The preceding paper of this series on “Promotion and Arrest of Mutarotation of Tetra-acetylglucose in Ethyl Acetate” described the most successful experiments that have yet been made with a view to controlling the facile isomeric changes which depend on the migration of a hydrogen atom with a concurrent re-arrangement of bonds in the molecule of an unsaturated (or cyclic) organic compound. These “desmotropic” or “prototropic" changes proceed normally only in presence of an amphoteric solvent, such as water or a mixture of pyridine and cresol, which is able simultaneously to remove a proton from one position in the molecule, and to supply a proton in the alternative position. The re-arrangement of single and double bonds, which accompanies the migration of the hydrogen atom, provides a conducting mechanism whereby an electron can be transferred through the intermediate conjugated system from one point of attachment of the mobile hydrogen atom to the other. This electronic migration makes it possible to neutralise the negative charge produced by the removal of a proton from the one point of attachment, together with the positive charge produced by the addition of a proton to the second point of attachment, so that the mobile hydrogen atom need not carry its electron with it, but can migrate in the form of an ion. The mechanism of the prototropic change can then be represented as a sort of internal electrolysis in which the acid and base act as negative and positive poles, whilst the conjugated double and single bonds of the organic molecule take the place of the electrolyte. Thus, if we represent the acid and base as HA and B, and the two forms of the prototropic compound as HS and SH, the reversible isomeric change can be represented by the following scheme: B + HS + HA ⇌
+
BH + SH + A̅. In this scheme the arrows show the direction in which the electron migrates
through the molecule
as a result of the bond-shifting, whilst the proton migrates in the same direction
through the medium
. Apart from the prototropic change, the action formulated above is a simple reversible neutralisation of an acid and base, which, if the prototropic compounds were eliminated, could be represented by the equation B + HA ⇌
+
BH + A̅,
e. g.
, NH
3
+ HAc ⇌
+
NH + A̅c.
The mechanism of chemical change. Part II.—Catalysis of the mutarotation of beryllium benzoylcamphor