseness with which a hydrogen atom 
is held depends on the nature of the other atoms or 
radicals associated with the same carbon atom.** The 
hydrogen atoms therefore are not to be regarded as 
retained in the molecule with the same degree of 
firmness; in other words, valency is not a constant 
to be measured in units. 
It will be gathered therefore that Arrhenius and 
Nef, from different points of view, support the idea 
that separation of hydrogen from the hydrocarbon 
precedes entry of the substituent into the molecule; 
Michael and Fliirscheim are concerned chiefly with 
the distribution of valency in the molecule, which 
determines whether a particular hydrogen atom shall 
be displaced by hydrogen or not; Kekulé’s hypothesis 
requires addition to precede substitution. Is there 
any experimental evidence to indicate where the 
balance of probability lies? I think it can be argued 
that the phenomena of substitution observed with 
optically active substances do: not lend support to the 
views of Arrhenius or of Nef, which imply actual or 
virtual dissociation, but that they point to the inter- 
mediate formation of an additive product, which 
undergoes scission as Kekulé supposed. 
additive product can be formed only if residual valen- 
cies be present in both carbon compound and adden- 
dum. 
The argument runs thus: Unless valency has fixed 
direction in space, a conception now abandoned if 
modern theories of valency be accepted, the conclu- 
sion seems to be inevitable that dissociation of the 
optically active compound :— 
CWXYZ into CWXY+Z, 
must lead to racemisation, the radicals W, X, Y, dis- 
tributing themselves in two-dimensional space, thus 
destroying the asymmetry; whence it follows that 
introduction of the substituent, V, into the molecule 
in place of Z can give rise only to an optically inactive 
product. Now, it is a well-established fact that a 
radical attached directly to the asymmetric carbon 
atom may be replaced by another without racemisa- 
tion following.?® Therefore, preliminary dissociation 
being excluded, Kekulé’s additive hypothesis remains. 
But the prolonged study of that remarkable reaction 
known as the ‘‘ Walden inversion”? by Emil Fischer, 
McKenzie, and other investigators, has led to results” 
23 A. Michael, Ber., 1901, xxxiv, 4028, covering reference to earlier 
papers. 
24 B. Fliirscheim, Trans. Chem. Scc., 1909, xcv, 721. 
=5 P. Walden, Fer., 1895, xxviii, 1297; W. Tilden and B. M. C. 
Marshall, Trans. Chem. Soc., 1895, Ixvii, 494. 
Such an- 
