Disubstituted benzene derivatives phases

As the capacity factors of p-CD complexes of disubstituted benzene derivatives are comparatively low and their differentation is also rather small - thus the final observed selectivity is mainly determined by the differences in the stability constants between p-CD complexes of ortho, meta and para isomers. The sequence of elution of ortho, meta and para isomers from RP column should be therefore reverse to the stabilities of their (3-CD complexes. This sequence should be opposite to that observed on the columns filled with P>-CD silica bonded phases (for the same compounds and under similar conditions). 

In fact, the results concerning J-CD activity in mobile phase solutions, concerning positional isomers of disubstituted benzene derivatives (23,28) seem to be consistent with those obtained by other authors using f-CD silica bonded phases (4-8). It should be mentioned that all of the above considerations concern mainly neutral molecules for the compounds undergoing dissociation, the equilibria are more complicated and the sequence of elution of isomers more variable. 

Cyclohexadienylidenes, disubstituted at the 4-position are expected to be kinetically more stable than the parent carbene, however, the rearrangement to benzene derivatives is still very exothermic. The gas phase chemistry of 4,4-dimethyl-2,5-cyclohexadienylidene Is was investigated by Jones et al.100,101 The gas phase pyrolysis of the diazo compound 2s produces a mixture of p-xylene and toluene, and by crossover experiments it was demonstrated that the methyl group transfer occurs intermolecularly via free radicals. Thus, the pyrolysis of a mixture of the dimethyl and the diethyl derivative 2s and 2t... 

Experimental Verification. The changes of capacity factor values k with (3-CD concentration in the mobile phase solution are illustrated in Figure 1 as the behavior of methylphenobarbital enantiomers on RP-18 columns (26). The similar influence of CD on k values was observed for all the studied compounds ( disubstituted benzenes, mandelic acid and its derivatives, some aromatic aminoacids, some barbiturates and hydantoins)(17-19,21-26,28). o(- or -CD additions were always followed by a decrease in the apparent capacity factor (k ) values. These results suggest that the adsorption... 

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