Cyclohexane, geometrical isomerism

We should compare this system with a 1,4-disubstituted cyclohexane such as 4-methylcyclo-hexanecarboxylic acid (see Section 3.4.4). There is a plane of symmetry in this molecule, so there are no chiral centres but geometric isomers exist, allowing cis and trans stereoisomers. The restrictions imposed by bridging have now destroyed any possibility of geometric isomerism. 

Unexpectedly, also the quantum yield for geometrical isomerization of 38a to give 39a was found to be remarkably low in cyclohexane (0 = 0.0007), though it increases in more polar solvents such as benzene (j> = 0.007) or chloroform (quantum yield of cis->trans isomerization in cyclohexane exceeds that of the intramolecular 4k + 2k cycloaddition when cis-dianthrylethylene 38a is irradiated in the presence of suspended crystalline 39a). 

TABLE 10 Quantum Yields of Geometrical Isomerization of l-(9-Anthryl)-2-phenylethylenes 68/69 in Cyclohexane and in Acetonitrile (95 ... 

Polar solvents also may affect the geometrical isomerization of ethylenes by affecting the efficiency of intersystem crossing [96,97]. Thus, even in the absence of polar substituents, solvent polarity can be of importance in the photochemical isomerization of aryl-substituted ethylenes. For example, cis-l-(9-anthryl)-2-(l-naphthyl)ethylene 74 in cyclohexane isomerizes to the trans-isomer 75 with a quantum yield of 0.05, while the quantum yield in acetonitrile solution is 0.31. Any trans->cis isomerization 75—74 in cyclohexane has not been observed, but the reaction does proceed with a preparatively useful quantum yield of 0.05 in acetonitrile [98],... 

The effect of solvent polarity on interpreted qualitatively Because the highly polar excited states enter into strong interaction with polar solvents, their tendency toward geometric isomerization (which requires complete reorganization of the solvate structure) is naturally very low. Data from Schulte-Frohlinde and co-workers [5] show substantial and different polar solvents, for example, /V./V -dimethylformamide (DMF) or ethanol, photostationary state is correspondingly shifted to the trans side [152], With... 

Know the meaning of chair conformation of cyclohexane, equatorial, axial, geometric or cis-trans isomerism, conformational and configurational isomerism. 

Goldstein also pointed out that BCH has a heat of formation 9 kcaFmol higher than that of 1,5-hexadiene at 250 C. Thus the transition state for the geometric and structural isomerizations are 43 and 44 kcal/mol, respectively, above 1,5-hexadiene. The estimated heat of formation of the likely intermediate, a non-interacting chan-cyclohexane diyl is 42 kcaFmol above 1,5-hexadiene so it can be an intermediate in... 

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