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.

* 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

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

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

FIGURE 115.—Symmetry elements of hydrate, d-rhamnose monohy

class 4.

drate, a-lactose, and stachyose.

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.

B=103°30' (Wolff) [3].

TABLE 64.-Crystal forms of sucrose

TABLE 65.—Angular values between faces of sucrose crystals

1 Calculated from the three italicized angles measured by Wolff.

Twinning: Frequent on the c axis.

Axial 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

quently the two optic axes of the crystal, lie in the plane of the stereographic projection.

Becke [8] found that the median line which coincides with one of the axes of the optical ellipsoid made an angle of 66°37' with the crystallographic c axis in the obtuse angle ẞ (sodium light), as shown in the stereographic projection (fig. 116). This value varies by about 6° from one end of the spectrum to the other, i. e., the position of the optical ellipsoid varies somewhat with the wave length of the light used to measure it.

Becke [8] measured the angle between the optic axes for sodium light and found it to be 48°0' (fig. 116). By calculation from the measured refractive indices along the three principal directions of the ellipsoid, he obtained the value 48°22'.

Merwin [9] has redetermined the three principal refractive indices over a more extended range of wave lengths, as shown in table 66.

TABLE 66.-Refractive indices of crystalline sucrose [9]

It will be noted from the stereographic projection (fig. 116) that one of the optic axes is almost exactly perpendicular to the a faces and the other is approximately perpendicualr to the r faces. Along these two axial directions, sucrose in the crystalline state rotates the plane of polarization. The most recent study of this interesting phenomenon has been made by Longchambon [10] who gives-15.6° per centimeter as the rotation along the axis nearly perpendicular to the a (100) faces, and +51° per centimeter along the other optic axis, the rotation being of opposite sign along the two directions. Other investigators have given somewhat larger values. The cohesive forces tending to bind the units of the crystal together are least along the direction perpendicular to the a faces, as evidenced by the fact that the crystal cleaves easily into sections parallel to the a faces. This perhaps accounts for the usual prominent development of these faces. Stammer [11], Schaaf [12], Bock [13], and Wulf [14] have studied

the effects of various impurities upon the form or relative development of faces of sucrose crystals.

Their results may be summarized as follows: For pure sugar the faces shown on quickly grown crystals are p, a, c, and r' of somewhat near equal development (fig. 117 and fig. 118a). On more slowly grown crystals, the faces r, q, and o' are also usually developed, the latter two on the left end, as shown in figure 117. On crystals that have been rounded by filing and by solution, there frequently develop the following faces in addition to those above: On the left end, b, o, and 2p, and on the right end, o', o, and The presence of molasses (containing raffinose) causes the faces r' to predominate over c, frequently to the complete exclusion of the latter. Likewise, considerable raffinose causes a long, slender prismatic development along the b axis, as shown on the extreme right of figure 118b. The presence of dextrose causes a thin plate-like development, the a faces greatly predominating over all others.

P.