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mercury, neutralized with sulfuric acid, treated with a small quantity of a decolorizing carbon, and filtered. The filtrate is diluted five-fold with warm alcohol, filtered after cooling, and evaporated in vacuo to a sirup. After cooling and standing, the sirup yields crystalline d-a-glucoheptitol almost quantitatively. The crystalline mixture is triturated with alcohol, filtered, and recrystallized from ethyl or methyl alcohol. Pure d-a-glucoheptitol melts at 127° to 128° C and is optically inactive.

(b) GLYCALS

General characteristics. The glycals are unsaturated derivatives containing two hydroxyls less than the parent sugars. Usually they are prepared by the reactions represented by the following equations:

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The glycals are of importance because they can be used for the preparation of new sugars and sugar derivatives. Oxidation of glycals with perbenzoic acid followed by treatment with water gives a mixture of the two epimeric sugars [11]; hence, by conversion of a sugar into its glycal and subsequent oxidation, the epimeric sugar may be prepared. Strangely, substitution of the hydroxyls greatly alters the proportions of the epimeric sugars produced. Thus the oxidation of triacetylglucal gives almost exclusively glucose derivatives, whereas the oxidation of glucal gives glucose and mannose, with mannose in predominating quantity [11, 12]. By treating glycals with perbenzoic acid in the absence of water, followed by the addition of methyl alcohol, methyl glycosides are obtained [11]. The products obtained by the perbenzoic acid oxidation usually contain small quantities of monobenzoyl derivatives [13].

Treatment of the glycals with cold aqueous sulfuric acid gives sulfuric esters which on hydrolysis yield desoxy sugars. Chlorine adds to the double bond to give a mixture of epimeric 1,2-dichloro derivatives, while hydrobromic acid appears to give 2-bromo derivatives [14]. Oxidation of the glycals with ozone splits the molecule at the double bond. Reduction with hydrogen in the presence of a catalyst yields hydroglycals [14]. The behavior of the glycals towards hydrogen chloride provides a convenient qualitative test: A pine splinter moistened with a glycal solution and exposed to hydrogen chloride gas turns green.

The tendency of the glycals to undergo intramolecular change is particularly noteworthy and should be kept in mind when working with these products. Boiling triacetylglucal with water results in the migration of the double bond to the 2,3 position and the hydrolysis of one acetyl group [15]. The product, diacetylpseudoglucal, on treatment with barium hydroxide, undergoes further rearrangement to give isoglucal and protoglucal [16].

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(1) TRIACETYLGALACTAL AND GALACTAL.

Method [13].-To a 12-liter flask surrounded by an ice-salt bath there is added 1,000 ml of water, 500 ml of acetic acid, and 100 g of zinc dust which is kept in suspension by the aid of a mechanical stirrer. During a period of 3 hours, ten 75-g portions of finely powdered bromotetraacetylgalactose, each dissolved in 300 ml of warm glacial acetic acid, are added. Water is added in quantities to keep the composition at approximately 50-percent acid, and 600 g of zinc dust is added in portions at intervals during this period. The temperature is allowed to rise slowly to room temperature over a period of 18 hours. Then the mixture is filtered,' and the filtrate is extracted four times with a total of about 11 liters of benzene. The washed extracts are evaporated at a pressure of 14 mm to a thick sirup. The weight of the sirup, obtained by the combination of two such preparations from a total of 1,370 g of the bromo-tetraacetylgalactose, is about 685 g. This sirup is then purified by distillation at 140° to 155° C at a pressure of approximately 0.05 mm. The distillate (500 g) is deacetylated by dissolving it in 4 liters of dry methyl alcohol containing 0.1 mole of barium methylate. After standing for 18 hours in the refrigerator, the solution is saturated with carbon dioxide and the barium carbonate is separated and discarded. The alcoholic solution is evaporated in vacuo to a thick sirup, which is dissolved in 200 ml of absolute alcohol and evaporated again. The resulting sirup is extracted with absolute ethyl alcohol. The insoluble residue is discarded and the extracts are concentrated to a sirup containing about 60 percent of solids. The galactal crystallizes readily from this solution. It is separated by filtration and recrystallized from hot ethyl acetate. About 200 g of the recrystallized product is obtained from 500 g of the acetate. The pure substance melts at 100° C.

NOTES

1 Care must be taken to avoid oxidation of the zinc dust, which takes place when air is sucked through the residue on the filter.

2 Sufficient barium methylate must be used to neutralize any acid present in the distillate and to leave an excess. This can be ascertained by diluting a small sample with water and adding phenolphthalein. An excess is indicated by a red color.

(c) REFERENCES

[1] H. J. Creighton, Trans. Electrochem. Soc. 75, 289 (1939).
[2] D. H. Killeffer, Ind. Eng. Chem. (News ed.) 15, 489 (1937).

[3] W. L. Ipatieff, Ber. deut. chem. Ges. 45, 3224 (1912).

[4] J. B. Senderens, Compt. rend. 170, 47 (1920).

[5] J. Böeseken and J. L. Leefers, Rec. trav. chim. 54, 861 (1935).

[6] M. Raney, U. S. Patent 1,563,587.

[7] M. L. Wolfrom, W. J. Burke, K. R. Brown, and R. S. Rose, Jr., J. Am. Chem. Soc. 60, 571 (1938).

[8] P. A. Levene and M. Kuna, Science 85, 550 (1937).

[9] H. Müller and T. Reichstein, Helv. Chim. Acta 21, 251 (1938).

[10] E. Fisher, Liebigs Ann. Chem. 270, 80 (1892).

[11] M. Bergmann and H. Schotte, Ber. deut. chem. Ges. 54, 440, 1564 (1921). [12] P. A. Levene and R. S. Tipson, J. Biol. Chem. 93, 631 (1931).

[13] W. W. Pigman and H. S. Isbell, J. Research NBS 19, 204 (1937) RP1021. [14] E. Fischer, M. Bergmann, and H. Schotte, Ber. deut. chem. Ges. 53, 509 (1920).

[15] M. Bergmann and W. Freudenberg, Ber. deut. chem. Ges. 64, 158 (1931). [16] M. Bergmann, L. Zervas, and J. Engler, Liebig's Ann. Chem. 508, 25 (1933).

XXXII. CRYSTALLOGRAPHY OF THE SUGARS

1. INTRODUCTION

All the sugars, being optically active, crystallize in one or another of the 11 enantiomorphous crystal classes. An enantiomorphous crystal is one which by reflection in a plane mirror yields an image like the object, but laterally inverted, and which cannot be made by rotation to resemble the first precisely, but behaves as a left hand does to a right hand. Both varieties of the crystals are known in many cases, familiar examples of which are right and left quartz and right and left tartaric acid. Also both right and left forms of many of the sugars have been prepared. By far the greater number of these fall into two crystal groups: Class 4, which has only one symmetry element, namely an axis of twofold or digonal symmetry; and class 6, which has three digonal symmetry axes intersecting each other at 90°. Class 4 is the only enantiomorphous class in the monoclinic system, and class 6 is the only one in the rhombic or orthorhombic system. These two classes only will be reviewed.

2. CHARACTERISTICS OF SYMMETRY CLASS 4 (MONOCLINIC SPHENOIDAL)

The monoclinic or monosymmetric system is characterized by three. axes of unequal length, two of which, a and c, are inclined to each other, but the third, b, is perpendicular to those two. This may be written

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Class 4 of this system is characterized by the fact that there are no planes of symmetry and that only one of the three axes is an axis of

symmetry, namely the b axis. Half of a complete revolution about this axis restores the original appearance of the crystal. (See fig. 115, where the heavy dots represent crystal faces and the ellipse represents

FIGURE 115.-Symmetry elements of class 4.

the digonal axis of symmetry.) It is known as the sphenoidal class, or digonal polar type, since the two ends of the b axis are of different crystallographic forms.

Table 63 shows the possible crystal forms of this class. Column 1 gives the form symbol; column 2, the Millerian index. representing the form; column 3, the number of faces which comprise the form; and column 4, the name of the geometrical figure which the form comprises. In class 4 belong: Sucrose, a-dextrose hydrate, d-rhamnose monohydrate, a-lactose, and stachyose.

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Crystal system: Monoclinic.

(a) SUCROSE

Class 4: Monoclinic sphenoidal; monoclinic hemimorphic. Symmetry type: Digonal polar, characterized by one digonal axis only. Habit: Prismatic.

Ratio of axes: a:b:c=1.2595:1:0.8782.

8=103°30' (Wolff) [3].

TABLE 64.-Crystal forms of sucrose

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Millerian Calcu-
face symbol lated
(100)-(001) 76°30'
(100)-(101) 46°15'
(100)-(101) 64°30'
(100)-(110) 50°48'
(001)-(101) 30°15'
(001)-(101) 39°00'
(001)-(011) 40°30'
(110)-(110) 78°28′
(110)-(110) 78°28'

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(011)-(011) 99°00'
(111)-(111) 115°12′
(111)-(101) 32°24'

98°00'

114°30'

32°52'

(110)-(001)

81°27'

43°53'

31°11'

19°26′

40°13'

10°22′

36°10'

14°1'

16°30'

33°30'

Calculated from the three italicized angles measured by Wolff.

Twinning: Frequent on the c axis.

Arial plane: b{010}

Cleavage: Along a {100}.

Solubility: Greater rate of solution on one end than on the other.
Double refraction: Negative.

a=1.537, 8=1.565, y=1.571.

Refractive indices.-The optical ellipsoid is a surface representing the refractive index whose major, intermediate, and minor axes are determined by the maximum, intermediate, and minimum refractive indices of the crystal. The position of the optical ellipsoid with respect to the reference axes is determined in part by the crystal symmetry. For sucrose the axis of the optical ellipsoid which coincides with the symmetry or b reference axis of the crystal happens to be the intermediate axis; hence, the maximum and minimum axes, and conse

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