NOTES
July, 1960
931
reported marking three combustion experiments from the cis isomer to the trans isomer, should give with each of the compounds. From their meager a difference of energy nearly twice that observed. data, one calculates a value of AH = +1.5 i As previously pointed O U ~ , the ~ ~explanation , ~ ~ lies 0.9 kcal./mole for the heat of isomerization, in in the fact that the six-membered ring requires the liquid state, of the trans isomer to the cis some distortion in order to bring together the isomer. Because of a number of uncertainties, it is desired bonds for the fusion of the two rings. difficult to recover an absolute value for the heat of This distortion is not much for the cis isomer but rombustion of the isomers from the little informa- is appreciable for the trans isomer. This introtion supplied. Within the limits of uncertainty, duces additional strain which keeps the energy the heat of isomerization derived from Becker of the trans isomer from becoming as low a$ I t and RothZ1is in accord with the value from the might otherwise become, and hence makes the present investigation. difference in energy of the two f o r m less than originally expected. Similar considerations may IX. Discussion be inquired into in the case of (a) the cis and trans The data of the present information yield isomers of decahydronaphthalene, formed by t~ AHo = 1.04 & 0.53 for the heat of isomerization like fusion of two six-membered cycloparaffiii of the i r a m isomer to the cis isomer in the gaseous rings, and (b) the cis and trans isomers of bicyclostate at 2 5 O , making the trans isomer more stable. [3.3.0]octane, formed by a like fusion of two fiw:is discussed by a number of i i i v e s t i g a t o r ~ , ~ membered ~,~~ cycloparaffin rings. a five-membered cycloparaffin ring and a sixMcCullough and co-workersz4 have nieawred mernbered cycloparaffin ring may be fused to- calorimetrically the entropies of the cis and trans gether through two adjacent carbon atoms com- isomers of hexahydroindaii and find the entropy mon to both rings either through (a) one equatorial of the cis form to be greater than that of the trans and one axial hoiid or (b) two equatorial bonds. form by 1.68 0.10 cal. deg. mole, for the liquid T i l the former case, the cis isomer of hydrindan is state at 25". From this value. and the heat formed and in the latter case the trans isomer. It of isomerization determined in the present investii p expected that. on the basis of the conformation, gation, the standard free energy of isomerimtioii the trans isomer would be more stable energeti- of the trans form t o the cis form is calculated to he (*ally,that ib, have a lower energy content, as shown by the experimental data. However, .i?-ithout AF02ss = 0.24 rt 0.52 kcal. 'mole (8) othtbr complicating factors, a simple substitution This rewlt indicates that, within the limits of 1111( i f one equatorial bond for one axial bond, in going certainty, the two isomerL: hare sub+taniidly tho (12) IT. G . Dauben and I
