300 LOWRY AND DRAM THE ROTATORY DISPERSIVE XXXI .-The R o t a t o y Dispersive Power of Organic Compounds. Part I X . Simple Rotatory Dis-persion in the Terpene Series. By THOMAS MARTIN LOWRY and HAROLD HELLING ABRAM. IN a paper on “The Form of the Rotatory Dispersion Curves,” published in 1913 (T. 103 1067) i t was shown: (a) That the rotatory dispmion in a large number of simple organic compounds may be expressed by the formula a=k/(h2-h02), where k is the (‘ rotation constant ” and h02 is the ‘‘ dispersion con-stant ” of the substance. ( b ) That this formula can be applied both to magnetic and to natural rotatory powers. ( c ) That a very simple method of testing the form of the dispr-sion curves is to plot the reciprocals of the rotatory powers against the squares of the wave-lengths.When the simple dispersion formula is valid the observations will plot out t a a straight line. ‘The validity of this simple dispersion formula was established in several ways. Thus: (1) Twenty-five hydrocarbons alcohols and acids for which the ratio a4358/a5461= 1.636 were grouped together and their magnetic dispersion ratios were averaged for six different wave-lengths ; these averages showed a remarkable agreement with the ratios calculated by means of the simple dispersion formula. (2) I n the same way the optical dispersion ratios of eight second-ary alcohols for which a,,,/ a5461 = 1.651 were found to agree clmely with ratios calculated by the simple formula. (3) A few optical and magnetic rotations of larger magnitude showed a similar close agreement in individual cases without the necessity f o r averaging which arises when the readings are small.(4) I n the case of a- and &methyl glucosides very concordant results were obtained when the two constants of the simple equation were calculated (a) from the mercury readings and a54617 ( 6 ) from the cadmium readings a5086 and q 4 3 8 (Lowry and Abram, T r m . Faraday Soc. 1914 10 108). These earlier observations showed that the simple dispersion formula can be applied very generally to compounds of simple structure such as the optically active secondary alcohols which contain only a single asymmetric carbon atom and also to com POWER OF ORGANIC COMPOUNDS. PART IX. 301 pounds such as the glucosides which contain several asymmetric carbon atoms associated with the simplest possible radicles f o r example hydrogen hydroxyl and the like.An opportunity has, however occurred recently of testing the validity of this same simple formula in the1 case of a large number of optically active compounds in which these elements of simplicity in the molecular structure are conspicuously absent. The striking results of this further test form the subject of the present communication. The new data now under consideration were provided by the observations of Prof. Rupe who in continuation of earlier experi-ments on the (‘Influence of Constitution on the Rotatory Power of Optically Active Substances ” (Annalen 1903 327 157; 1909 369, 311; 1910 373 121; 1913 395 87 136; 1913 398 372; 1914, 402 149) has published a series of measurenients of the optical rotatory power of (1) twelve derivatives 6f methylenecamphor, (2) merhhol and eleven of its esters (3) myrtenol and eleven of its esters (4) three hydrocarbons derived from citronellaldehyde, (5) camphor pulegone and carvone (Annalen 1915 409 327).Unlike the previous series of measurements which were confined to observations with sodium light the last series included in the case of almost every compound readings for four different wave-lengths in the visible region of the spectrum. It was therefore possible to study not only the optical rotatory power of the various com-pounds but also the character of their rotatory dispersion. The four wave-lengths selected from a continuous spectrum, were A = 6563 5898 5463 and 4861 corresponding closely with the Frauntofer lines C = 6563 D = 5893 F= 4861 and the green mercury line Mg 5461.I n order to preserve a convenient sequence of lettering these four wavelengths were described as C B E? and F ; but as the symbol E has long been applie’d t o the Fraun-hofer line of wave-length 5270 the’ symbol Q is used below for the green (quicksilver) line in the series which thus becomes C D, Q F . Thel various substances were examined either in the pure state as liquids a t 20° or dissolved in benzene a t 20° since this solvent was found to have no very great influence on the rotatory powers; several substances were examined both in the pure state and in solution. When the experimental work was approaching completion the data were handed over for detailed analysis to Dr.A. Hagenbach, Professor of Physics in the University of Basle. The important deductions which he was able to make are set out in a paper on ‘‘ Rotatory Dispersion in Homologous Series ” (Zeitsch. physikal. N %z LOWRY AND ABRAM THE ROTATORY DIBPERSIVE Chem. 1915 8@ 570). The chief points of this paper are as (1) The dispersion ratis ap/qo is practically constant in each series of closely-related compounds. Compoupda which differ in any marked degree from the average are regarded as “relatively anomalous.” (2) A similar statement may be made in reference to all the six ratios d a a r upla, QF/ag aQlaw rFQ/aol d a a 88 WBS shown by tbbul&iog $hew ratios for (i) eight derivatives of methylene-urtmphor (ii) menthol and se,ven of its esters (iii) three hydro-carbons from citronellaldehyde.(3) It follows therefore that if the dispersion law for one member pf the series be given by the equation a= +(A) the rotatory dispersion in every compound may be expressed by equations, such as: fOllQW8 : [all = Q14 (A) -[a21 = c24) (A) -[as1 = C34(h) * This proportionality of rotatory powers was demonstrated by tabu-lating the ratios a2/al u3/a1 a / a l eta in each of the three series of uompounds. Attempts were made t o determine mainly by graphical methods, the nature of the unknolwn function +(A). Thus the equations of Biot, and of Stefan, were1 tested by plotting a against 1 /A2. Cb=B/A2, a=A+B/Xa, Boltzmann’s equation : ~ = A / A ~ + R / x ~ or ahzcA+B/h2, was tested by plotting aha against 1 1 ~ 2 .I n neither case was an exact linear law disclosed. The equations of Lommel and the two-term equation which Drude used t o express the rotatory power of quartz could not be tested in this way; but the empirical equa-tions : p = A + B / h n and loga=A+B/h were tested by plotting log a against log 1 / A and against 1 / x ; sub-sequently log a was also1 plotted against A and a against 1 / X (&4nnaZen 1915 409 349 351) but again without disclming any simple linear relation between these quantities. The nature of the function in the equation a=+(X) thus remained still undis-covered. The present paper serves to supply this deficiency by showing that in almost every case the new dispersion data can b POWER OF ORGANIC COMPOUNDS.PART IX. 303 expressed by a simple equation of the type first put forward by Drude. The fact that Drude’s equation was not used by Hagenbach and in general failed during many years to secure the practical recog-nition which it deserves may be accounted f o r in two ways. I n the first place the equation was put forward as an approximation only in a very general form: containing an indefinite number of arbitrary constants. The arbi-trary constants A12 h22 As2 . . . in the denominator were deduced from measurements of refractive dispersion and it was not even suggested that they could be derived (in a still mare satisfactory way and often with much greater exactness) from the rotations themselves. In the second place Drude demonstrated the validity of his equation only in one single case namely that of quartz, the equation f o r which took the form 76 k a= 1 - __ A2 -x,2 A2‘ No data whatever were given for optically active liquids and the magnetic rotatory dispersion in carbon disulphide and in creosote (!) was expressed by a different formula also depending on measurements of refractive dispersion.The first extensive prac-tical application of Drude’s formula was therefore made less than six years ago in the second paper of the present series. The easiest (although perhaps the least exact) method of testing the simple dispersion law .a = 7c/ ( ~ 2 - ~ 2 ) is to plot the reciprocals of the rotations against the squares of the wavelengths. The dramatic effects which are produced by plotting 1 /a against ~2 are shown by comparing- the straight lines of Figs.1 to 4 with the broken lines* or curves which were given by all other methods of plotting. It is specially reniarkable that plotting a against 1 1 ~ 2 (Stefan’s formula) should give curves where1 plotting 1 /a against ~2 (Drude’s formula) gives very exact straight lines. These lines indicate clearly that where half a dozen other relationships have failed the simple Drude formula gives a t once a satisfactory expression of the experimental data.? Hagenbach appears t o have plotted his curves on the assumption that the ‘‘ E ” line used by Rupe was the Fraunhofer line cS2,,, and not the mercury line u5461. t More exact data may perhaps compel the use of additional terms as in the case of quartz which requires one two or three terms according to the range and accuracy of the data employed but there are no indications of this in the data examined hitherto.N” 304 LOWRY AND ABRAM THE ROTATORY DISPERSWE A marked exception mcurs in the case of pulegone which gives a smooth full curve and evidently shows ‘ I complex ” rotatory dis-persion. Diphenylrnethylenecamphor C,,H,,O:CPh, the dis-persion ratios of which are much lower than those of all the related compounds gives a curve; so also does menthyl b-phenylcinnamate, C,,H,,O*CO*CH:CPh, the rotatory dispersion of which must be complex since the dispersion ratio alF/ac = 1.72 falls below the Rotatory dispersion in derivatives of rnethylenecarnphor. Notice curvature in the case of the diphenyl derivative.minimum value uF/uC = XcZ/XFz = 1.818 beyond which hO2 would become negative and A an imaginary quantity; the fact that both these compounds contain the group :CPh can a t present only be regarded as a coincidence. All the other compounds appear as a result of this rough graphical analysis over a narrow range of wave-lengths to give simple rotatory dispersion. A more exact test of the’dispersion formula is given by numerical calculation. The following table shows that the specific rotations POWER OF ORGANIC COMPOUNDS. PART IX. 306 observed and calculated of a series of typical compounds lie well within the range of possible experimental errors. TABLE I. Specific Rotations 0 bserved and Cdculated. E,thylidenecamphor,* [a]= 47.322 / (hz - 0.0829). Obs.......... 136.37' 178.58' 219.31' 308.49' Calc. ......... 136-05 178.58 219.55 308-49 0 - C ......... +Om32 f - 0.24 f Hydroxymethylenecamphor,* [a] = 22*843/ (hz - 0.0874). Obs. ......... 66-53 87-66 108.57 153.41 Calc. ......... 66.53 87-70 108.26 153.41 0-c ......... f -0.06 +Om33 f Benzylmethylenecamphor [a] = 33.431 / (h2- 0.0887). Obs. ......... 97.87 129.00 156.26 Calc. ......... 97-75 129.00 156.39 0-c ......... +0.12 rt -0.13 Menthol,* [a] = 15.0681 (A2 - 0*0236). Obs. ......... 37.01 46-58 54.78 Cdc. ......... 37.01 46.47 54.82 0-c ....... *. f +Om11 -0.04 Menthyl benzoate,* [a] = 29*364/ (A2 - 0.0255). Obs. ......... 72-41 91.10 107.76 Calc. ......... 72.46 91.10 107.59 0 - C ......... -0.05 f +0-17 Myrtenol [a]= 14*700/(h2-0~0316). Obs........ .. 36-83 46.49 55-04 Calc. ......... 36-83 46-48 55-09 0-c ......... f $0.01 -0.05 Myrtenyl benzoate a = 11.505 / (A2 - 0.0341). Obs. ......... 29.01 36.67 43.51 Cdc. ......... 29.01 36-67 43.52 0-c ......... f -0.01 * Dissolved in benzene. 226.50 226.60 f 70.84 70.84 f 139-30 139-30 rt: 71.81 71.81 f 5d.90 56.90 f In view of the fact thatl the readings for solutions in benzene were multiplied by ten to convert them into specific rotations, whilst hhe others wem approximately doubled the agreement shown above is practically perfect 306 LOWRY AND ABRAM THE ROTATORY DISPERSIVE Even clearer evidence of t'he validity of the simple dispersion formula is afforded by a study of the average dispersion ratios observed and calculated f o r groups of relate'd compounds.Three such groups were' avelraged by Hagenbach namely : benzene) uF/aC = 2.310. (a) Eight derivatives of methylenecamphor (dissolved in ( b ) Seven esters of menthol (pure or in benzene) alF/ao =1*920. FIG. 2. ENTHOL ,i-= PHEN$ ACETATE. 1 I ESTERS. I 1 15 0-40 0.36 0.30 0.25 Rotatory dispersion in menthol and its esters. Notice curvatture in the case of 8-phenylcinnamate. ( c ) Three derivatives of citronellaldehyde (in the pure state) , To these there are now added average ratios for: (da) Six derivatives of methylenecamphor (in the pure state), (e) Ment.ho1 and seven esters (dissolved in benzene) aP/uc = a,/a = 1.991. ctpIQc = 2.303. 1.911 POWER OF ORGIAWIC! COMPOUNDS. PART IX. 307 (f) Myrtenol afid eight esters (in the pure state) up/aC =1*958.The close agteement betweeh the observed ahd calctilated values of these ratios is shown in table 11. Rotatory dispersion in myrtenol and its esters. TABLE 11. Dispersion. Ratios 0 bserved and Calculated. (a) Obs. ...... 2-310 1-751 1.415 1.G33 1.237 1-319 h,2= aplac. aplag. aFfa8. aglttc. ayfan. anlac. {Gala. ... 2.310 1-752 1.418 1.629 1.236 1-318}0.0879 ( b ) Obs. ...... 1.920 1.529 1.294 1.488 1.181 1.259) AU2= { cfalc. ... 1.921 1-529 1,294 1.484 I.181 2.257f0.0250 Obs ...... 1.991 1.561 1.313 1.6.15 1.1818 1.274 A@%= {Cali. ... 1.992 1.569 1.316 1.513 1.191 1-269)0*0401 (d) Obs. ...... 2.303 1.756 1.423 1.619 1.234 1.3121 Aoz= {Calc. ... 2.303 1.748 1.417 1.626 1.234 1.318 /0.0871 (c) jobs. ......1.911 1.521 1.289 1.482 1.180 1.256 A,’= [Calc. ... 1.911 1.522 1.292 1.479 1.179 1.255}0.0227 ( f ) fobs. ...... 1.958 1.550 1.306 1-499 1.186 1.264\ A 2= 1CaIc. ... 1.958 1.550 1.306 1-499 1,186 1.264j0.833 308 LOWRY AND ABRAM THE ROTATORY DISPERSIVE This agreement is nearly as clme as in the case of the data by which the validity of the simple Drude formula was first estab-lished and even the largest differences are usually less than the average errors of the individual ratios. The “simple” character of the rotatory dispersion could therefore only be called in question i f data were available of greater exactnws or over a wider region of the spectrum. A further opportunity of testing the validity of the simple dis-FIG. 4. R o m y dispersion in derivatives of camphor pulegone and carvone.Notice curvature in the case of pulegone. persion law is provided by the inclusion in a more recent paper by Prof. Rupe (Helu. Chint. Acta 1918 1 452) of dispersion data for four sample,s of camphylcarbinol, a compound containing three asymmetric carbon a b as corn POWER OF ORGANIC COMPOUNDS. PABT IX. 309 ponents of a complex ring system. observed and calculated rotations is shorn in table 111. The agreement between the TABLE 111. Rotatory Dispersion in Camphylcarbirtol. First sample [a] = 15*980/ (A2- 0.10220). ~ = 6 5 6 3 5898 5463 4861 Obs. ...... 48-64' 65-24' 81-87' 119.17' C ~ C . ......... 48-64 65.05 81.43 119-17 0-c ...... +0-19 +0*14 f Obs. ......... 46.31 62-11 77-67 113.48 Calc. ......... 46.31 61-93 77.53 113.48 0-c ......f +O.lS +0.14 f Obs. ......... 46.45 62.22 77-74 113.90 C ~ C . ......... 46-45 62.13 77-79 113.90 0-c ...... f +049 -0.05 f Second sample [aj=15*213/(h2- 0.10223). Third sample [a]=15'252/(A2-0*10239). Fourth sample [a] = 16.10/(h2- 0*10304). Obs. ......... 49-13 65-73 82.44 120.82 C ~ C . ......... 49.13 65.76 82.39 120.82 0-c ...... f -0.03 +0.05 f It will be observed that the sample having the highest rotatory power which was also probably the purest gives a remarkably close agreement the differences being in opposite directions and amounting only to a few hundredths of a degree or abuut 1 part in 2000. This exact agreement suggests that the simple dispersion law may b'e of value as a test of purity and that deviations from it may in some cases justify a further examination of the chemical composition of the material used for the measurements.It is of interest to notice the chemical character of the comr pounds to which the ('simple" dispersion formula has now been applied. They are as follows: CHMe, I ~H,*O*CO*R 1. Methylene camphors. 2. Menthol eshrs. 3. Myrtenol esters. CMe2:CH*CHz*CH2*CHMe*CH:CHR. 4. Hydrocarbons from citronellaldehyde 310 L0WR.P AND’ ABBAM THE ROTATOkY DISPEkgIVE Nearly all are complex ring compounds or loaded with double bonds. The fact that the simple formula applies to compounds of such complex structure is remarkable evidence of the broad and sound basis on which the formula rests. Ghuructeristic W’ave-Zengt hs.-In Drude’s simple equation the rotatory dispersion is defined completely by the ‘‘ dispersion con-stant”’ ~ 2 ; this is the square of a wave-length which is that of a hypothetical absorption band usually in the ultra-violet region of the spectrum.This wave-length defines the whole course of the dispersion curve and is independent of the particular wavelengths used to determine itl; thus the value of A may be deduce’d equally well from the mercury ratio a4,,/a5.iF,1 from the cadmium ratio a,,,,,/~ry;~~ or from the ratio aF/a derived from the data now under discussion. A preliminary study of these data by Prof. Rupe had however disclosed the fact that in the case of the methylene-camphors u,/aC =a, whilst in the case of the citronellaldehycke hydrocarbons a p - a = a that is for each series there is a (‘ characteristic wave-length,” A (Zeitsch.physikal. Chem. 1915, 89 SSl) for which the rotation is equal to the difference between the rotations for the F and C lines. This wave-length is not a fundamental constant of the dispersion curve like the Ao2 of Drude’s equation since it depends on the two standard wave-lengths for example F and C which are select’ed as determining the differ-ence; but it usually lies within the limits of tohe visible spectrum and affords a picturesque method of setting out the essential features of the dispersion curve. By assuming the va1idit.y of Stefan’s formula Hagenbach showed that this waverlength can be deduced from the expression or taking in all the four rotatory powers, Drude’s equation on the other hand which is the one that actually fits the curves gives for the (( characteristic wave-length ” the expression XC2 - (n - 2)Ap2 - 0.4307 - 0*2363(n-3) (n - 1)2 (n - 1)2 where is the dispersion ratio a,,,/a,.Thus for t,he citronell-aldehyde hydrocarbons for which n = 2.00 this equation gives ha2=hc2 as was observed experimentally whsn it was found that aF - ac = a,. Ad2 = - POWER OF ORGANIC COMPOUNDS. PART IX. 311 Constant Rotatory Dispersion in Homologous Series.-In the compounds now under consideration new radicles are introduced into t-he molecule atl points which are separate’d from the asym-metric carbon atoms by a considerable chain including in every case either an oxygen atom or a double bond. A constant dis-persion ratio is therefore observed from the beginning and any substance of which the rotatory dispersion differs largely from the average of the series is noteworthy and exceptional.The only conspicuous exceptions amongst some thirty-six compounds under consideration in the present research were found in two substances containing the group :CPh,. These have now been shown t o differ from the others also in giving complex instead of simple dispersion curves so that the rule appears to apply without excq-tion to all compounds showing simple rotatory dispersion. A different state of affairs prevails however in the secondary alcohols of Pickard and Kenyon which have a “growing chain” attached directly to the asymmetric carbon atom. The dispersion is here always simple but the dispersion constant varies i n the different series and only assumes a steady value in each s e r i ~ when the “growing chain” of carbon atoms has definitely estab-lished itself as the heaviestl radicle attached to the asymmetric carbon atom (Lowry Pickard and Kenyon T.1914 105 101). The lowest homologues usually show an exceptionally high rotatory dispersion but this is not accompanied by any change in the type of the dispersion curve and is therefore entirely distinct from the “ anmalous rotatory dispersion,” of which an exact definition was given in a former paper of the present series (T. 1915 107 1195). It would be a real misfortune if substances which are perfectly normal in their rotatory dispersion were to be regarded even as ‘ I relatively anomalous ” whenever they happen to differ slightly from their homologua and it is hoped that this unnecessary and misleading description will be abandoned. Summary, a = Ic/ (12 - A,?) can be applied to express the rotations produced by a large number of compounds of the terpene series including (a) derivatives of methylenecamphor including camphylcarbinol ( b ) menthol and its esters ( c ) myrtenol and its esters and fd) hydrocarbons derived from citronellaldehyde. Pulegone and two compounds containing the group :CPh, show complex rotatory dispersion. It is shown that the simple dispersion formula GUY’S HOSPITAL, LONDON 833. [Received March 20th 1919.