Retentivities on carbon

Jackson, P. T., Schure, M. R., Weber, T. P., and Carr, P.W., Intermolecular interactions involved in solute retention on carbon media in reversed-phase high-performance liquid chromatography, Anal. Chem., 69(3), 416, 1997. 

Table 11. Representative Organic Chemicals and Typical Retentivities on Activated Carbons...

Both regio- and stereospecificiiy may be influenced by the catalyst and by alkali. Raney nickel opens ce>2,3-diphenylbul-2-ene epoxide with retention of configuration to give cr3 f/iro-2,3-diphenylbutan-2-ol, whereas palladium-on-carbon gives the inverted threo isomer. If a small amount of alkali is added to nickel-catalyzed reductions, nickel too gives the threo isomer (d5). 

Figure 3-15. Catalyst activity retention vs. carbon on regenerated catalyst [12].

W.R. Moore and R.C. Hoseney, Influence of shortening and surfactants on retention of carbon dioxide in bread dough, Cereal Chem., 1986, 63, 67-70. 

PAN can be hydrolyzed quantitatively to N02 in alkaline solution. Thus, at least partial retention of PAN on alkali-coated denuders is likely. Ferm and Sjodin (42) reported up to 10% retention of PAN on an open-tube denuder coated with Na2C03. Facile conversion of N02 to N03 on carbonate-impregnated filters by oxidation with 03 has been reported (21), so similar conversion on alkali-coated denuders is likely. The use of glycerol in the alkaline denuder coating has been shown to decrease substantially the extent of nitrite oxidation (55). Slow oxidation of N02 to N03" in extracts of the denuder coating is also probable, a factor that can probably be minimized by low-temperature storage and by minimizing delays before analysis. 

The use of tandem denuders, each with the same coating, permits at least partial correction for these sources of error in HN03 measurement. Species retained with low efficiency on carbonate coatings (e.g., NO and PAN) are collected in about equal amounts by successive denuders. If it is assumed that oxidation during and subsequent to sampling occurs equally on successive denuders as well as in their corresponding extracts, the N03 from the second denuder extract can be subtracted from that in the front denuder extract to obtain a measure of atmospheric HN03. However, this technique does not correct for the efficient retention (and subsequent oxidation) of atmospheric or artifact HONO. 

Oxidative Addition of Alkyl Halides to Palladium(0). The stereochemistry of the oxidative addition (31) of alkyl halides to the transition metals of group VIII can provide information as to which of the many possible mechanisms are operative. The addition of alkyl halides to d8-iridium complexes has been reported to proceed with retention (32), inversion (33), and racemization (34, 35) via a free radical mechanism at the asymmetric carbon center. The kinetics of this reaction are consistent with nucleophilic displacement by iridium on carbon (36). Oxi-... 

Carbon-based material on a silica template has been pioneered by Knox (34). It can be used at any pH. However, the mechanism of retention on this support is quite different from that for the average alkyl-bonded silica (35). Further information on reversed-phase retention can be found in Ref. 36. 

The [2 + 2] cycloreversion reaction proceeds stereospecifically with retention at carbon atoms, its rate obeys first-order kinetics and it depends on the stereochemistry and number of substituents at the ring carbon atoms. It is therefore concluded that the reaction mechanism is a concerted [2S + 2a] cycloreversion. 

Schmid P, Suter-Hofmann ME, Schlatter C. 1989. A simplified clean-up procedure for the rapid determination of PCDDs and PCDFs based on retention on activated carbon AX-21. Chemosphere 18 1741-1748. 

Substitution at certain unsaturated centers has little direct stereochemical interest, because there is no choice, e.g. substitution at aromatic, acetylenic, and carbonyl carbons must go with retention. On the other hand, stereoselection is possible at ethylenic and allenic carbon, phosphorus (P—O, P=S) and sulfur (S=0) centers. There appear to be important mechanistic differences between substitutions at unsaturated carbon and phosphorus or sulfur. All SE, SH, SN substitutions at such carbon atoms appear to proceed in at least two steps, while those at phosphorus and sulfur may go in one or more steps. For the SN process, comparative data are available here, substitution at unsaturated carbon proceeds with retention, while at phosphorus and sulfur inversion predominates. Substitution at unsaturated phosphorus and sulfur sites was sufficiently similar to other saturated centers that it was considered with them. Because of these mechanistic differences, we shall examine substitutions at unsaturated carbon more closely. 

For LC a similar relationship should apply if the retention mechanism shows the expected theoretical dependence on carbon number. The situation is more complex since the partition coefficient is a function of many intermolecular forces. Several papers have been published showing a homologous series retention like that described for GC.5 In principle then, the retention index concept should also apply in those cases. However, little interest has been shown in developing an index for LC, probably because the paraffins are not usually run by LC and the modes of analysis by LC are much more variable and complex, so that the data are not as widely usable. 

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