COMMUNICATIONS TO THE EDITOR
Sept., 1940
2567
than that of the synthetic optically inactive form methyl, due to the large. number of electrons. with the double bond conjugated to the benzene Conclusion of complete calorimetric measurements ring. The hexahydrocannabinols have less mari- in this Laboratory down to 12'K. on silicon tetrahuana activity. Comparative pharmacological methyl permits an accurate estimate of the values with that of a highly potent product ob- hindering potential. The entropy has been calcutained by Dr. J. C. Matchett through fraction- lated using the following frequencies and disation of purified red oil in a molecular still and tances': used as a standard by Dr. S. h e w e a t the Cornell Carbon skeleton: IwI = 598, 2w2 = 202, 3 w 4 , ~ , = ~ 239, Medical School in tests according to his procedure 307,8,9 = 800 cm.-l. CHa internal: '6(a) = 1264, Q(u) = 1427, I P ( T ) = of "Bioassay by Approximation" ( J . Am. Pharm. Assoc., 28, 427 (1939); J . Phurm. Exptl. Therap., 2905, %(u) = 2963 em.-'. CHa rocking: = 950 cm.-l. 66,23 (1939)) are shown in Table I. Distances: Si-C = 1.93 A,, C-H = 1.09 A. The frequency 3w7,~,9 is estimated from the frequencies 696 and 831 ern.-' which are apMean value parently a result of resonance degeneracy. The potency max. deviation rocking frequencies are estimated by analogy 1.00 with ethane. The calculation of the entropy a t 0.00 227.00'K. and at 299.8'K. (the normal boiling 2.15 * 0.66 1.75 * .25 point) is summarized in Table I along with the 0.70 * .,lo corresponding calorimetric values.
TABLE I BIOASSAY O F TETRAHYDRO AND HEXAHYDRO CANNABINOLS Substance
Min. Max. Potency above below
Standard red oil ........ Cannabidiol ........ Tetrahydrocannabinol, - 165O 1.50 2.80 Tetrahydrocannabinol, -240" 1.60 2.00 Hexahydrocannabinol, -70" 0.60 0.80 Tetrahydrocannabinol (syn.) .13 .27 Hexahydrocannabinol (syn.) .10 .20
.20 * .07 .15 * .05
The acetates and methyl ethers of the two tetrahydrocannabinols were colorless, highly viscous oils. Tetrahydrocannabinol [a]a 4 ~- 164' gave an acetate [ a ] " ~-167' and methyl ether [a]a 2 ~ - 166'; tetrahydrocannabinol [a]3 0 -2240' gave an acetate [a]a 4 -229' ~ and methyl ether [ a I 3 * D -226'.
TABLE I THE ENTROPYOF SILICONTETRAMETHYL IN THE IDEAL GAS STATE FROM MOLECULAR AND SPECTROSCOPIC DATA 227.0°K., e. u.
~
Translational and rotational (free) 72.40 Vibrational ( 3 0 7 , ~ , 9 ) 0.23 Vibrational (8S = 950) .28 7.59 Vibrational (other modes)
Total NOYESCHEMICAL LABORATORY ROGERADAMS Calorimetric UNIVERSITY OF ILLINOIS S. LOEWE (Sr - S) X 4 (experiURBANA, ILLINOIS D. C. PEASE INCOLLABORATION WITH TEIE e. K. CAIN mental) TREASURY DEPARTMENT R. B. WEARN (S - S) X 4 (V = 1280, NARCOTICS LABORATORY R . B. BAKER I = 5 . 3 x 10-40) D. C. HANSWOLFF WASHINGTON, RECEIVED AUGUST13, 1940 HINDERED INTERNAL ROTATION OF METHYL GROUPS: THE ENTROPY OF SILICON TETRAMETHYL
Sir : We have recently completed an investigation which indicates that the potential hindering internal rotation of methyl groups is probably due to hydrogen repulsions. A study of the entropy of gaseous silicon tetramethyl yields a potential of 1280 calories, compared to 4800 calories for tetramethylmethane. If the potential were mainly due to lack of cylindrical symmetry in the bond orbitals caused by electron interactions, a higher potential than 4800 calories might be predicted for silicon-tetra-
80.50 77.94 2.56 2.56
299.S°K., e. 11.
75.73 0.64 .OS .S5 10.59
* 0.08 *
* 0.16 .30
87.81 86.33
* 0.46
1.48
*
0.1 .2
=t
* +
*
0.3 .6
* 0.9
1.73
The heat of vaporization was obtained temporarily from the measured vapor pressure equation and approximate state data. Hence the larger error a t the higher temperature where the correction to the gas volume is larger. The potential of 1280 =t 160 calories was obtained from Pitzer's tables2 to fit the experimental discrepancy, (S, - S) X 4. At the lower temperature if the 950 frequency were in error by 200 cm.-', and if w7,8,9 should really be a t 696 em.-', the entropy would only be raised by 0.7 e. u. and the potential would then be 1600 calories.
THESCHOOL OF CHEMISTRY AND PHYSICS THEPENNSYLVANIA STATE COLLEGE J. G. ASTON STATE COLLEGE, PA. R. M. KENNEDY RECEIVED AUGUST23, 1940 (1) Rank and Bordner, J . Chcm. Phys., 3, 248 (1935). (2) Pitzer, ibid., 6, 469 (1937).
