131Having determined from the effect of stirring that the ligand anion is concentrated in the interface, however, one can reasonably assume that the locus for the formation of the 1:l complex is the interfacial region. Calculating the correct value of the ligand anion concentration in the interface from the observed differences in P H ,~~ values (vide supra) and assuming that since the metal ion is not surface active, its concentration in the interface is the same as it is in the bulk aqueous phase, we obtained values for rate constants that were eminently reasonable, i.e., essentially constant values for all the alkylated dithizones. Log kl for Ni and Zn are 5.13 and 8.08, respectively, which are consistent with those obtained for these metals in a broad cross section of ligand-substitution reactions. Essentially the entire cause of the unusual kinetic behavior, as well as of the unusual extraction equilibrium behavior, is the excess concentration of the ligand anion in the interfacial region, which is greatly enlarged by high-speed stirring.Our studies are now directed toward characterizing the nature and the behavior of unsupported liquidliquid interfaces, as well as the chemical reactions that occur in such an environment, an area of research that seems certain to be of significance to surface science in general as well as to extraction chemistry in particular. support of the National Science Foundation. Much of the research described here was conducted with theThe reversible concentration change in organic phase, which was caused by stirring, suggests a significant participation of the liquid-liquid interface in the equilibrium. The interfacial component is less than 3% for dithizone and then rises rather rapidly from 26% in MeDz to 82% in EtDz, to 99.9% in BuDz, and to all but 7 X in hexyldithizone. These changes are indicattive of the dramatic change in the heterogeneous acid-base equilibrium constant (up to a factor of lo5) that occurs because the anionic alkyldithizonate (but not the neutral ligand) is surface active and is adsorbed into the interfacial region.39The extraction kinetics of nickel and zinc with the alkyldithiz~nates~~ closely resembled those with dithizone and its analogues studied earlier,30 in that the rate was first order each in metal and ligand and inverse first order in hydrogen ion. However, if one were to assume that the mechanism remained the same, Le., that the rate-determining step was the formation of the 1:l complex from metal and ligand anion in the aqueous phase, one would estimate increases in the second-order reaction rate constant with increasing size of alkyl group until, with the hexyldithizone, the values for both Ni and Zn would exceed the diffusion-limited rate constant for bimolecular reaction! (39) Watarai, H.; Freiser, H. J. Am. Chem. SOC. 1983, 105, 191. (40) Watarai, H.; Freiser, H.One of the most conceptually simple and earliest studied chromophores is that of the carbon-iodine bond. The lowest lying transition involves promotion of one of the n electrons of io...

When all patients are pooled, S-100 appears to be suitable in the assessment of incidence, course and outcome of cognitive deficits. We suspect that in some surgical procedures, such as urological surgery, S-100 appears to be of limited value in detecting POCD. Neuron specific enolase did not reflect neuropsychological dysfunction after noncardiac surgery.

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655 Chart I1 LOW ENERGY Chair a 0 Twist ('Zh) ('Zd) HIGH ENERGYbetween the calculatedlo and experimental values of this energy difference. Rearrangement of meso-1 can be envisioned as occurring by one of several mechanistic pathways. Two such possibilities, dissociation-recombination and sequential 1,3-~igmatropic rearrangement, can be ruled out. Thermolysis of meso-1 in liquid cumene failed to produce detectable quantities of either 1 -methylcyclopentene or methylenecyclopentane. Furthermore, when the thermolyses were run in the gas phase, 4 could 4 N not be detected, despite the fact that 4 was shown to be stable to the reaction conditions. These findings imply that both dland meso-1 react by a [3.3] sigmatropic shift; [ 1.31 sigmatropic rearrangement and homolytic dissociation are higher energy processes.It is now recognized that there are two symmetry allowed variants of both the low and high energy [3.3] sigmatropic rearrangement.' The low energy form involves a conrotatory twisting of adjacent methylenes while, in the high energy form, rotation is disrotatory. The two variants of each form refer to the manner in which the conrotatory or disrotatory modes are coupled. The four possibilities are shown in the figures in Chart I1 (mirror images excluded). An intriguing aspect of the Cope rearrangement of diene 1 is the recognition that the most accessible conformations for rearrangement are not the chair and boat forms, but rather the less conventional twist (dl-1) and plane (meso-1) forms. In view of the similarity of the activa-H Twist (dl-2) Plane (meso-1) w tion parameters of 1 and parent 1,5-hexadiene, the twist and plane mechanisms must be considered viable alternatives to the chair and boat forms in thermal rearrangements of conformationally flexible 1,5-hexadienes.

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