182.
p-Dimethylaminobenzaldehyde (PDMAB)
JOHN REFFNER and WALTER C. McCRONE, McCrone Associates, Chicago 16, 111.
1.532 for the light vibration direction,
X. This determination was possible p-Dimethylaminobenzaldehyde
P
-
.4Rh HTDE
DI3lETHYL.4hlINOBENZ.4LDE-
(PDMAB) is a n interesting compound crystallographically as well as analyticall!-. I n particular, the determination of the refractive indices represents a real challenge to the ingenuity of the crystallographer. P D M A B is soluble in every refractive index medium tried (Cargille liquids, methylene iodide, iodonaphthalene, Aquaresin, and dimethyl diphenyl silicone). It reacts 17-ith any halides such as A d 3 , ilsBr3,and K2Hg14, which might be added to any of the above media to raise the refractive indices. Finally, addition of PDLIAB to any index medium lowers the refractive index of the medium by a very large percentage. Pure methylene iodide with a refractive index of l . i 4 dissolves considerable PDMAB and then shows a refractive index of 1.61. K h e n the methylene iodide is first saturated with sulfur and with copious amounts of iodine to give a refractive iiidex of 1.86, the index falls Even lower, to 1.56, n-lien saturated with P D X I B . Actually i t as possible to make a direct determination of only one of the principal refractive indices, alpha. This deterininntion n-as made using Aquaresin to which some KzHgId had been added to raise the index. The resulting medium showed a refractive index of
only because the Aquaresin used n as very thick and therefore contained little water in which the KZHgI4 added could ionize and react with the PDRIAB. Unfortunately, the Aquaresin could not be handled nhen more KBHgId was added unless water was added, and this immediately resulted in a reaction between PDMAB and K2Hg14. The principal refractive indices, beta and gamma, were, however, determined by the following steps: 1. The optic axial angle, 2E, was carefully measured on the 001 tablets and gave a value of 74' (+). 2. Beta prime, the component of beta in the 001 face, was then measured b y several indirect methods. First, in a saturated solution in methylenc. iodide and on 001 views showing alpha and beta prime, the angle between these two vibration directions that gave a Becke line going both ways simultaneously was determined. From this angle and the known value of alpha, beta prime was estimated to be 1.65. Kext a well-oriented crystal lying on the 001 face and showing a nell-formed hemiorthodome, 201, n a s carefully measured to give the nidth of the projection of the 501 face on the 001 face and the birefringence for that thickness of crystal by counting fringes n i t h sodium light. The knom n interfacial angle (001 A 501 = 52") with the width of the projection of the 201 face on the 001 face then permitted a calculation of the thickness of the crystal corresponding to the bircfringence, beta prime
Figure 1. Orthographic projection of a typical crystal ofp-dimethylaminobenzaldehyde
\ 201 3TO
'100
minus alpha, on that view. With these figures the birefringence could be calculated and subsequently beta prime. A value of 1.656 was obtained. 3. From the value of 2 E (74') an approximate value of 2V was estimated (44"), assuming beta n-ould be about 1.60. 4. We nom have two relationships between beta and gamma: one based on alpha and 2V and the other on beta prime and the extinction angle. The latter can be used by taking the equation for an ellipse having gamma and beta a. major ayes and beta prime (1.65) 25' from beta. Solution of these t n o equations gives beta = l .585 and gamma = 2.02. 5. K i t h this value of beta, 21.' was then recalculated to be 45', which, however, caused no significant change in beta and gamma. The estimated error of i 0 . 0 2 for beta is a measure of con-
Powder Diffraction lines for PDMAB d
(Obsd.)
Z
6.09 5.467 5.139 4.756 4.287 3.798 3.669 3.445 3.341
10
100 90 10 10 10
3.241 3.078 2,919 2.825 2.753 2.557
10 80 5 5 5 20
2,430 2 367 2.286
10
