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PRESIDENTIAL ADDRESS. 41] 
tution and physical properties, to the elucidation of which an increasing amount 
of research is being directed. The double linkings in our formule no longer 
represent two units of valency in terms of hydrogen, nor are they now used to 
indicate polarity of the central atom or distribution of the valency in space; 
Werner’s conception of valency accounts, as the phrase goes, for the concen- 
tration of re-activity at that part of the molecule where unsaturation exists, and 
it is of service when different degrees of unsaturatedness are displayed by 
compounds which, on the older view, would be expected to show similarity in 
chemical behaviour.* With your permission I propose briefly to review our 
knowledge of that type of chemical change known as substitution from the 
standpoint of residual valency. 
' Substitution in the Paraffin Series. 
So far back as 1839 the fact was discovered that replacement of hydro- 
gen by chlorine in the acetic acid molecule does not lead to any essential 
modification in the properties of the acid. It is not a little remarkable, there- 
fore, that although much of the progress in organic chemistry has been achieved 
by substitutions of the most diverse types, we are still unable to say that 
agreement has been reached with regard to the nature of the processes by which 
this replacement of one radical by another in a molecule is brought about. 
Never has attention been concentrated more closely than now on the study of 
what, for want of a better phrase, is termed the ‘mechanism of chemical 
reactions "—the processes which are covered and hidden by the sign of equality 
used, inaptly, in chemical equations—but the integrating mind, to the need for 
which Professor Frankland alluded on a recent occasion,® has not yet been 
evolved to reconcile the uncertain or contradictory answers vouchsafed to much 
patient experimenting. Organic chemistry is not singular in this respect: as 
much might be said about controversies not yet settled which concern them- 
selves with such every-day phenomena as the chemistry of the candle-flame 
or of the rusting of iron. 
It is a commonplace that Kekulé, to whom theoretical chemistry owes so many 
fruitful suggestions, was of the opinion that substitution is not a process in 
which what may be called a direct exchange of radicals occurs, but is preceded 
by the temporary union of the molecules of carbon compound and addendum, 
followed by disruption into two new molecules, the substituted carbon compound 
being one of them. It is clear, then, from the standpoint of Kekulé’s hypothesis, 
that some degree of unsaturation is to be looked for in all carbon compounds and 
in all addenda. Hence, the paraffin hydrocarbons which furnish derivatives 
only by substitution, and, under the older, more rigid view of valency pro- 
pounded by Kekulé himself, are typically saturated compounds, supply the 
exceptions to prove the general validity of the hypothesis that addition precedes 
substitution. 
Before examining the case of these hydrocarbons, however, some advantage 
may be gained if the behaviour of other groups of compounds be examined in 
the light of the idea underlying Kekulé’s view. By reference to the literature, 
it is evident that since the beginning of this century attention has been con- 
centrated on the phenomena of substitution in the important group of carbonyl 
compounds, particularly the ketones and acids, which in many cases yield 
halogen substitution derivatives of one type. Thus methyl ethyl ketone when 
brominated in sunlight yields two bromoketobutanes of the constitution shown in 
* As an example of the unsatisfactory character of the doubly linked 
formula to which the older meaning was attached, the following may be quoted : 
unsym.-Diphenyldichloroethylene, like ethylene, combines molecularly with brom- 
ine, but tetraphenylethylene does not ; 
CH, 0 (C H,)s C(CoH)s 
CH, CCl C(C, Hs). 
yet a similar structure has been assigned to each (Biltz, Annalen, 1897, 296, 219). 
® Pp. I. Frankland, Proc. Chem. Soc., 1913, 29, 101. 
2 
