7884
purchased from Miles Laboratories, Inc., Elkhart, Ind. 465 14. Solute concentrations in the cholesteric M. mesophases were normally between and In summary, LCICD of noncomplexing achiral molecules has been observed in lyotropic cholesteric mesophases composed of PBLG and PBDG in a variety of helix supporting solvents. LCICD of anthracene in lyotropic cholesteric mesophases is distinctly different from that found in thermotropic cholesteric mesophases which is tentatively attributed to a variation in the ability of the two cholesteric mesophase types to order anthracene single molecules or possibly the intervention of the previously suggested mechanism b. Acknowledgment. Stimulating discussions with Drs. W. H. H. Gunther, G. Johnson, H. Gibson, and J. E. Kuder are gratefully acknowledged.
R = C0,H) followed by LiAIH., reduction of the ester (1, R = CH2C02CH3)to the alcohol (1, R = CH2CHIOH), mesylate (mp 55”) formation, and hydride displacement with LiAIH,.
+-+ Ii
1
2
The rate of ring opening of these two alkyl derivatives (1, R = CH, and CzH5)to the dienes ( 2 , R = C H a and C2Hj)was measured both in the gas phase and in solution (over the temperature range 136.5-238.6’) as previously d e ~ c r i b e d . ~ ”Least-squares calculations on the results gave the following Arrhenius equations for (20) Rochester Institute of Technology Co-op. the 4-methyl derivative: log (k,/sec-l) = (13.86 f F. D. Saeva,* G. R. OlinZ0 0.03) - (36.90 f. 0.07),/6; log (kJsec-l) = (13.55 f 0.31) - (35.50 f 0.13)/6; log (kblsec-’) = (13.64 f Xerox Corporution, Rochester Research Center Webster, New York 14580 0.30) - (35.68 f 0.13)/6. The following equations Received July 18, 1973 were derived for the 4-ethyl derivative: log (k,/sec-l) = (13.68 f 0.29) - (36.19 f 0.13)/6 and log (kblsec-’) = (13.55 f 0.15) - (35.87 f 0.06)/0, where 6 = Ring Opening of Bicyclo[2.2.0]hexanes. Effect of 2.303RT kcal/mol, error limits are least-squares deviaAlkyl Group Substitution upon Interpretation of tions, and the subscripts g, a, and b refer to the gas Radical Stabilization Energies phase, tetrachloroethylene, and o-dichlorobenzene as solvent, respectively. These may be compared to acSir : tivation energies for l-chlorobicyclo[2.2.O]hexane (1, Without exception, the substitution of an alkyl R = H ) of 35.42, 34.50, and 34.60 kcal/mol and Argroup for a hydrogen atom in a series of aliphatic hyrhenius log A values of 13.49, 13.21, and 13.25 in the drocarbons will decrease the a-bond strength. This gas phase, tetrachloroethylene, and o-dichlorobenzene decrease is made up of a contribution from increased as solvent, respectively.* steric (gauche) interactions in the parent molecule and Both gas and liquid phase results are consistent with an increased stability of the radical formed in the series a small but significant increase of 1.2 f. 0.5 kcal/mol tertiary > secondary > primary.2 The picture is not in the activation energy when the bridgehead hydrogen so clear for cyclic hydrocarbons, as in several cases atom is replaced by a methyl group; there is an addialkyl substitution increases the activation energy for a- tional small increase in the activation energy on subbond scission. Within the approximations of the stituting an ethyl group for the bridgehead hydrogen biradical mechanism, O’Neal and Benson2 have shown atom. that radical stabilization energies derived from cyclic We have previously obtained excellent group addicompounds are in reasonable agreement with those tivity in the following 1,4-disubstituted bicycle[? .2.0]derived from acyclic c o m p ~ u n d s . T ~ o explain a large hexane series: H,H, H,Cl, Cl,CI, and CI,H, CI,CO2increase found in the activation energy for the ring CH3, COzCH3,C02CH,-this assumed no 1,4 interopening of 1,1,3,3-tetramethylcyclobutane,Cocks and actions in the transition c ~ m p l e x . ~ From the absence Frey4 introduced a further variable into the mechanism, of any interactions between the pairs CI,C02CH8 and that of steric interactions in the ring opening of cycloC02CH3,C02CH3it may be concluded that likewise butanes. Our studies on the bicyclo[2.2.0]hexane systhere will be no 1,4 interaction between the smaller tem have now reached a stage where we can conclude CI,CH3 and CI,C2H; groups. The increase in activation that our results are not in agreement with the above energy on alkyl substitution must be an intrinsic proppostulates. These results are presented in their present erty of the alkyl group and not a result of steric interform due to the possible effect on the large volume of action. lo work which incorporates the above assumptions. (6) E. N. Cain and R. I
