LACTIC ACID, a chemical term, which, though originally invented to designate the particular acid contained in sour milk, has now, through the discovery of other acids isomeric with and very similar to that acid, acquired a generic, in addition to its original specific, meaning.
1. Lactic Acid properly so called, Fermentation Lactic Acid, Ethylidene Lactic Acid.— Scheele (Transactions of Stockholm Acad., 1780) was the first to isolate this acid (from sour milk) and establish its individuality. About twenty-four years later Bouillon Lagrange, and, independently of him, also Fourcroy and Vauquelin maintained that Scheele’s new acid was nothing but impure acetic. But this notion was combated by Berzelius, and finally refuted (in 1832) by Liebig and Mitscherlich, who by the elementary analyses of lactates proved the existence of this as a distinct acid....
In the article Fermentation (vol. ix. p. 97) it is explained how lactic acid is produced from milk-sugar and from ordinary glucose by “lactic fermentation.” The most convenient process for the preparation of the acid is Bensch’s. A solution of “invert-sugar” (see vol. ix. p. 96) is produced by dissolving 6 parts of cane-sugar and τ ⅛th part of tartaric acid in 35 parts of boiling water and allowing to stand for two days. There is then added τ 1 gth part of. foul cheese, 8 parts of sour milk, and 2⅜ parts of carbonate of zinc, and the mixture kept at 40° to 45° C. for eight to ten days. The sugar, C 6 H 12 O 6 , ferments into lactic acid, C 3 H 6 O 3 , which, by the carbonate present, is converted into lactate of zinc. But part of the lactate is invariably lost through “butyric fermentation” with evolution of hydrogen, which latter converts part of the sugar into mannite, C 6 H 14 O 6 . The fermented liquid is heated to boiling, strained clear, and allowed to cool, when the lactate of zinc separates out in crystalline crusts, which are purified by recrystallization from hot water. The free acid is obtained by decomposing the hot aqueous solution of the salt with sulphuretted hydrogen and filtering off the sulphide of zinc. The filtrate is evaporated on a water bath to a syrup, which is treated with ether. Mannite and other impurities remain, and the acid passes into the filtrate, from which the ether is easily expelled by distillation,and subsequent evaporation in an open basin. What ultimately remains is a thick colourless syrup, which, in ordinary chemical parlance, goes as lactic acid, although it is at best only an approximation to the hydrate, C 3 H β O 3 .H 2 O. The extra H a O is easily enough removed by continued evaporation, but no portion of it can be thus got rid of without the acid C 3 H 6 O 3 itself suffering dehydration into lactic anhydride, C 6 H 10 O 5 , and lactide, C 6 H 8 O 4 , by successive subtractions of H a O from 2C 3 H 6 O 3 .
The behaviour of lactic acid solution to basic reagents and of the acid itself to alcohols (in the presence of dehydrators) is strictly that of a raonobasic acid C 3 H 6 O 3 ; i.e., so much lactic is strictly equivalent to one molecule of acetic acid, and as the latter is proved to be CH 3 .C00H, lactic acid must be assumed to be (C 2 H 5 O) CO OH. But the radicle C 2 H 5 O (unlike the CH 3 of acetic acid) still contains one hydrogen atom, which, although not replaceable by metals, can be replaced by acid radicles such as acetyl C 2 H 3 O or, conjointly with the oxygen atom, by Cl,Br,L Thus, for instance, lactic ether, (C 2 H 5 O)COOC 2 H 5 , when treated with chloride of acetyl, C 2 H 3 O.C1, gets converted into acetyl-lactic ether,[C 2 H 4 (C 2 H 3 O)O]COO(C 2 H 5 ), with formation of hydrochloric acid. By the action of hydriodic acid the same II conjointly with the O of the radicle is replaced by iodine with formation of water. In a word, lactic acid, besides being an acid analogous to, for instance, acetic acid, CH 3 COOH, is at the same time an alcohol analogous to ordinary spirit of wine, C 2 H 5 .OH, as shown by the formula HO—C 2 H 4 —COOH. This two fold character of our substance explains the readiness with which it passes into anhydrides. Lactic acid the acid acts upon lactic acid the alcohol; the replaceable H in the former unites with the OH of the latter, and the two rests combine into an ether which is lactic anhydride. Thus :— (C 2 H 4 0H)-COO H 1
(C 2 H 4 COOH)-"" oh---
The slanting lines show the mode of combination after the reaction. The anhydride, as we see, still contains an OH and COOH, and a repetition of the group within its molecule leads to lactide, C 6 H 8 O 4 . This latter body could be presumed to be formed from one molecule of lactic acid:
H0(C 2 H 4 )C00H = (C 2 H 4 )C00 + H,0H; but the vapour density determination proves the molecular weight to be in accordance with the larger formula C 6 H 8 O 4 .
Admitting, as well we may, that lactic acid is a compound of COOH and OH with C 2 H 4 , what is this C 2 H 4 itself? This question has been satisfactorily answered. When lactic acid is distilled rapidly, it breaks up into formic acid, H.C00H, and aldehyde, (CH 3 )(CO)H. Conversely when aldehyde is treated with hydrocyanic acid and muriatic acid it is converted into lactic acid, in two steps, thus: (1) the C = O in the aldehyde combines with the (NC)H, the H going to the 0 and the (NC) by its C to the C, to form the group (NC)—C—0—H; and (2) the NC of this group, by the action of the water, gets converted into a COOH, with formation of NH 3 , which combines with the muriatic acid. NC + 2H 2 O = NH 3 + C00H. The (CH s ) and H in the original aldehyde retain their places, so that what we obtain ultimately must be
(CH 3 )- (CH)<qøq H and this consequently is the structure of lactic acid. The radicle (CH 3 )(CH) is called ethylidene, to distinguish it from “ethylene” which is (CH 2 )(CH 2 ).
2. Paralactic or Sarcοlactic Acid.— This acid was discovered by Berzelius in the juices of flesh. It is almost identical with ordinary lactic, but differs from it in this that it (and its salts) turn the plane of polarized light, and also in this that the sarcolactates in general are more readily soluble than ordinary lactates, and contain different proportions of crystal water from these. Thus, for instance, we have for the zinc salts
Ordinary. Sarco-.
Zn(C 3 H 5 O 3 ) 2 · 3H 2 O. Zn(C 3 H 3 O 3 ). 2H 2 O.
Soluble in 60 parts of cold Soluble in 17 parts of cold and in 6 parts of boiling water. water.
The isomerism of the two acids used to be explained by assuming that the sarco-acid contained ethylene in lieu of the ethylidene of the ordinary acid, thus:
(OH)—(CH s )-(CH 1 )-C00H, i,e., that the OH and COOH were attached to different carbon atoms. But this has been proved by Erlenmeyer to be a mistake. The sarco-acid has precisely the same structure as ordinary lactic acid. It is a case of absolute (i.e., of unexplained) isomerism.
3. Hydracrylic Acid.— From glyceric acid by the action of hydriodic acid we obtain ∕3-iodopropionic, which, when treated with water and oxide of silver, exchanges its iodine for OH:
I. H 2 C-CH 2 -COOH fr ivGs
(OH). H a C-CH 2 -COOH, which is hydracrylic, an ethylene-lactic acid. That this really is so was proved by Erlenmeyer, who obtained it by the action of water ( + HC1) on undoubted ethylenc-eyanhydrine (OH)—(C 2 H 4 )—(CN). As suggested by the formula, it differs markedly in its reactions from the two more properly so-called lactic acids.
All lactic acids, when heated with hydriodic acid in sealed-up tubes, pass ultimately into (the same) propionic acid, (CH 3 )-(CH 2 )-COOH. (w. D.)
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