Direct Titration of
cis-GlyCOlS
with lead Tetraacetate
RICHARD E. REEVES, Southern Regional Research Laboratory, ,Yew Orleans La.
HE rapid oxidative cleavage by lead tetraacetate of czsTglycol groups located on five-membered rings has been known from the xork of Criegee ( 2 , 5’) and Hockett and eo-workers (6, 7). It has now been found that this reaction is sufficiently rapid to allow direct titration of such groups. The titration can be follomd potentiometrically with lead and platinum electrodes. The reactivity with lead tetraacetate of true cis-glycols is so much greater than that of the other types of glycols that the end point of the titration is not appreciably displaced by the presence of the other glycols. Aliphatic glycols, trans-gl) cols on fivemembered rings, and cis- and trans-glycols on ordinary pyranoside rings have failed to interfere a i t h the titration. In the titration of methyl a-D-niannofuranoside (Figure 1, A ) , the end point corresponded to the addition of 1 mole of lead tetraacetate, although the glycoside contained an aliphatic glycol in addition I
I
I
I
to the cis-glycol on the five-membered, ring. Erythritol anhydride (cis-3,4-dihydroxytetrahydrofurane)and 1,4-anhydromannitol also each consumed exactly 1 mole of lead tetraacetat,e in the titrations. In separate experiments it was found t,hat ethylene glycol, methyl a-D-glucopyranoside, methyl a - ~ mannopyranoside, or 1,6-dibenzoyl-2,5-anhydrosorbitoldid not interfere a-ith the titration of erythritol anhydride (Table I and Figure I, B ) . Reducing sugars, on the other hand, do displace the end point, presumably because of their ability to forni furanose rings containing cis-glycols. It now appears that cis-hydroxyl groups on a pyranoside ring can also be titrated directly with lead tetraacetate, provided they are oriented in the true cis position, a condition requiring an unusual “boat” conformation of the ring instead of the ordinary trans or “chair” form. Thus the a-glycol at position 3,4- of methyl 2,6-anhydro-a-~-altropyranoside (8) can be titrated, whereas in methyl a-D-altropyranoside the same group cannot be so titrated. EXPERIMENTAL
Titrations were carried out on 0.1 to 0.25 millimole of cisglycol in 20 to 30 ml. of purified acetic acid or acetic acid-water solution. The lead tetraacetate reagent was prepared with purified acetic acid and standardized against thiosulfate in the manner described by Hockett, Dienes, and Ramsden ( 5 ) . The lead electrode was cleaned before each titration in 0.5 ‘I’nitric acid, and rinsed Kith distilled water. A titrimeter of the type described by Buras and Reid ( 1 ) was employed after being calibrated in millivolts with a potentiometer. The lead tetraacetate reagent may be added rapidly a t the beginning, but as the end point is approached should be added a t approximately 0.2 to 0.4 ml. per minute.
r r
i - A
’1200
I j i i
1000
Although there was no doubt, about the position of the end point, the observed potentials xere not closely reproducible from one titration to another. They were affected by the rate of stirring when pure acetic acid was the solvent; and the initial values depended, apparently, upon the condition of the lead electrode. The potentials immediately following the end point may be unstable because of secondary reactions.
b, i 0
1 2 3 4 5 0.1 N L E A D TETRAACETATE, ML.
Figure 1.
6
The erythritol anhydride \\-as prepared by refluxing inactive erythritol with 50% sulfuric acid by the method of Henninger (4). It was a colorless sirup JThich distilled at 120’ (bath temperature) a t 0.8 mm. and gave correct carbon and hydrogen analyses. The physical properties of the other substances employed xere in close agreement with those recorded in the literature.
Potentiometric Titration of Cis-GlycOls
Lead electrode negative with respect to platinum A . Meth3l a-D-mannofuranoside (0.1 mmole) in 2 5 % acetic acid solution B . Erythritol anhydride (0.25 rnmole cis-glycol) and 1,6-dibenzoy1-2,5-anhydrosorbitol (0.27 mmole transglycol) in 5 0 % acetic acid solution
ACKNOWLEDGMENT
h sample of methyl 2,6-anhydro-a-~-altropyranojide was supplied by S. IC. Richt,myer.
Tahle I. Poteiitionletrir Titration of cis-Glycols with Lead Tetraacetate, with and without Added Substances cis-Glycol Name Erythritol anhydride Erythritol anhydride
26 26
Erythritol anhydride
26
Erythritol anhydride
26
Erythritol anhydride
26
Mg.
Added S u b s t a n c e Name AIg. Solvent Sone HO.4c Methyl a194 HOAc glucoside Ethylene 1100 25% HOAc glycoi 11ethJ.l L ~ - D 100 25% HO.4c m a n n o..
Methyl a-D-mannofuranoside 1,4-Anhydromannitol Methyl 2,R-anhydroa-D-altropyranoside Values determined -added.
19.2
pyranoside 1,6-Dibenzoyl2,5-anhydrosorbitol None
16.8 13.3
Sone None
Moles of Reagent per Mole of cis-Glycol0
0.98 1.01 1.02 1.01
100
50% HO.4c
1.00
..
257, HOAc
1.00
..
HOAc 50% HOAc
1.03 1.03
..
LITERATCRE CITED
(1) Buras, E. PI.,and Reid, J. D., IND. EKG.CHID,!., ASAL. ED.,17, 120-5 (1945). (2) Criegee, R., Blichner, E., and Walther, W., Ber., 73B, 571-5 (1940). (3) Criegee, It., Kraft, L., and R a n k , B.. A4nn.,507, 159-97 (1933). (4) Henninger, Snn. chim.phus. (B), 7 , 223-33 (1886). (5) Hockett, R. C., Dienes, M . T., and Rarnsden, H. E., J . Am. Chem. Soc., 65, 1474-7 (1943). (6) Hockett, It. C., Flctcher, H. G., Jr., Sheffield, E . L., Goepp, R. M.,Jr., and Soltaberg, Sol, Ibid., 68, 930-5 (1946). (7) Hockett, R. C., Nickerson, 11. H., and Reeder, IT‘. H., 111, Ibid., 66, 472-4 (1944). (8) Rosenfeld, D. A , , Itichtmyer, ?;. K., and Hudson, C. S., Ibid., 70, 2201 (1948).
graphically from plot of 4 mv./A ml. versus ml.
RECEIVED April 2, 1948. Presented before t h e Division of Sugar Chemistry a n d Technology a t the 113th Meeting of t h e AMERICASCHEMICAL SOCIETY, Chicago, Ill.
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