THE BORIC ACID PROBLEM* BY WILDER D. BANCROFT A S D H E R B E R T L. DAVIS
h recent investigation of the structure of malic and of tartaric acids led us to a study of the remarkable effects which boric acid exerts on the optical rotation and certain other properties of these compounds and of various other compounds containing hydroxyl groups. Some of these phenomena were studied by van’t Hoff’ who gave the formulation which is still in large use. “Kow that the increase of rotation through ring formation has been established, the very considerable rise of rotation observed on addition of boric acid is seen in another light. Such is the effect of this addition that, as is well known, it was only by this means that activity could be demonstrated in the case of mannite, sorbite, arabite, etc. If we consider now the more recent observations, especially those of Magnanini, we see in the first place that the proved diminution of the number of molecules involves the hypothesis that an addition product is formed. I n the next place, in view of the fact that only polyatomic alcohols (including erythrite) and oxy-acids are affected by boric acid, while mannite with six hydroxyl groups demands three molecules of boric acid, there must be two hydroxyl groups connected with one boric acid molecule, and we come of necessity to the hypothesis that the following ring is formed:
-c-0
nrhich is in harmony with the other properties (acid character, depression, conductivity).” The structure of tartar emetic was also heldtobeof this same type where the two hydroxyl groups could be those attached to the asymmetric atom or to one of these and one of the carboxyl hydroxyls. This is the structure which is inferred in the explanation of Holleman.2 “Addition of boric acid to an aqueous solution of pyruvic acid causes a marked rise in the value of the conductivity of the organic acid. This phenomenon is characteristic of a-hydroxy-acids, and indicates each rnolecule of pyruvic acid to be in union with one molecule of water, in accordance with the * This work is part of the programme now being carried out a t Cornell University under
a grant from the Heckscher Foundation for the Advancement of Research established by
riugust Heckscher a t Cornell University. ’ “ T h e Arrangement of Atoms in Space,” I j I (1898). * “ A Textbook of Organic Chemistry,” 275 (r92j).
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WILDER D. BASCROFT A S D HERBERT L. DAVIS
structural formula CH,.C(OH),.COOH.” Later Hollenian returns to a more general treatment of these phenomena, p. 418. “Mention has been made of the influence exerted by boric acid on the conductivity of hydroxy derivatives. The results obtained by Boeseken in his investigation of the effect of this acid on the polyhydric phenols possess a general significance. Of the three dihydroxybenzenes, catechol alone has its electric conductivity in aqueous solution greatly augmented by the addition of boric acid. With pyrogallol the effect is similar, but not with the other polyhydric phenols. X seminormal solution of boric acid was found to have a conductivity of 2 5 . 7 X IO*, that of a similar solution of catechol being 13.6 X IO-^. A solution containing both substances in seminormal concentration had the conductivity 5 5 5 . 2 X IO^, the sum of the conductivity values for boric acid and catechol separately being only ( 2 5 . 7 13.6) X IO-^ = 39.3
x
+
10-6.
“The conductivity of a seminormal solution of resorcinol was found to be x IO-^, and that of an equivalent solution of boric acid and resorcinol to be 25.0 X IO-^ instead of ( 2 5 . 7 5 . 7 ) X IO-^ = 31.4 X ro4. For catechol there is an enormous increase in conductivity, but for resorcinol a slight diminution. “Both catechol and pyrogallol have two hydroxyl-groups in union with two carbon atoms directly attached, but this fact does not explain the increase of conductivity, for glycol, C H 2 0 H’ CH20H, lacks the characteristic. An explanation is furnished by assuming the hydroxyl groups of these two phenols to be situated in the same plane as the carbon atoms, so as to make possible the formation of a ring system of the type 5.7
+
=
=
c-0
II
c-0
\
/
B.OH
with a degree of dissociation much higher than boric acid alone. “The influence exerted by boric acid on the conductivity of polyhydric alcohols in aqueous solution obviously affords an aid in the determination of the configuration of these substances. Applied to glycol, this method indicates the two hydroxyl-groups not to be in corresponding positions, but as in the configuration H 0 . C H 2 . The conductivity of boric acid is raised by I
H&*OH glycerol, erythritol, mannitol, dulcitol, and sorbitol, indicating the presence in each of these substances of a t least two hydroxyl-groups in corresponding positions. ” By this method Boeseken was able to decide the probable formula of the a- and 8-dextrose, concluding that the a-dextrose is the form containing two hydroxyl groups on adjacent carbon atoms and on the same side of the plane of the ring structure.
248r
THE BORIC ACID PROBLEM
Another application of this procedure is described by Cohen.’ “Boeseken and Coops have, however, discovered a method for determining the relative positions of the hydroxyl groups, namely, by the increased conductivity of a solution of boric acid, which seems to be manifested wherever in polyhydroxy compounds the pairs of hydroxyl groups are grouped on the same side of the molecule. Sow, as active tartaric acid increases, whilst mesotartaric decreases the conductivity, it would appear that the carboxyl groups tend t o repel one another. The arrangement of the groups round the two asymmetric carbon atoms, as seen in looking along the bond joining the two latter, may be represented in the following way: H
OH COOH
H
COOH
H
I
!
‘.COOH
OH
COOH
H
OH
OH Mesotartaric acid (hydroxyl groups opposite)
Tartaric acid (hydroxyl groups adjoining)
I n the discussion of configuration determination, Cohen says, p. 288: “Boeseken has introduced an ingenious method for distinguishing cis and trans I . z cyclohexandiols. The cis compounds increase the conductivity of boric acid and also condense with acetone, forming compounds of the type --c-0
-4-0
whereas the trans isomers do not.” One of the most recent developments in this field is the isolation of substances which appear to be compounds in which boron must be assumed to have a coordinated valence of four and to possess optical activity. Cohen, p. 339, says: “A similar compound of salicylic acid and boric acid
[B (CeH4
