The content of this article represents the completion of the work he did earlier on optical conversion.
An optical antipode could only be obtained if the recemic form of the substance was obtained first. Once the racemic form was available, one intervened according to the three classical modalities already identified by Pasteur to isolate one or both enantiomers. Walden discovers the possibility of switching from an optical antipode to its opposite in a direct way. First of all, he recalls the results obtained in previous works
In my last communication I had shown that it is possible, starting from asparagine, to obtain an l-bromosuccinic acid through the direct action of bromine and nitric oxide, while – after conversion to malic acid – by means of phosphorus pentabromide to obtain a d-bromosuccinic acid, that is, to reproduce from the same compound now one, now the other optical antipode. [ see blog article First Part]
He perfects this discovery starting not from asparagine, which as he claims may appear complex, but from aspartic acid. He confirms the results he had obtained with asparagine. He emphasizes that the results are obtained using low temperatures
In order to eliminate and limit as much as possible all violent interventions in the positions of the atoms, as well as all changes in their reciprocal arrangements, all reactions were carried out at low temperatures, on the other hand it was precisely the work at higher temperatures that had provided my predecessors with only racemic forms.
At this point he poses the crucial problem
Now, since in spite of everything both optical antipodes were obtained from the same active starting material, the question had to be raised: do the two right and left isomers of halogenosuccinic acid really correspond to a single original malic acid or, if not, from which malic acid do the right halogenosuccinic acid derive and from which the left halogenosuccinic acid?
And he adds
To resolve these issues, it was necessary to complete the conversion of brominated or chlorinated succinic acid obtained into malic acid.
The replacement of the chlorine atom with the hydroxyl group is more complex than Walden imagined. He describes the difficulties encountered and the successful attempt to overcome them, showing us how great his obstinacy and experimental ability was. It uses silver oxide prepared in situ from silver nitrate and potassium carbonate. From the optical antipode l-chlorosuccinic acid obtains l-maleic acid, i.e. the same natural optical antipode. By analogy, he also extends the same result to l-bromosuccinic acid. He then moved on to the conversion of d-chlorosuccinic acid into malic acid, which he had obtained from l-malic acid by chlorination using phosphorus pentachloride [see blog article First Part]. This time it gets d-malic acid instead.
Let us now consider that our right malic acid comes from right chlorosuccinic acid, which in turn was obtained (via PCl5) from left malic acid; As a result, with the help of phosphorus pentachloride (as well as phosphorus bromide), a transformation of the left isomer into its right antipode has been carried out.
The transformation of l-malic acid (levorotary) into d-malic acid (dextrorotary) suggests the reverse path to Walden. Starting this time from d-malic acid, obtained as before, it obtains the formation of l-chlorosuccinic acid.
Since now, as demonstrated above, l-chlorosuccinic acid corresponds to l-malic acid, the transformation of d-malic acid into the corresponding l-malic acid has also taken place here.
Walden summarizes the overall results of his work
We therefore have before us the following cyclic process: starting from l-malic acid we obtain (through PCl5) a chlorosuccinic acid which is dextrorotary; by replacing chlorine with a hydroxyl group in this right-hand acid, we obtain a malic acid which is also rotating to the right and represents the optical antipode of the starting material; then treating this d-malic acid again with phosphorus pentachloride, a left-rotating chlorosuccinic acid is obtained, the optical antipode of the acid mentioned above; In turn, this left chlorosuccinic acid (replacing the halogen with a hydroxyl group) produces a left-rotating malic acid, i.e. it converts back into the original material used.
It then opens up to further investigations
This cyclic process provides us with the method mentioned at the beginning for the direct conversion (without prior racemization) of an optical isomer into its antipode, when the active asymmetric carbon atom is directly bonded to a starch or hydroxyl group or to a halogen; the converting agent used is phosphorus pentachloride (and pentabromide); It does not seem unlikely that other agents may also exert a similar peculiar effect.
Graphical representation of the Walden cycle malic acid/ chlorosuccinic acid
Another cycle discovered in 1897
Walden would report in a later article in Diese Berichte in 1897 another result that would confirm the direct conversion between two enatiomers. He discovered a cycle of transformations between the enantiomers of malic acid and the enantiomers of chlorine or bromine succinei acids, in which this time the action of a potassium hydroxide solution caused the inversion of the enantiomers. Recall that in the previous article he had used pentachloride or phosphorus bromide as transforming agents. At the end of his article he summarizes the results obtained:
The final synthesis shows the following: 1. in chemical terms, the effect of silver oxide (silver carbonate) is identical to that of potassium hydrate, in both cases there is a substitution of halogen by hydroxyl [in halogen acid succine]; however, 2. This effect is just the opposite with regard to the subsequent polarimetric behavior of the resulting oxygenated acid, as 3. the direction of rotation does not seem to have changed, i.e. that of active halogenated acid, if it is hydroxylated by silver oxide, while 4. it is reversed if the introduction of hydroxyl takes place via potassium hydrate.
But much more clarifying than the path taken is a nice graph with which he concludes the exposition of the results obtained:
