38 SECTIONAL ADDRESSES 
it leaves behind a positive or negative charge. In order that this type of 
isomeric change may proceed, it is essential that these opposite charges 
should be neutralised. ‘The electronic theory of valency allows us to 
recognise that this is done by the rearrangement of bonds (or ‘ desmo- 
tropy ’) (32) which accompanies prototropic change, since a valency 
electron is thereby transferred through the interior of the molecule, to 
neutralise the charge of the proton, which is transferred through the 
amphoteric solvent. 
The migration of a hydrogen atom, to which the most fertile types of 
mutarotation are due, was thus linked up to an extended definition of 
acids and bases, which I set out in 1923 (33), at a time when it must have 
been in the minds of many other workers, and which was described more 
fully by Brénsted a few months later (34). ‘Thus, if we define an acid 
and a base as a proton donor and acceptor respectively, 
‘ i . 
B + HA=—=BH +A (where B is the base and HA the acid), 
the migration of a proton in a prototropic compound under the combined 
action‘ of a base B and an acid HA can be expressed by the equation 
+ — 
B--HS +HA= BH +SH-+A, 
used by Brénsted and Guggenheim (35), where HS and SH represent 
the two isomeric forms of the substrate. In an amphoteric medium such 
as water, catalysis by bases and acids can be represented by equations in 
which water plays the part either of an acid or of a base, thus : 
+ _— 
Catalysis by a base: B + HS + HOH=~BH + SH + OH 
+ — 
Catalysis by an acid: H,O 4+HS +HA=~H,0+SH+A. 
Finally, the possibility of autocatalysis must be recognised. Thus, since 
nitrocamphor is a strong acid, it may itself act as the acidic component of 
the catalyst ; mutarotation may than proceed by adding only a base, 
which in these conditions may become a complete catalyst for the 
mutarotation. 
The process of isomeric change, as set out above, can be regarded as 
an electrolysis of the organic molecule between positive and negative 
poles, provided by the acid and basic components of the amphoteric 
solvent. [his mechanism has therefore been described (36) as an 
* electrolytic theory of catalysis by acids and bases.’ Similar conditions, 
however, prevail in all conjugated systems, and these can now be formu- 
lated in general terms, as systems in which opposite charges at the ends 
of the system can be neutralised by a migration of valency electrons 
through the system (37). 
ROTATORY DISPERSION, 
At the time when the earlier measurements of mutarotation were made, 
it was customary to measure the optical rotations of organic compounds 
only for the yellow sodium line. Work on rotatory dispersion had indeed 
been suspended almost completely since the death of Biot in 1862, and 
