It is interesting to examine the various orders of elution of the terpene hydrocarbons from the four stationary liquid phases employed in this study. On polar columns. the two p-menthanes elute rapidly, despite their higher boiling points (Tables I1 to 117 (21). possible explanation for this may be that neither of these compounds is very polarizable, and therefore, there is very little bonding interaction between them and the stationary liquid phases. If this is the case, the trans, having a lower boiling point than the cis, should elute’first, as it does. The trans has the diequatorial Conformation, while the cis exists predominantly in the axial methyl, equatorial isopropyl conformation [the energy difference between the latter more stable conformation and the equatorial methyl and axial isopropyl conformation is about 1 kcal. (20)l. If significant bonding interaction did take place, one would expect the diequatorial form to participate more readily than others (j), and thus elute last. Klouwen and ter Heide ( I S ) have reported a series of terpene polarizabilities. Fmploying polar stationary liquid phases, they conclude that cyclohexadiene derivatives with conjugated double bonds inside the six-membered ring-e.g., a-terpinene and a-phellandrene-show a lower polarity than the isomer with isolated ethylenic linkages in the ring-i.ci., y-terpinene. They found a-phellandrene less polarizable than a-terpinene, and state that most of the bicyclic members of the terpenes do not differ appreciably in polar character (except for sabinene, which behaves like a monocyclic terpene). These authors conclude that terpenes with exocyclic ethylenic double bonds are more easily polarized than those with endocyclic double bondse.g., 01- and /?-pinene. Sabinene was rclported to have a stronger polar character than /?-pinene. A\ll of the above conclusions are in agreement with the orders of elution described in the present study. Caniphrne elutes after cis-p-menthane on LA\C-2-R446 and before it on the other two polar columns. Also the order d3-carene-myrcene is reversed on n-butylcyclohe.;yl phthalate columns (’Table IV). p-Cymene, a nonterpenoid compound n-ith a carbon skeleton similar to u-limonene, appears to be more strongly polarizable than Dlimonene, 8-phellnndrene, and y-terpinene
Table V.
Peak 1 2 3 4 5
A \
6
ka
Chromatography of Terpene Mixture on Apiezon L a t 71 (’ C..
Compound a-Pinene Camphene trans-p-Menthane @-Pinene czs-p-Menthane D-T,imonene @-Phellandrene yTerpineiie
?.P., C. 156 159 6 170 164 171-2 177 7 178 183
Vi/Vg 0 476 0 559 0 641 0 667 0 687 1.00 1 03
Q 16Q* 14 3,100 15 3.600 27 11,700 19 5,800 20 6,400 23 8.500 19 5.800 1 24 23 8,500 Average 6,700
Si? ( X lo3)
R
174 0 147 0 40 5
30 0 456 0 30 0 204 0
2 G 4 0 0 77 0 60
10 4 0 57 4 7
Pee Table I for operating conditions.
The order of elution on Xpiezon L appears to be that of the boiling points of these hydrocarbons, with the euception of trans-p-menthane (Table V). This would indicate that even on a socalled nonpolar column, this separation is not completely free of some bonding effects (19). The affinity of the terpenes for the LAC-2-R446 column over the other columns used in this study is graphically demonstrated b y reference to Figures 1 and 2. The analysis time on the LXC-2-R446 is roughly twice that of the Tween-20 column despite the fact that they are both the same length and the Tween-20 column rvas operated a t a significantly lower temperature (Table I), From this study one may conclude that the separation of the terpene hydrocarbons is greatly enhanced by employing capillary columns; that column efficiency as expressed by 16 &2 is not the sole factor determining degree of separation; and that selection of the proper stationary liquid phase is still by far the most significant factor in obtaining maximum resolution
(9) Golay, 11.J. E.. Ihzd., p. 36. (10) James, A . T., A \ . . ~ L CHEM. . 28, 1564 (1R.Sfi’l ,--__,. (11) Jones, W. L., Iiieselbach, It., Ibid., 30, 1590 (1958). 112) Kirchner. J. G.. Miller. J. AI.. Keller, G. J., Ibzd., 23, 420 (1951). (13) Klouwen, 11. H., ter Heide, R., J . Chromatog. 7,297 (1962). (14) Lipsky, S. R., Landowne, R. A,, Biochem. Bzophys. Acta 27, 666 (1958). (15) McWilliam, I. G., Deaar, R. A., “Gas Chromatography 1958,” D. H. Desty, ed., p. 142, Academic Press, New York, 1958. (16) McWilliam, I. G., Dewar, R. A,, S a t w e 181, 760 (1958). (17) Second Research Conference in Gas Chromatography Cniv. of California, Los Angeles, Jan. 29-30, 1962. (18) von Rudloff, E , Can. J . Chetn. 38, 631 (1960). (19) Wehrli, A., Kovats, E., Helv. Chim. ilcta 42. 2728 11959). (20) Win&ein, S:,Holness, X. J.. J . Ani. Chem. SOC. 77, 5562 (1955). (21) Zubvk, IT. J., Conner, A. Z., SAL. CHEJI.32,912 (1960).
RECEIVED for review July 5 , 1962. Accepted August 31, 1962. Kork supported by a grant from the Research and Development Department of the Sunkist Growers, Inc., Ontario, Calif.
LITERATURE CITED
(1) Ambrose, D., Keulemans, -4.I. AI., Purnell, 1., H., ANAL. CHEM.30. 1582
(1958).
(2) Bernhard, R. A., Food Res. 25, 531
11960).
Correction
(3j Bernhard, R. A , , J . Assoc. O$ic. A g r .
Chemists 40,915 (195ii. ,
(4) Clements, R. L., bczence 128, 899
(1958). ( 5 ) Dauben, W.G., Bozak, R. E., J . Org. Chem. 24. 1596 (19591. (6) Desty, D. H.,‘Xntu;e 179, 242 (1957). ( 7 ) Desty, D. H., Gold;p, A., “Gas Chromatography 1960, R. P. W. Scott, ed., pp. 162-83, Butterworth. Washington,-D. C., 1960. ( 8 ) Dijkstra, G., de $Fey, J., “Gas Chromatography 1958, D. H. Desty, ed., p. 56, Academic Press, Sew York, 1958.
Conductivity Method Determination of Urea
for
In this article by Kei-Tsung Chin and K y b e Kroontje [ k i a ~CHEx . 33, 1757 (1961)], on page 1758, column 3 , paragraph 5 , the sentence should read: “The experiments reported n ere conducted at 27“ C. Kith a conductivity cell having a cell constant of 0.30723 cm.+’
VOL. 34, NO. 12, NOVEMBER 1962
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