| V. | |||
|---|---|---|---|
| C | H3 | ||
| │ | |||
| C | H2 | ||
| ╱ | |||
| O | |||
| ╲ | |||
| C | O | ||
| ╱ | |||
| O | |||
| ╲ | |||
| C | O | ||
| ╱ | |||
| O | |||
| ╲ | |||
| C | H2 | ||
| │ | |||
| C | H3 | ||
The immediate precursor of alcohol and carbon dioxide is here seen to be the anhydride of ethoxycarboxylic acid (V), whilst that of lactic acid is lactic anhydride (IV). (Baeyer does not appear, as recently stated by Meisenheimer [[1907], p. 8], Wohl [[1907, 2]], and Buchner and Meisenheimer [[1909]] to have suggested that lactic acid was an intermediate product in alcoholic fermentation, but rather to have represented independently the course of the two different kinds of fermentation, the alcoholic and the lactic.)
It was subsequently pointed out by Buchner and Meisenheimer [[1904]] that Baeyer's principle of oxygen accumulation might be applied in a different way, so that a ketonic acid would be produced, the decomposition of which, in a manner analogous to that of acetoacetic acid, would lead to the formation of two molecules of lactic acid, from which the final products alcohol and carbon dioxide might be directly derived, as shown in the following formulæ:—
| C | HO |
| · | |
| C | H(OH) |
| · | |
| C | H(OH) |
| · | |
| C | H(OH) |
| · | |
| C | H(OH) |
| · | |
| C | H2(OH) |
| C | OOH |
| · | |
| C | H(OH) |
| · | |
| C | H2 |
| · | |
| C | O |
| · | |
| C | H(OH) |
| · | |
| C | H3 |
| C | OOH |
| · | |
| C | H(OH) |
| · | |
| C | H3 |
| ─ | ─── |
| C | OOH |
| · | |
| C | H(OH) |
| · | |
| C | H3 |
| C | O2 |
| ─ | ─── |
| C | H2·OH |
| · | |
| C | H3 |
| ─ | ─── |
| C | O2 |
| ─ | ─── |
| C | H2·OH |
| · | |
| C | H3 |
A scheme based on somewhat different principles has been propounded by Wohl [Lippmann, [1904], p. 1891], and has been accepted by Buchner and Meisenheimer [[1905]] as more probable than that quoted above. Wohl and Oesterlin [[1901]] were able to trace experimentally the various stages of the conversion of tartaric acid (I) into oxalacetic acid (III), which can be carried out by reactions taking place at the ordinary temperature, and they found that the first stage consisted in the removal of the elements of water leaving an unsaturated hydroxy derivative (II) which in the second stage underwent intramolecular change into the corresponding keto-compound (III): [p101]
| C | OOH | C | OOH | C | OOH | |||||
| · | · | · | ||||||||
| C | H(OH) | H | C | (OH) | C | O | ||||
| · | − | · | = | ║ | ⇌ | · | ||||
| C | H(OH) | OH | C | H | C | H3 | ||||
| · | · | · | ||||||||
| C | OOH | C | OOH | C | OOH | |||||
| I. Tartaric acid | II. | III. Oxalacetic acid. | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
This change differs in principle from that assumed by Baeyer, inasmuch as the second stage is not effected by the re-addition of water, but by the keto-enol transformation, which is now usually ascribed to the migration of the hydrogen atom, although the same result can theoretically be arrived at by the addition and removal of the elements of water. The analogy of this process to what might be supposed to occur in the conversion of sugar into carbon dioxide and alcohol was pointed out by Wohl and Oesterlin, and subsequently Wohl developed a theoretical scheme of reactions by which the process of alcoholic fermentation could be represented. In the first place the elements of water are removed from the α and β carbon atoms of glucose (I) and the resulting enol (II) undergoes conversion into the corresponding ketone (III), which has the constitution of a condensation product of methylglyoxal and glyceraldehyde, and hence is readily resolved by hydrolysis into these compounds (IV). The glyceraldehyde passes by a similar series of changes (V, VI) into methylglyoxal, and this is then converted by addition of water into lactic acid (VII), a reaction which is common to all ketoaldehydes of this kind. Finally, the lactic acid is split up into alcohol and carbon dioxide (VIII):—